high school science part i

1

Measurement in Science andTechnology

We all make use of measurements in our daily life. A milkman measures milk, a shopkeepermeasures rice or pulses, a farmer measures his field, a tailor measures the cloth beforetailoring and so on. Everybody measures something or the other with different types ofdevices. It is seen that if we know what to measure and how to measure it, we can do manythings well in life. Measurement is one of the basic life skills necessary for everyday life. Itis also useful and essential in the learning of science and technology.

There is a constant need for measurement in our everyday life. Let us find out littlemore about the process of measurement. What does this process of measurement involve?Which tools are used for accurate and precise measurement? On which factors aremeasurement techniques based? This lesson will make you aware of several such aspects ofmeasurement. In this lesson you will study about different measurement systems includingthe ancient system of measurement and the SI units. You will also learn about the methodsof measurement of various physical quantities like length, mass, time, area and volume.

OBJECTIVES

After completing this lesson, you will be able to:

• cite examples of the uses of various parts of our body and senses to measure length;• state the limitations of the use of body parts and senses for measurement and justify the

need for a standard to measure anything exactly;• describe the Indian and various other measurement systems used in the ancient times;• define a physical quantity with examples;• differentiate between fundamental and derived units;• write S.I. units of different fundamental physical quantities;• use multiples and submultiples of different units;• define the least count of a measuring instrument;• name the various devices and instruments used to measure length, mass and time stating

the standard in each case;• measure area of regular and irregular figures;• measure volume of regular and irregular solids.

: 4 : Measurement in Science and Technology

1.1 HISTORICAL BACKGROUND OF MEASUREMENT1.1.1 Body parts and senses used for measurementSince ancient times, people used their senses and body parts to measure various things.They did this because it became necessary for them in their daily life to deal with otherpeople. Let us find out how senses and body parts help us in measurement.(a) Use of our body parts and senses for measurementWe have five senses, which help us to find out about the things around us. These senses areseeing, hearing, smelling, tasting and touching. For example, if we see a tall and a shortbuilding or a tall and short person we can feel the difference in their heights. Similarly, if wetouch a body we can feel the hotness or coldness of the body. Thus, our senses do help usto guess or estimate the height, length and hotness or coldness of a body and other thingsaround us. Here, estimation means a rough measurement made by our senses.(b) Use of body parts for measurementIn ancient days, long before measuring instruments were invented, people actually useddifferent parts of their body to measure length. Figure 1.1 shows various parts of our body,which were used and can still be used to carry out various measurements. But since thesemeasurements are dependent on the size of the person, they may vary from person toperson. The length of the cubit, for example, depends on the arm length of the measurer.Thus, cubits had different lengths. To have a better understanding, let us perform an activity.

Fig. 1.1 Use of body parts for measurement

ACTIVITY 1.1Aim : To understand the accuracy in the use of body parts for measurement.What is required?A ruler, a measuring tape.What to do?! With the help of a ruler, measure the length of various parts of your body like

the arm or the palm, which are normally used for measurement.! Repeat the measurements for your friend or for a younger brother and sister

also. You can use a measuring tape also for this activity.! Compare the measurements.What do you observe?You will find that there is a difference in the measurement of your body parts withthose of your friends.

Hand

Thumb

First finger

Hand span

Measurement in Science and Technology : 5 :

(c) Limitations of our senses and body partsThough we use our senses and body parts for various measurements, we cannot trust themto measure exactly and accurately. Can you depend on your eyes to judge accurately theheight or lengths of different objects? Look at figure 1.2a. Which circle is larger-A or B?Well, both are of the same size. Larger circles around the central one make it appearsmaller. Small circles around the central circle make the other appear larger.

Fig. 1.2 Limitations of our senses and body parts in measurement

There are many more such instances where objects can fool our eyes. Now look atfigure 1.2b and tell which line segment is larger. Verify your estimation by measuring eachline segment with the help of a scale.

In the above mentioned cases we tried to guess the length or size by seeing i.e. tried togive an estimate, which may or may not be correct. Thus, the use of senses or body partsfor measurement does not provide

! accuracy of measurement,! reliability of measurement,! uniformity of measurement,

The limitations of the use of senses and body parts have made us to develop somedevices and instruments for accurate measurements.

1.1.2 Indian measurement system

a) Indian measurement system in the ancient periodMeasurement plays an important role in our lives. We have been using measurement rightfrom the pre-historic time. Let us have a brief look into the historical development ofmeasurement system in India. In ancient periods, the lengths of the shadows of trees orother objects were used to know the approximate time of the day. Long time durationswere expressed in terms of the lunar cycles, which even now is the basis of some calendars.In India, excellent examples of measurement practices in different historic periods areavailable. Our ancient literature reveals that in India different types of measurement practiceswere followed in different periods. For example, about 5000 years ago in the ‘Mohenjodaroera’, the size of bricks all over the region was same. The length, breadth and width of brickswere taken as a standard and were always in ratios of 4:2:1.

Similarly around 2400 years ago during the Chandragupta Maurya period there was awell-defined system of weights and measures. The government at that time ensured thateverybody used the same weights and measures. According to this system, the smallest unitof length was 1 Parmanu. Small lengths were measured in anguls. For long distancesYojana was used. One yojana is roughly equal to 10 kilometres.

(a) Estimating the size of the circle

A B

(b) Estimating the length of a line segment


The Indian medicine system, Ayurveda, also had well-defined units for themeasurement of the mass and volume. The measurement system was strongly followedto ensure the proper quantity of medicine for particular disease.

Different units of measurements used in the period of Chandragupta Maurya8 Parmanus = 1 Rajahkan (dust particle from the wheel of a chariot)

8 Rajahkans = 1 Liksha (egg of lice)

8 Likshas = 1 Yookamadhya

8 Yookamadhyas = 1 Yavamadhya

8 Yavamadhyas = 1 Angul

8 Anguls = 1 Dhanurmushti

(Reference: Kautilaya’s Arthashastra)

b) Indian measurement system in the medieval periodIn the medieval period also the measurement system was in practice. As described in Ain-i-Akbari by Abul Fazl-i-Allami, during the period of Moghul Emperor Akbar, the gazwas used as the unit of measuring length. Each gaz was divided into 24 equal parts andeach part was called Tassuj. This system was extensively used to measure land pieces, forconstruction of buildings, houses, wells, gardens and roads. You should know that, the gazwas widely used as a unit of length till the metric system was introduced in 1956. Eventoday in many parts of our country, particularly in the rural areas, gaz is being used as a unitof length.

c) Indian measurement system during British periodIn order to bring about uniformity in the system of measurement and the weights used, anumber of efforts were made during the British period. The British rulers wanted to connectIndian weights and measures to those being used in Great Britain at that time. During thisperiod the inch, foot, and yard were used to measure length whereas grain, ounce, pounds,etc. were used to measure mass. These units and weights were used in India till the time ofIndependence in 1947. The essential units of mass used in India included Ratti, Masha,Tola, Chhatank, Seer and Maund. Raatti is a red seed whose mass is approximately 120mg. It was widely used by goldsmiths and by practitioners of traditional medicine system inIndia.

Relation between various units of mass used during the British period

8 Ratti = 1 Masha

12 Masha = 1 Tola

5 Tola = 1 Chhatank

16 Chhatank = 1 Seer

40 Seer = 1 Maund

1 Maund = 100 Pounds troy (exact)

CHECK YOUR PROGRESS 1.1


1. Name the smallest unit of length during the Chandragupta Maurya period.2. List out our body parts normally used for measurement.3. In which period was ‘gaz’ used as a unit to measure length?

1.2 THE MODERN MEASUREMENT SYSTEM

In order to overcome the limitations of senses and body parts, and to bring about a worldwideuniformity in the measurement system, the need for exact measurement was felt. For this, astandard of measurements had to be developed which everybody everywhere accepts.

The problem of measuring lengths exactly was first solved by the Egyptians in 3000B.C.They did this by inventing the standard cubit. They realized that the length of the armactually did not matter as long as people of Egypt were concerned. Then they mademeasuring sticks exactly of the same length as that of standard cubit. In this way they madesure that the cubit was the same length all over Egypt. That is really how measurement iscarried out today. In fact, for each measurement a standard is chosen. Every measuringinstrument has to be compared with that standard. The present measurement system, whichis accepted world-over, has its origin in the French Revolution. You will study the details ofthe modern system of measurement, in the following sections.

1.2.1 Fundamental quantities and units

You have read that measurements are concerned with quantities like length, mass, time,density etc. Any quantity which can be measured is called a physical quantity. Out of thedifferent physical quantities, there are seven physical quantities in terms of which otherphysical quantities can be measured. These fundamental physical quantities are length, mass,time, electric current, temperature, luminous intensity and amount of substance. Suchquantities are considered to be the basic or fundamental physical quantities.

If you are asked to measure the quantity of a given amount of milk, you will express thevolume of milk in some accepted units of volume. Likewise, if an engineer measures thelength of a road that connects two cities, he should express the distance in an accepted unitof length. Such a procedure makes life more comfortable. If there were no common unitsaccepted by all, life would be miserable. Such units are much more essential in scientificmeasurements to facilitate communication of information at international level.

Any measurement of a quantity includes a reference standard or unit in which thequantity is measured and the number of times the quantity contains that unit. Thus, whenwe say that the length of a rod is 4 metres, the rod is four times the metre, which is the unitof length. Metre is the standard length that is adopted as a standard for comparison whilemeasuring length. In the process of measurement the accepted reference standard which isused for comparison of a given quantity is called a unit.

1.2.2 The SI units

Scientists have developed and used several systems for expressing the units of physicalquantities. However, all measurements in any system are based on the units of the basic orfundamental physical quantities. The units of the fundamental or basic quantities that areindependent of each other are called fundamental units.


Keeping in view the importance of the proper units for measurement, there have beenattempts over centuries in several developed civilizations to suggest standard units ofmeasurements at international level. In the year 1967, the XIII General Conference onWeights and Measures rationalised the MKSA (Metre, Kilogram, Second, Ampere)system of units and adopted a system based on six basic units. It was called the SystemeInternationale de unites known as SI units in all languages. In 1971, the GeneralConference added another basic unit to the SI units i.e., mole for the amount of substance.

The fundamental units in different systems are different. The international system ofunits, known as SI units, are commonly used for all scientific purposes. This system hasseven basic units for seven physical quantities, which are given in Table 1.1.

Table 1.1: SI units and their symbols

Physical quantity Unit Symbol

Length metre m

Mass kilogram kg

Time second s

Temperature kelvin K

Amount of substance mole mol

Electric current ampere A

Luminous intensity candela cd

Perhaps you may be confused by mass and amount of substance and also with luminousintensity as given in Table 1.1. The mass of a body is the amount of matter contained in thebody, while a mole is the amount of any substance equal to its molecular mass.

1 mole of HCl = 36.46 g2 moles of HCl = 36.46 x 2 = 72.92 g

Luminous intensity is the amount of light emitted by a point source per second in aparticular direction.

The yard and mile as units of length are still in use in USA.

Units of length still in use in USA1 mile = 8 furlongs1 furlong = 220 yards1 yard = 3 feet1 foot = 12 inches1 yard = 0.9144 m (exactly)1 inch = 2.54 cm (exactly)1 mile = 1.61 km

The guiding principle in choosing a unit of measurement is to relate it to commonman’s life as far as possible. As an example, take the unit of mass as kilogram or the unit of


length as metre. In our day-to-day business we buy food articles in kg or tens of kg. Webuy cloth in metres or tens of metres. If gram had been chosen as the unit of mass ormillimetre as unit of length, we would be unnecessarily using big numbers in our daily life.It is for this reason that the basic units of measurements are very closely related to our dailylife.

1.2.3 Standard units of fundamental quantities

Once we have chosen the fundamental units of the SI, we must decide on the set of standardsfor the fundamental quantities.

a) Mass: The SI unit of mass is kilogram. One kilogram is the mass of a particularcylinder made of Platinum–Iridium alloy, kept at the International Bureau of Weightsand Measures in France. This standard was established in 1887 and there has been nochange because this is an unusually stable alloy. Prototype kilograms have been madeout of this alloy and distributed to member states. The national prototype of India is theKilogram no 57. This is preserved at the National Physical Laboratory, New Delhi.

b) Length: The SI unit of length is metre. Earlier the metre (also written as meter) wasdefined to be 1/107 times the distance from the Equator to the North Pole throughParis. This standard was abandoned for practical reasons. In 1875, the new metre wasdefined as the distance between two lines on a Platinum-Iridium bar stored undercontrolled conditions. Such standards had to be kept under severe controlled conditions.Even then their safety against natural disasters is not guaranteed, and their accuracy isalso limited for the present requirements of science and technology. In 1983 the metrewas redefined as the distance travelled by light in vacuum in a time interval of 1/299792458 seconds. This definition establishes that the speed of light in vacuum is299792458 metres per second.

c) Time: The SI unit of time is second. The time interval second was originally defined interms of the time of rotation of earth about its own axis. This time of rotation is dividedin 24 parts, each part is called an hour. An hour is divided into 60 minutes and eachminute is subdivided into 60 seconds. Thus, one second is equal to 1/86400th part ofthe solar day. But it is known that the rotation of the earth varies substantially withtime and therefore, the length of a day is a variable quantity, may be very slowly varying.The XIII General Conference on Weights and Measures in 1967 defined one second asthe time required for Cesium–133 atom to undergo 9192631770 vibrations. Thedefinition has its roots in a device, which is named as the atomic clock.

d) Temperature: The SI unit of temperature is kelvin (K). The thermodynamic scale onwhich temperature is measured has its zero at absolute zero, and has its lower fixedpoint corresponding to 273.15 K at the triple point of water (0o C). One unit ofthermodynamic temperature (1K) is equal to 1/273.15 of the thermodynamictemperature of the triple point of water.

e) Electric current: The SI unit of electric current is the ampere (A). One ampere isdefined as the magnitude of current that when flowing through two long parallel wires,each of length equal to 1 m, separated by 1 metre in free space, results in a force of 2x 10-7 N between the two wires.

f) Amount of substance: The SI unit of amount is mole (mol). One mole is defined as


the amount of any substance, which contains, as may elementary units, as there areatoms in exactly 0.012 kg of C-12.

g) Luminous intensity: The SI unit of luminous intensity (I) is candela (Cd). The candelais defined as the luminous intensity, in a given direction, of a source that emitsmonochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensityof 1/683 watt per steradian in that direction.

1.2.4 Derived units

The basic units or the fundamental units are independent of each other. The units of allother physical quantities can be expressed in terms of these basic units. Such units arecalled derived units. Thus, the units, which are obtained by the combination of fundamentalunits, are known as derived units. For example, area can be expressed in terms of the basicunit of length, as given below:

You know the area of a surface is the product of length and breadth. Therefore, the unitof area will be equal to the product of the unit of length and the unit of breadth (rememberthat breadth is also length).

Unit of area = metre x metre = (metre)2

Thus, the unit of area is m2.Similarly, volume is equal to length x breadth x height of the object.

Therefore, the unit of volume = unit of length x unit of breadth x unit of height

= metre x metre x metre = (metre)3

Thus, the unit of volume is m3.

The derived units of other physical quantities are also found in the same way. Some ofthe commonly used derived units are given in Table 1.2.

Table 1.2 : SI units and symbols of some derived units

Physical quantity SI Unit Symbol

Area square metre m2

Volume cubic metre m3

Density kilogram per cubic metre kg/m3

Velocity metre per second m/s

Acceleration metre per square second m/s2

Force kilogram metre per square second kg m/s2

(also called Newton) (called N)

Work kilogram square metreper square second kg m2/s2

(also called Joule) (called J)

There are some other commonly used derived units with special names. They are givenin the Table 1.3.

Table 1.3: Some commonly used derived units


Physical quantity Special name Symbol SI UnitForce Newton N kg m/s2

Pressure Pascal Pa N/m2

Energy joule J NmPower watt W J/s

1.2.5 Multiples and sub-multiples of units

Sometimes the measurement of physical quantities can give very large or very small numbers.The smaller and larger units of the basic units are multiples of ten only. They strictly followthe decimal system. These multiples or submultiples are given special names. These arelisted in Table 1.4.

For example, the mass of the earth and mass of the electron are found to be as follows:

Mass of earth (M) = 5,970,000,000,000,000,000,000,000 kg

Mass of an electron (me) = 0.000,000,000,000,000,000,000,911 kg

You will notice that it is not a convenient way to express the mass of earth or the massof an electron. It takes up space and time to read it. Thus, for convenience, large numbersor very small decimals are expressed in an abbreviated form. The abbreviations in commonuse are based upon the powers of ten as given in the Table 1.4.

Table 1.4: Representation of large and small quantities in powers of ten

Large quantities Small quantities

100 = 1 1 = 100

10 = 101 0.1 = 10–1

100 = 102 0.01 = 10–2

1,000 = 103 0.001 = 10–3

10,000 = 104 0.0001 = 10-4

100,000 = 105 0.00001 = 10–5

1,000,000 = 106 0.000001 = 10-6

Thus, 103 = 10 ×10 × 10 = 1000

and, 001.01000

1

10

110

33 ===−

Example 1.1: Suppose a large ship has a mass of nine hundred thousand kilograms. Expressit in powers of ten.Solution: Given, mass of ship = 900,000 kg

Thus, in powers of ten, the mass of ship = 9 × 105 kgExample 1.2: Express the number 0.00034 in terms of powers of ten.Solution: 0.00034 = 3.4 × 10 – 4

This concept has been used to express multiples and submultiples of basic units of


measurement – again for the purpose of convenience. For example, let us take the SI unitof length, i.e. metre. Its multiples and submultiples would be:

Multiple Sub-multiple1000 metres = 1 kilometre 1/1000 metres = 1 millimetre

or 103 m = 1 km or 10–3 m = 1 mm

As the metric system uses the base 10, the change from one unit to the another is veryeasy and it uses simple prefixes to denote multiples or submultiples of the basic units. Forexample, prefix kilo always means 1000 whether it is kilometre (1000 m) or kilogram(1000 g), kilowatt (1000 W) or whatever. Similarly, the prefix centi always means 1/100while the prefix milli always denotes 1/1000. A list of prefixes for multiples and submultiplesis given in Table 1.5.

Table 1.5: Prefixes for multiples and submultiples

Name Symbol Equivalentdeca da 101

hecta h 102

kilo k 103

mega M 106

giga G 109

terra T 1012

deci d 10–1

centi c 10–2

milli m 10–3

micro µ 10–6

nano n 10–9

pico p 10–12

CHECK YOUR PROGRESS 1.21. What are the characteristics of a physical quantity?2. Differentiate between fundamental and derived units.3. What is the difference between mass and amount of a substance?4. Derive the unit of the following quantities:

(i) Force = Mass × acceleration(ii) Pressure = Force/Area

5. Represent 237 nm in metres.1.3 MEASUREMENT OF QUANTITIESWe use measurements of different types in our daily life. For example, while buying cloth,we measure its length and while buying milk or kerosene we measure its volume. But foraccurate and precise measurement, we have to follow certain methods. Let us study someof them.

Let us consider a physical quantity, say length. We know that its standard of measurementis metre. Measuring sticks with the same length as the standard metre have been madewhich we commonly call as the metre stick. This one metre long stick is divided into 100equal parts, i.e. into 100 centimetres. Each centimetre is further divided into 10 millimetres.Thus, the smallest division on a metre scale is 1 millimetre. This is the least count of the


metre scale. Thus, the minimum or the least quantity that can be measured by a giveninstrument is called its least count. For example, the least count of a metre scale is 1 mmor 0.1 cm. A metre scale cannot measure lengths less than 1mm. The least count of anymeasuring instrument is, thus, very important. We must always quote the result of ameasurement only up to the least count of the measuring instrument used. Besides, themethod and the selection of proper measuring device for a particular measurement are alsovery important.

1.4.1 Length and its measurementAs we have studied in the last section, length is the distance between two points and it ismeasured in terms of metres. Different types of devices are used to measure lengths. Forexample to measure the length of your table, you will use a ruler or measuring tape. But tomeasure the diameter (thickness) of a wire, you will require a screw gauge. These deviceshad been made by comparing them with a standard length called standard metre. The standardmetre is a fixed length decided by scientists and accepted by all.

a) Using a scale to measure lengthTo measure the length of a given line segment AB (Fig 1.3), the metre scale is kept alongthe line segment with its graduations parallel to it. The metre scale must be so placed thatits divisions are as close as possible to the line segment to be measured. Its zero end ismade coincident with one end of the line segment. Note the point where the other end ofthe line segment lies. Suppose, it lies beyond the 2 cm mark and is coincident with thesecond small division after it. Since each of these marks is 1 mm, the total length of the linesegment is

2 cm + 2 mm = 2 cm + 0.2 cm = 2.2 cm

Fig. 1.3 To measure the length of a line segment using a metre scaleRemember that while looking at the reading on a scale, we must keep our eyes in front

of and in line with the reading to be taken. In case of a metre scale, it is not always possibleto make the zero mark on the scale coincident with one end of the line to be measured. Withrepeated use, the ends of measuring scale get somewhat worn-out and ill defined. In suchcases, we keep the metre-scale with any of its divisions other than zero coincident with oneend of the line. Suppose we place the scale (ruler) in such a way that the two ends of the linesegment coincides with 2.0 cm and 4.2 cm marks, respectively (Fig 1.4). Then, the lengthof the line segment is

4.2 cm – 2.0 cm = 2.2 cm

That is, it is the difference between the readings on a scale at its two ends.

Fig. 1.4 The length of a line segment is the difference between the readings on a scale at its two ends.

If we have to measure a larger length, such as length of a playground, we use a measuring

1 2 3 4 5 6 7 8 9 10 11 12

1 2 3 4 5 6 7 8 9 10 11 12

A B

A B


A B

C D

A B

ND

50

20

15

10

9

E G

Q

Sugar1 Kg

Sugar1 Kg

tape that may be 10 m, 15 m or 50 m long.

Sometimes, we are required to measure very small lengths, say (less than 1 mm) likethe diameter of a thin wire, the dimensions of fine machine parts, etc. We cannot use ametre scale for such measurements. For such distances, measuring instruments like thevernier callipers and the screw gauge are used. Vernier callipers, as shown in Fig 1.5, is aninstrument used to measure the length or thickness of a solid body up to 0.01 cm accurately.A screw gauge as shown is Fig. 1.6, is an instrument used to measure the length or thicknessof a solid body up to 0.001 cm. accurately However, each measuring instrument is limitedto a certain accuracy of measurement which depends on its graduation. To measure thethickness of a wire or a metallic sheet we require screw gauge.

Fig.1.5 Vernier callipers Fig.1.6 Screw gauge

To measure large distances like the distance of your school from your house, or distancebetween two cities or the distance between the earth and moon, we use indirect methodsof measurement. For example, to measure the distance between two cities, we will measurethe average speed of certain vehicle, say a car, and the time taken by it to cover that distance.The product of the speed and time will give the required distance.

1.3.2 Mass and its measurementLike length there are many other measurements, which we make in our daily life by usingdifferent measuring standards and instruments. For any object, say this book, if some bodyasks you to answer the question, “How much stuff is there in it”? It means he is trying tofind out the mass of the object. As you have studied earlier in this lesson, mass of a body isdefined as the amount of matter contained in the body. The standard mass chosen by thescientists is called kilogram. This standard is used to compare the masses of unknownbodies.

In order to measure the mass of different bodies different types of balances or scalesare used. The most common is the one we see with the shopkeepers and vendors. Whatstandard masses are used by shopkeepers to measure quantities? What do their balancelook like? Have you seen a balance like the one in figure 1.7(a) or 1.7(b).

ACTIVITY 1.2Fig. 1.7 (a) The shopkeeper’s balance (b) A modern balance

(a) (b)


Aim : To make a sensitive balance to compare the masses of light objects.

What is required?

A tall bottle like a squash bottle or an oil bottle, two square pieces having each sideabout 15 cm in length cut from a sheet of a chart paper, a few drinking straws, pins,sewing thread, gum, plasticine (or wet atta)

What to do?

! Use the square papers to make pans as shown in Fig.1.8.

! Draw a small square at a distance of 2 cm from the edges.

! Fold the paper along the outline of the inner square.! Fold again along dotted lines and fix the paper to outer side of the scale pan

with gum.! Pass lengths of sewing thread through the centre of the four sides. Make a knot

so that four stands are of the same length.! Measure the drinking straw and find its mid-point. Pass a pin through this point.! Balance the pin on a piece of small rubber (eraser) which is glued or fixed with

cello tape to the bottle cap.! Tie the pans near the two ends of the straw in such a way that they are at equal

distances from the mid-point, i.e. the pin.! Check to see if they are balanced, otherwise use little bits of atta or plasticine on

the pans.! This balance can be used to compare the weights of small objects like paper

clips and buttons.! Try and find out the amount of water loss when leaves dry up by weighing them

when green and drying them on a hot plate and re-weighing.

Fig. 1.8 Method to make a sensitive balance

A shopkeeper’s balance, however, does not provide accurate measurement of massesthat is needed. In some cases, for example, to find the mass of a piece of gold or the

Pin

Stapil

Straw

Pan Pan


composition of chemicals required to make aspirin, etc. For accurate measurement ofmasses a physical balance is used. Figure 1.9 shows a physical balance. Known massesfrom a standard box are used with this balance.

Fig. 1.9 A physical balance

1.3.3 Time and its measurement

Time is measured when you answer questions like, how long does it take to reach Delhifrom Bombay? How long do the fruits last? When does the school start?

All these questions relate to happenings of two events with a gap between them. Forexample, if someone says, “It took me 17 hours to travel by train from Delhi to Bombay”,she is thinking of a measurement of the time interval between a first event (i.e. leavingDelhi) and a second event (i.e. arriving Bombay). She may have measured this intervalwhich is a time interval by looking at her watch when she departed and when she arrived.Thus, when we measure time we measure the interval of time between two events.

Sundial

Long long ago, people noticed that shadowswere long in the morning and evening, and werethe shortest when the sun was directly overheadat noon. From these observations they learnt totell the hour of the day. Based on it, the world’sfirst timepiece – the sundial was made. Thesundial was a hemispherical opening in a blockof stone or wood. It had an upright rod, calledgnamon fixed in the center of the opening (Fig.1.10). The shadow of the gnamon travelled overthe day, telling the time of the day. But thesundial had certain limitations. Can you thinkof them?

Time is measured in seconds (s), minutes (m), hours and days with stop watches andclocks. Our early ancestors used the alternation of the day and night as a clock. They did

Fig. 1.10 A sundial


this because this phenomenon repeats itself at regular intervals of time. As such, theyconsidered this as a standard with which they used to compare an unknown time interval.Such a system, which repeats itself at regular intervals of time, is called periodic system.

The measurement of time is really the comparison of an unknown time interval with thestandard time interval of a periodic system. Based on this, instruments like sundials, waterand sand clocks were used in early times to measure time intervals.

In fact, water clock was the ancestor of our mechanical clocks. Let us perform anactivity to understand the working of a water clock.

ACTIVITY 1.3

Aim : To use a water clock to measure your pulse or your friend’s pulseWhat is required?Water, beaker, a paper cup and a pinWhat to do?! With the help of a pin make a very small hole in the bottom of the cup.! Place your finger over the hole and fill the cup with water.! Hold the cup over a sink or a larger beaker and remove your finger from the

hole. The water should drip from the hole and you should be able to count thedrops easily. If water runs out instead of dripping, get another cup and try tomake a smaller hole.

! After preparing the water clock (Fig. 1.11), use your middle finger and lightlyfeel your pulse.

! You start counting your pulse, you tell your friend to start counting the dropsfrom the cup at the same time. Both of you have to start and stop at the sametime.

! Record the time taken by the heart to beat 15 times in terms of ‘drops’.! Repeat this with your friend. Is there a difference in the pulse rate between you

and your friend?

Fig. 1.11 Working of a water clock

These clocks of early times however, were inconvenient to use because the sundial couldnot be moved form one place to another place and sand and water clocks had to be attendedregularly.The real advancement in the construction of clocks came with the introduction of thependulum. Let us see how pendulum helped us in measuring time.The pendulum—A tool to measure time


A

CB

Stone

Fig. 1.12 A simple pendulum

(a) Stop Watch (b) Electronic Watch (c) Quartz Clock

Fig. 1.13 A pendulum clock

Tie a small stone to one end of a long can be used as a stringand hang it with the help of the other end to a firmsupport. This may be used as simple pendulum. Pull t h estone gently to one side and let it go. The stone begins t omove to and fro, i.e. oscillates (Fig. 1.12). Make sure that itdoes not move in circles.

When the pendulum was at rest, it was at A. Thisposition is called the mean position. When it swings, itmoves form A to B, back to A, from A to C and back to A. In this way it completes one fullswing. Each swing is called one oscillation. The distance from A to B or from A to C iscalled amplitude of the oscillation. Amplitude of a pendulum is the maximum distance thependulum moves away from the mean position while it is oscillating. The time taken forone oscillation is called the time period of the pendulum.

Once your pendulum has started swinging steadily you can use your stopwatch or awristwatch with seconds hand to find out your pendulum’s time period. For this, you maycount how long your pendulum takes to make 20 oscillations and then from it, the time forone oscillation can be calculated.

Pendulum clock

The pendulum was used as a time controller in clocks. I n1656, Christian Huygens, a Dutch scientist, made t h efirst pendulum clock, which was regulated by amechanism using a ‘natural’ period of oscillation.Although, Galileo had invented the pendulum andnoticed that the time taken by the weights hangingfrom a chain or rod to swing back and forth is exactly t h esame amount of time. The whole system wasenclosed in a case and thus became the grandfatherclock. The length of pendulum and the acceleration dueto gravity at a place determined the time taken in oneoscillation.

Though with the discovery of pendulum clocks, time keeping became almost accurate,but it had certain limitations like acquiring large space, and difficulty in movement from oneplace to the other. Therefore, spring watches were discovered. Such watches have a flatsteel-bound spring, which is coiled tight by winding the spring. As the time passes thespring uncoils moving the hour and minutes hands attached to it. Thus, it tells us the time.

With the advancement of science and technology and to meet the need of more accuratetime measurement, quartz clocks and atomic clocks came into existence.


Fig. 1.14 Different types of clocks

Quartz clocks

Quartz clocks came into existence in 1929 when quarts crystal rings were used in themechanical clock. But they became popular in 1970. The rings were connected to anelectrode in a circuit. When a current is passed through the circuit, the crystal vibrates ata regular frequency. This helps us to measure time. The quartz clocks lose one second inevery 10 years.


1. You are given some words like pans, beam, pointer, weights, objects. Use these wordsto fill up blanks in the following paragraph, which gives a general description of abalance.“A balance has two_____________supported from a rigid_____________ At the centerof the support there is a _____________which is free to move. In one pan the_____________to be measured are placed. In the other pan _____________ are placedone by one to balance both the pans.

2. Estimate the length of this page of your book in the following ways:(i) by just looking at it (i.e., seeing)(ii) with the help of your fingers(iii) by using your ruler (in cm)

3. Why were the clocks of early times inconvenient to use?1.4 MEASUREMENT OF AREAThe concept of area finds considerable use in our day to day life. For example, we have toconsider the area of the top of the table while buying glass or mica for it. Similarly, thefarmer has to consider the area of his field while estimating about the crop yield and so on.Now, the question arises what ‘area’ is?

In fact, the area of a figure can be defined as the surface enclosed by the figure or theextent of the surface of the figure. Like every other physical quantity, we need a unit of areaalso, for its measurement. The area of a square of side 1m is taken as SI unit of area, whichis one square metre, and it is abbreviated as 1 m2. To measure areas, we often use the unitscm2, mm2, km2, etc. Also knowing that 100 cm = 1m, we have

104 cm2 = 1m2

and 1 km2 = 106 m2, 1 m2 = 10–6 km2

Now, let us see, how the areas of different types of figures are measured.1.4.1 Areas of regular figuresTo measure the areas of regular geometrical figures like a rectangle, a triangle, or a circle,

we have well-known formulae. Some of these are given inTable 1.6

Table 1.6: Formulae to calculate the areas of somegeometrical figures

Figure Area

Rectangle length ×breadth


Triangle ½ ×base × height

Circle π× (radius)2

Parallelogram base × altitude

Using these formulae, we can calculate the required area.For example, if you are asked to find the area of a rectangularplayground whose sides are given as 50 m and 60 m, youcan easily calculate the area by finding the product of thetwo sides of the playground.

1.4.2 Area of irregular figures

You can easily find the areas of regular figures by usingformulae. But the problem arises in the case of irregularfigures. Because, an irregular figure does not have anydefined length, breadth, etc. We cannot, therefore, use anyformulae to calculate its area. In such cases, we make use ofgraph papers having squares of side 1 cm each as shown inFig. 1.15. First, we draw the outline of the given figure of irregular shape on that graphpaper. Then we count the number of complete squares in it and the number of incompletesquares. While counting the incomplete squares, we count only those squares that lie halfor more within the figure; the other incomplete squares are neglected. The total number ofsquares thus counted gives the approximate area of the given irregular surface in cm2.

In order to measure the areas of the irregular figures of very big size like field orplayground, we split them into regular-shaped figures. Then the area of each figure iscalculated and added to find the total area.

CHECK YOUR PROGRESS 1.41. By what factor will the area of a rectangle increase if all its sides are increased 3 times?2. A circular tabletop has a radius of 1.4 m. What is the area of mica needed to cover it?3. How will you measure the area of the leaf of a plant?4. The area of a figure is 60 m2, what is its value in cm2?

1.6 MEASUREMENT OF VOLUME

You would have seen that all the materials occupy certain space. The total space occupiedby any piece of matter is referred to asits volume. The SI unit used for volumemeasurement is the volume of a cube ofside 1m each. We call this unit as onecubic metre, abbreviated as 1m3. Tomeasure smaller or larger volumes, weuse other appropriate units like cm3,mm3, or km3.

Now, let us study how to measure thevolume of different types of bodies.

Table 1.7: Volume of regular solids

Solids Volume

Cube (side)3

Cuboid Length × breadth × height

Sphere (4π/3)× (radius)3

Cylinder π(radius)2× height

Fig. 1.15 Method to find thearea of an irregular figure


1.6.1 Volumes of regular solids

To measure the volumes of regular solids like cube, sphere or cylinder etc., we have wellknown formulae. Some of such formulae these are given in Table 1.7.

You would have seen a milkman or a kerosene dealer using volume-measuring vesselsas shown in Fig. 1.15 These are generally cylindrical or conical in shape and have theircapacity in litres. A litre is one-thousandth part of the SI unit of volume, i.e. m3.

1 litre = 10-3m3

a) Taking a reading of liquid level in a measuring cylinderIt is observed that liquids like water form a concave meniscus as shown in Fig. 1.17a, while

those like mercury form a convex meniscus Fig. 1.17b. Now, question arises how to takecorrect readings of the liquids in such cases. We must keep our eyes in line with the flatmiddle part of the liquid while taking a reading. If we just look at the measuring cylinderand water level we will get a wrong reading.

1.6.2 Volume of irregular solids

In order to measure the volume of irregular solids,we follow an indirect way of measurement. Forthis purpose, we use a graduated cylinder or anoverflow can. Let us see, how?

a) Using graduated cylinder

For small solids, we half-fill the given graduatedcylinder with water and note the reading. Now, dipthe solid in it after tying it with a thread as shownin Fig. 1.17. You will notice that the water levelrises in the cylinder. Note this reading also. Thus,the difference in the readings of the water levelbefore and after insertion of the solid gives the

1,000

1 Litre

500 250

1/4 Litre1/ Litre2

a b

10

20

30

40

50

60

70

80

90

100

Concave meniscus

Convex meniscus

Fig. 1.16 Volume measuring vesselsFig. 1.17 (a) Liquid with concave meniscus (b)

Liquid with convex meniscus

Fig. 1.18 Measuring the volume of a solidusing graduated cylinder

10

20

30

40 40

50 50

60 60

70 70

10

20

30


volume of the solid.

We cannot use water if the given solid is a pieceof water soluble material, such as rock salt. In such acase, we must use a liquid in which the given solidneither dissolves nor reacts chemically.

b) Using an overflow can

If the given solid is so large that it cannot be dippedin a graduated cylinder, then we use a large overflowcan with a spout. We fill the overflow can with watertill it starts overflowing as shown in Fig 1.19.

We wait till no more drops overflow. We thenplace a clean graduated cylinder below the nozzle ofthe overflow can and dip the given solid in it. Some water overflows and collects in thegraduated cylinder. The volume of water overflown is carefully noted. This is equal to thevolume of the given solid.

CHECK YOUR PROGRESS 1.51. Why do we need a suitable oil while determining the volume of a piece of rock salt

using a graduated cylinder?2. How many cm3 will be there in one litre?3. What is the shape of the meniscus of milk in a cylinder?4. What is the volume of a sphere of radius 7 cm?

LET US REVISE

! Measurement is basically a process of comparison and involves two things: a numberand a unit.

! The unit of physical quantity is a standard value of it in terms of which other quantitiesof that kind are expressed.

! There are seven fundamental quantities amount of subsances namely length, mass, time,temperature, amoung of substances light intensity and electric current.

! There are seven SI units and a number of derived units.

! A metre scale is used to measure large lengths. To measure small lengths, we usevernier callipers or screw gauge.

! Area is measured in square metre (m2) and graph papers are used for estimating theareas of irregular figures.

! The total space occupied by any piece of matter is called its volume. It is measured incubic metres (m3). The unit ‘litre’ is also used to measure the volume of liquids.

! Standard measuring vessels are used to measure volumes of liquids like milk, keroseneoil, mobile oil at petrol pumps, etc.

! In the laboratory, we use graduated cylinder and an overflow can to measure the volumeof large irregular bodies.

TERMINAL EXERCISES

10

20

30

40

50

60

70

80

Fig. 1.19 Measuring the volume of a solidusing an overflow can


A. Multiple choice type questions.1. Which of the following is not an SI unit?

(a) Metre (b) Pound(c) Kilogram (d) Second

2. If the mass of a solution is 10µg, it is the same as(a) 10-6g (b) 10-12g(c) 10-9g (d) 10-3g

3. A line segment was measured using a scale. One end of the line segment coincided withthe 1.3cm mark on the scale. The other end coincided with 7.2 cm mark. The length ofthe line segment is(a) 1.3cm (b) 7.2cm(c) 8.5cm (d) 5.9cm

4. Rajesh travelled from city A to city B by car. The average speed of the car was 70 km/h. It took 4h 30min to cover the distance. The distance between the two cities is(a) 315km (b) 280km(c) 2100km (d) 17.5km

B. Descriptive type questions.1. What are the limitations of using our senses and body parts for measurement?2. Define the following key concepts

(i) Estimation(ii) Standard of measurement(iii)Standard metre(iv)Time interval(v) Pendulum

3. Name the SI units used to measure length, mass, time and temperature.4. Give four examples of periodic systems?5. Define amplitude and time period of a pendulum.6. Airplane pilot cannot use his senses to guide his plane through thick clouds. He must

depend on the plane’s instruments. Why?7. In a village 100 acres of land was distributed among ten farmers. The farmers were

very happy because all of them got equal-sized plot of land. How did the Head of thePanchayat manage to do this?

8. Goldsmith uses a balance to measure gold ornaments. Why does he use an instrumentfor this purpose?

9. In 100 metre race, you must have seen that for each athlete the judge looks at a stopwatch to measure the ‘time’ required by the athlete to complete 100 metres. What doesthis ‘time’ mean?

10. Describe the method for finding out the area of a leaf.11. Measure the diameter of a glass marble by using a scale and two wooden blocks.

Which other instrument can be used for finding it more accurately? Why?12. A thin wire is closely wound on a pencil with its successive turns in contact with each


other. If turns of the wire occupy a total distance 2 cm, what is the diameter of the wire.Which other instrument can be used for more accurate result?

13. How much volume of petrol is needed to fill a spherical tank of radius 2.1 m?14. Why a standard reference is taken as a unit?

ANSWERS TO CHECK YOUR PROGRESS

1.11. Parmanu2. Arm, angul, cubit, etc.3. During the period of Moghul emperor Akbar.

1.21. It can be measured and is a subject of study through our five senses.2. a) Fundamental units are only seven in number whereas derived units are very large in

number.b) Fundamental units are independent of each other but derived units are obtained

from fundamental units.3. Mass of a body is the amount of matter contained in a body while the amount of

substance is equal to its molecular mass.4. Unit of force = Unit of mass x Unit of acceleration = kg ms-2

5. Unit of pressure = Unit of force/Unit of area = kg ms-2 / m2 = kg m-1s-2

6. 237nm = 237 x 10-9m = 2.37 x 10-7m

1.31. pans – beam – pointer – objects – weight2. Do as in section 1.3.1.3. They were heavy and bulky and could not be taken from one place to another.

1.41. 9 times2. 6.16 m2

3. refer section 1.4.24. 600000cm2

1.51. We cannot use water because rock salt will dissolve in water but not in oil.2. 1000 cm3

3. concave4. 1437.33 cm3

GLOSSARY

Area of a figure: the surface enclosed by a figure or the extent of the surface of afigure.

Derived units: Units that are obtained by the combination of fundamental units.Fundamental units: The units of fundamental or basic quantities that are independent

of each other.Least count: The minimum or least quantity that can be measured by a given instrument.Physical quantity: Any quantity that can be measured.Periodic system: A system that repeats itself at regular intervals of time.Unit: The accepted reference standard which is used for comparison of a given quantity.Volume: The total space occupied by any piece of matter.

Structure and Properties of Matter : 25 :

2

Structure and Properties of MatterAll the objects around us whether living or non-living are matter. Water we drink, food weeat, air we breathe, chair we sit on, are all examples of matter. Matter is anything thathas mass and takes up space. Matter appears in a huge variety of forms such as rocks,trees, computer, clouds, people, etc. Matter embraces each and everything around us.Therefore, in order to understand the world, it would be necessary to understand the matter.

Each pure kind of matter is called substance. Here, pure we mean the same throughout. Thus, aluminium is one substance and water is another. Please remember, the scientificmeaning of substance is a little different from its every day meaning and we shall discussit a little later in this lesson.

OBJECTIVES


define various states of matter as solid, liquid and gas, and distinguish one from theother based on their properties;classify the matter based on their composition as element, compound and mixture;differentiate between atoms and molecules;state Dalton’s atomic theory and explain various laws of chemical combinations;define isotopes, atomic mass and molecular mass;express chemical reaction in form of a balanced chemical equation;define mole concept and molar quantities such as molar mass and molar volume;apply mole concept to a chemical reaction and show a quantitative relationship betweenmasses of reactants and products;define Gay Lussac’s law of combining volume and Avogadro’s law;solve numerical problems based on various concepts covered above;

2.1 CLASSIFICATION OF MATTER

Earlier Indian and Greek philosophers and scientists attempted to classify the matter in theform of five elements - Air, Earth, Fire, Sky and Water. This classification was more ofphilosophical nature. In modern science, however, there are two main ways of classifyingthe matter :

i) Based on physical states: All matter, at least in principle, can exist in three states,solid, liquid and gas.

: 26 : Structure and Properties of Matter

ii) Based on composition and properties: The classification of matter includes elements,compounds and mixtures.

2.1.1 PHYSICAL STATE OF MATTERA given kind of matter may exist in different physical forms under different conditions.Water, for example, at one atmospheric pressure, may exist as solid, liquid or gas withchange of temperature. Sodium metal is normally solid, but it melts to a silvery liquidwhen heated to 98 oC. Liquid sodium changes to a bluish gas if the temperature is raised to883 oC. Similarly, chlorine, which is normally a gas can exist as a yellow liquid or solidunder appropriate conditions. These three different forms of matter differ from each otherin their properties. Solids are rigid with definite shapes. Liquids are less rigid than solidsand are fluid, i.e. they are able to flow and take the shape of their containers. Like liquids,gases are fluids, but unlike liquid, they can expand indefinitely.

Can you think of other differences between a gas and liquid? A gas can be compressedeasily whereas a liquid cannot. You might be aware, natural gas is compressed and suppliedas fuel for vehicles in the name of CNG (compressed natural gas). It is not possible tocompress a liquid. It is still more difficult to compress a solid. All these three forms ofmatter (solid, liquid and gas) are generally referred as states of matter. Taking fluidity/rigidity and compressibility, we can write characteristic properties of solid, liquid and gasin the Table 2.1.

Table 2.1: Characteristics of different states of matter

States of matter Fluidity/rigidity Compressibility

Solid Rigid NegligibleLiquid Fluid Very low

Gas Fluid High

As mentioned, a substance can exist in three forms depending upon temperature andpressure. Water at room temperature (25 oC) exists in liquid form and at 0 oC and 1atmospheric pressure as solid. If we go on increasing temperature of water at constantpressure, more and more of it will go into vapour form and at 100 oC will start boiling,. Ifwe continue heating at this temperature (100 oC), entire liquid water will be converted intovapour. This is true with most of the liquids. Definitely melting and boiling points ofdifferent substances will be different. Can you think why this variation in their meltingpoint and boiling point occurs? You will study later on that intermolecular forces aredifferent in different liquids, and therefore their boiling points and melting points aredifferent. In gaseous form, intermolecular forces are very weak and unable to keep moleculestogether in aggregation. However, in case of solids, these forces are very strong and capableof keeping molecules in fixed positions. This is the reason solids are rigid and hard andcannot be compressed. Liquids have properties intermediate to solid and gases asintermolecular forces between molecules in liquid are definitely more than gases and lessthan solids but strong enough to keep the molecules in aggregation (Fig. 2.1). Due to weakintermolecular forces in gases, molecules in gases can move freely and can occupy anyspace available to them. This property of gases is responsible for their effusion/diffusion.Molecules in gases are far apart and therefore when pressure is applied they can be broughtcloser and gases can be compressed.


Fig. 2.1. Gases, liquids and solids (a) Bulk appearance and (b) the molecular picture.

ACTIVITY 2.1

Fill gas in a balloon and tightly tie its mouth. Now hold it with both hands andcompress. What do you find? Balloon can be compressed easily.

2.2 CLASSIFICATION OF MATTER BASED ON COMPOSITION - ELEMENTS,COMPOUNDS AND MIXTURESAnother method of classification of matter is based on its composition. A substance ismatter that has a definite or constant composition and has distinct properties. Examplesare aluminium sliver, water, carbon dioxide, nitrogen, oxygen etc. Substances differ fromone another in composition and can be identified by their properties like colour, smell,taste, appearance, etc. Aluminium has uniform composition. Similarly water has uniformcomposition. No doubt there are also matter which do not have uniform composition.Such matter are called mixtures. Some examples of mixtures are air, soft drink, milk, andcement. Mixtures are either homogeneous or heterogeneous. Suppose you add 5g ofsugar to water kept in a glass tumbler. After stirring, the mixture obtained is uniformthrough out. This mixture is homogeneous through out and is called solution. Air issolution of several gases (oxygen, nitrogen, water vapour, carbon dioxide etc). Supposeyou mix sand with iron filings, sand grains and the iron filings remain visible and separate.This type of mixture in which the composition is not uniform, is called a heterogeneousmixture. If you add oil to water, it creates another heterogeneous mixture because theliquid thus obtained does not have a uniform composition.

We can create homogeneous and heterogeneous mixtures and if need arises we canseparate them into pure components by physical means without changing the identities ofthe components. We can recover sugar from its water solution by heating and evaporatingthe solution to dryness. From a mixture of iron filings and sand, we can separate iron filingsusing magnet. After separation we can see that the components have the same compositionand properties as they did to start with.

2.2.1 ElementsOxygen and magnesium, these two substances which have uniform composition throughout are elements. Antoine Laurent Lavoisier (1743-94), a French chemist was first toexplain an element. He defined an element as basic form of matter that cannot bebroken down into simpler substances even by chemical means. Elements serve as thebuilding blocks for various types of other substances, starting from water up to extremelycomplex substances like protein. Oxygen, nitrogen, magnesium, iron, gold all are example

Solid

Liquid

Gas

Vibratingparticle Container

Solid Liquid Gas

(a)

(b)


Matter

MixtureSeparation by

Physical methodsPure

substances

Homogeneousmixtures

Heterogeneousmixtures

CompoundsElementsSeparation by

Chemical methods

2.8%

All others

Magnesium

Calcium

Oxygen45.5%

Silicon27.2%

5.3%4.7%

6.2%

8.3%

Iron

Aluminum

Oxygen65%

Carbon18%

Hydrogen10%

Nitrogen3%

Calcium1.6%

Phosphorus1.2%

All others1.2%

of element which you have already studied in your lower classes. Today more than 112elements are known and we know various details about them. An element consists ofonly one kind of atoms. These elements are represented by suitable symbols, as you musthave read in your previous classes. Fig. 2.2 shows the most abundant element in earthcrust and in the human body. As can be seen from the figure, only five elements (oxygen,silicon, aluminium, iron and calcium) comprise over 90 per cent of Earth’s crust. Out ofthese five, oxygen is the most abundant element in our body.

Fig.2.2 (a) Elements in Earth’s crust (b) Elements in human body

2.2.2 Compounds

Most elements can interact with one or more other elements to form compounds. Acompound is a substance that consists of two or more different elements chemicallyunited in a definite ratio. A pure compound, whatever its source, always contains definiteor constant proportions of the elements by mass. As you have read, water is composed oftwo elements: hydrogen and oxygen. Property of water is completely different from itsconstituent elements: hydrogen and oxygen which are gases. Similarly when sulphur isignited in air, sulphur and oxygen (from air) combine to form sulphur dioxide. All sampleof pure water contain these two elements combined in the ratio of one is to eight (1: 8) bymass. For example, 1.0 g of hydrogen will combine with 8.0g of oxygen. This regularityof composition by mass will be discussed later on as law of constant composition). Thiscomposition does not change whether we take water from river of India or of United Statesor the ice caps on Mars. Unlike mixtures compounds can be separated only by chemicalmeans into their pure components.

In conclusion, the relationship among elements, compounds and other categories ofmatter are summarised in Fig. 2.3. We have just read that elements are made of one kind ofatoms only. Now we shall discuss how concept of an atom emerged and how far thisforms the basis of our other studies in science.

Fig. 2.3 Classification of matter

(a) (b)


Atoms of element X Atoms of element Y Compound of element X and Y


1. Which of the following matter fall(s) in the category of substance?(i) Ice (ii) Milk (iii) Iron (iv) Air (v) Water (vi) Hydrochloric acid

2. Which one of the following is solution?(i) Mercury (ii) Air (iii) Coal (iv) Milk

2.3 DALTON’S ATOMIC THEORY

In the fifth century B.C. Indian philosopher Maharshi Kanad postulated that if one goes ondividing matter (Padarth), he would get smaller and smaller particles and a limit will comewhen he will come across smallest particles beyond which further division will not bepossible. He (Kanad) named the particles Parmanu. More or less during the same periodGreek philosophers, Leuappus and Democritus suggested similar ideas. This idea was notaccepted at that time but it remained alive. Not much experimental work could be doneuntil Lavoisier gave his law: Law of conservation of mass and law of constantproportions sometimes in 1789. English scientist and school teacher, John Dalton (1766-1844) provided the basic theory about the nature of matter: All matter whether element,compound or mixture is composed of small particles called atoms.

Dalton’s theory can be summarized as follows:

Elements are composed of extremely small indivisible particles called atoms.All atoms of a given element are identical, having the same size, mass and chemicalproperties. The atoms of one element are different from the atoms of all other elements.Compounds are composed of atoms of more than one element. In any compound theratio of the numbers of atoms of any two of the elements present is either an integer ora simple fraction.A chemical reaction involves only the separation, combination, or rearrangement ofatoms; it does not result in their creation or destruction.

In brief, an atom is the smallest particle of an element that maintains its chemicalidentity throughout all chemical and physical changes. Most of the earlier findings andconcepts related to law of conservation of mass and law of constant proportions (Fig. 2.4)could be explained to a great extent. Dalton’s theory also predicted the law of multipleproportions. However, today we know that atoms are not truly indivisible; they arethemselves made up of particles (protons, neutrons, electrons, etc), which you will learnlater on.

Fig. 2.4 Law of constant proportions


H O

water2 NH

Ammonia3 CH

Ethyl alcohol3CH OH2

Cl Cl

P

PP

P

S S

S

S

S

S

S

Cl2

S

P4 S8

Chlorine Phosphorus Sulphur

Modern technology has made it possible to take photograph of atoms. The scanningtunnelling microscope (STM) is a very sophisticated instrument. It can produce image ofthe surfaces of the elements, which show the individual atoms (Fig.2.5).

Fig.2.5 Image from a scanning tunneling microscope

Now let us see how atoms and molecules are related with each other.

2.4 ATOMS AND MOLECULES

We have just seen, the first chemist to use the name ‘atom’ was John Dalton. Dalton usedthe word ‘atom’ to mean the smallest particle of an element. He then went on explaininghow atoms could react together to form molecules; which he called ‘compound atoms’.Today we know what a molecule is. A molecule is an aggregate of two or more than twoatoms of the same or different elements in a definite arrangement held together by chemicalforces or chemical bonds. We can also define a molecule as smallest particle of an elementor of a compound which can exist alone or freely under ordinary conditions and shows allproperties of that substance (element or compound). A molecule will be diatomic if thereare two atoms, for example, chlorine (Cl

2), carbon monoxide, CO; will be triatomic if

there are three atoms, for example, water (H2O) or carbon dioxide, (CO

2), will be tetratomic

and pentatomic if there are four and five atoms respectively. In general, a molecule havingatoms more than four will be called polyatomic. There are eight atoms in a molecule ofsulphur and nine atoms in a molecule of ethyl alcohol and we write formulas as S

8 and

C2H

5OH respectively (Fig. 2.6). Only a few years back, a form of carbon called buck-

minsterfullerene having molecular formula, C60 was discovered. The details you will studyin lesson 20.

Fig. 2.6 Atomic structure of some molecules


2.5 CHEMICAL FORMULAE OF SIMPLE COMPOUNDS

A molecule is represented by using symbols of elements present in it. This representation iscalled molecular formula of the compound. Thus, a molecular formula of a substance tellsus how many atoms of each kind are present in one molecule. In Fig. 2.6, you will find thatatoms in a molecule are not only connected in definite ways but also exhibit definite spatialarrangements. Properties of molecules depends upon the ways atoms are connected and onspatial configuration of the molecules. CO

2 and H

2O both are triatomic molecules but they

have entirely different properties. CO2 is a linear molecule and is a gas but H

2O is a bent

molecule and a liquid. Sodium chloride (common salt) contains equal number of sodiumand chlorine atoms and is represented by the formula, NaCl. Sulphuric acid, H

2SO

4 contains

three elements : hydrogen, oxygen and sulphur.

2.5.1 Valency and formulation

Every element has a definite capacity to combine with other elements. This combiningcapacity of an element is called its valency. In normal course, hydrogen has 1, oxygen has2, nitrogen has 3 and carbon has 4 valency. Valency of an element depends upon how itcombines with other elements. This will depend upon the nature of the element. Sometimean element shows more than on valency. We say element has variable valency. For example,nitrogen forms several oxides: N

2O, N

2O

2, N

2O

3, N

2O

4 and N

2O

5. If we take valency of

oxygen 2 then valency of nitrogen in these oxides will be 1,2,3,4 and 5 respectively. Verysoon, you will learn in lesson 3 that valency of an element depends on its electronicconfiguration. Valency of F, Cl, Br and I is normally taken as 1. In NaCl, valency of Na isalso 1. All alkali metals such as K, Cs, Rb have 1 valency. Valency of oxygen is 2 and thatof phosphorus is 5, we can write the formula of phosphorus pentaoxide as P

2O

5.

Thus, we can write the formula of water (H2O), ammonia (NH

3), carbon dioxide (CO

2),

magnesium oxide (MgO), phosphorus pentaoxide (P2O

5), hydrochloric acid gas (HCl),

phosphorus tribromide (PBr3) etc. if we know the elements constituting these compounds

and their (elements) valencies. Since valencies are not always fixed (as P has differentvalencies in P

2O

5 and in PBr

3 in the above example), sometimes we face problem. Writing

formula of a compound is easy only in binary compounds (i.e. compound made of onlytwo elements). However, when we have to write formula of a compound which involvesmore than two elements (i.e. of polyatomic molecules), it is somewhat cumbersome task.You will learn later on that basically there are two types of compounds: covalent compoundsand ionic compounds. Covalent compounds are of the type H

2O, NH

3 etc. An electrovalent

or ionic compound is made of two charged constituents. One positively charged called‘cation’ and other negatively charged called ‘anion’. Here again we should know thecharge (valency) of both types of ions for writing formula of an ionic compound.Compounds like sodium nitrate (NaNO

3), potassium chloride (KCl), potassium sulphate

(K2SO

4), ammonium choride (NH

4Cl), sodium hydroxide (NaOH) etc. are made of two or

more than two elements. For writing the formula of the compounds we should know thecharge (valency) of positively and negatively charged constituents of the compounds insuch cases. Remember in an ionic compound, sum of the charges of cation and anionshould be zero. A few examples of cation and anions along with their valency are providedin Table 2.2.


Table 2.2 Valency of some common cations and anionswhich form ionic compounds

Suppose you have to write the formula of potassium sulphate which is an electrovalentcompound and made of potassium and sulphate ions. Here, charge on potassium ion is +1and that on sulphate ion is –2. Therefore, for one sulphate ion two potassium ions will berequired. We can write,

[K+]2 [SO

42-]

1 = K

2SO

4

Similarly for writing formula of sodium nitrate, charge (valency) of sodium ion is +1and that of nitrate ion is -1, therefore, for one sodium ion, one nitrate ion will be requiredand we can write.

[Na+]1 [NO

3–]

1 = NaNO

3

Now, it is clear that digit showing charge of cation goes to anion and digit showingcharge of anion goes to cation. For writing formula of calcium phosphate we take chargeof each ion into consideration and write the formula as discussed above as,

[Ca2+]3 [PO-3

4]

2 = Ca

3(PO

4)

2

Writing formula of a compound comes by practice therefore write formula of as manyionic compounds as possible based on the guidelines given above.

2.5.2 Empirical and molecular formula

Molecular formula of a substance is not always identical with the simplest formula thatexpresses the relative numbers of atoms of each kind in it. Simplest formula of an elementis expressed by using its symbol as O for oxygen, S for sulphur, P for phosphorus and Clfor chlorine. Molecular formula of these substances are O

2, S

8, P

4 and Cl

2 respectively.

The simplest formula of a compound is called its empirical formula. The empiricalformula of a compound is the chemical formula that shows the relative number of atoms ofeach element in the simplest ratio. In contrast, the molecular formula tells us the actualnumber of atoms of each element in a molecule. It may be the same as the empiricalformula or some other integral multiple of the empirical formula. Empirical and molecularformulae of a few compounds are given in Table 2.3.

Anions Valency Cations Valency

Chloride ion, Cl- -1 Potassium ion, K+ +1Nitrate ion, NO-

3-1 Sodium ion, Na+ +1

Carbonate ion, -2 Magnesium ion, Mg2+ +2Sulphate ion, SO

42- -2 Calcium ion, Ca2+ +2

Bicarbonate ion, HCO-3

-1 Aluminium ion, Al3+ +3Hydroxide ion, OH

--1 Lead ion, Pb2+ +2

Nitrite ion, NO-2

-1 Iron ion, Fe3+(Ferric) +3Phosphate ion, PO3

4_ -3 Iron ion (Ferrous) Fe2+ +2

Acetate ion CH3COO- -1 Zinc ion, Zn2+ +2

Bromide ion, Br-

-1 Copper ion, Cu2+ +2 Iodide ion, I- -1 Mercury, Hg2+ (mercuric) +2Sulphide ion, S2- -2 Ammonium ion, NH

4+

+1


Table 2.3: Empirical and molecular formulae

Substance Empirical formula Molecular formula

water H2O H

2O

ammonia NH3

NH3

ethane CH3

C2H

6

hydrogen peroxide HO H2O

2

carbon dioxide CO2

CO2

hydrazine NH2

N2H

4

Formula of an ionic substance is always an empirical formula. For example, NaCl isempirical formula not a molecular formula of sodium chloride. You will study later onthat ionic substances do not exist in molecular form.


1. Give one evidence of modern technology which supports Dalton’s atomic theory.2. Write formula of the following compounds

(i) Ferric phosphate(ii) Barium chloride (v) Magnesium sulphate(iii) Calcium carbonate (vi) Sodium phosphate(iv) Phosphorous tribromide (vii) Sulphur trioxide

3. Write differences between an atom and a molecule.4. Write empirical formulae of the following molecules:

C2H

4, HCl, HNO

3

2.6 LAWS OF CHEMICAL COMBINATIONS

French chemist, Antoine Laurent Lavoisier (1743-1794) experimentally showed thatmatter can neither be created nor destroyed in a chemical reaction. This experimentalfinding was known as law of conservation of mass. In fact, this could be possible due toprecise measurement of mass by Lavoisier. Law of conservation of mass helped inestablishing the law of definite composition or law of constant proportions. This lawstates that any sample of a pure substance always consists of the same elements combinedin the same proportions by mass. For instance, in water, the ratio of the mass of hydrogento the mass of oxygen is always 1:8 irrespective of the source of water. Thus, if 18.0 g ofwater are decomposed, 2.0 g of hydrogen and 16.0 g of oxygen are always obtained. Also,if 2 g of hydrogen are mixed with 16.0 g of oxygen and mixture is ignited, 18.0 g of waterare obtained after the reaction is over. In the water thus formed or decomposed, hydrogento oxygen mass ratio is always 1:8. Similarly in ammonia (NH

3), nitrogen and hydrogen

will always react in the ratio of 14:3 by mass.

John Dalton thought about the fact that an element may form more than one compoundwith another element. He observed that for a given mass of an element, the masses of theother element in two or more compounds are in the ratio of simple whole number orintegers. In fact this observation helped him in formulation of his fundamental theory


popularly known as Dalton’s ‘Atomic theory’ which is discussed in Section 2.3. Let ustake two compounds of nitrogen and hydrogen : (i) ammonia (NH

3) and (ii) hydrazine

(N2H

4). In ammonia, as discussed above, 3.0 g of hydrogen react with 14 g of nitrogen. In

hydrazine, 4.0 g of hydrogen react with 28 g of nitrogen or 2.0 g of hydrogen reacts with14.0 g of nitrogen. It can be seen that for 14 g of nitrogen, we require 3.0 g of hydrogen inNH

3 and 2.0 g of hydrogen in hydrazine (N

2H

4). This leads to the ratio

That is, masses of hydrogen which combine with the fixed mass of nitrogen in ammoniaand in hydrazine are in the simple ratio of 3:2. This is known as law of multipleproportions.

2.6.1 Gay Lusaac’s law of combining volume and Avogadro’s hypothesis

The French chemist Gay Lusaac experimented with several reactions of gases and cameto the conclusion that the volume of reactants and products in large number of gaseouschemical reactions are related to each other by small integers provided the volumes aremeasured at the same temperature and pressure. For example, in reaction of hydrogen gaswith oxygen gas which produces water vapour, it was found that two volumes of hydrogenand one volume of oxygen give two volumes of water vapour

To be more specific, if 100 mL of H2 gas combines with exactly 50 mL of O

2 gas we

shall obtain 100 mL of H2O vapour provided all the gases are measured at the same

temperature and pressure (say 100 oC and 1 atm pressure).

As you know, the law of definite proportions is with respect to mass. Gay Lussac’sfindings of integer ratio in volume relationship is actually the law of definite proportionsby volume.

The Gay Lussac’s law was further explained by the work of Italian physicist and lawyerAmedeo Avogadro in 1811. Avogadro’s hypothesis which was experimentally establishedand given the status of a law later on, states as follows:

The volume of a gas (at fixed pressure and temperature) is proportional to the numberof moles (or molecules of gas present). Mathematically we can express the statement as

V ∝ nYou will study in section 2.8 that 1 mole of a substance is 6.022 × 1023 particles/

molecule of that substance.

Where V is volume and n is the number of moles of the gas. (It is clear from therelationship that more volume will contain more number of molecules). Avogadro’s lawcan be stated in another simple way

“Equal volumes of all gases under the same conditions of temperatureand pressure contain the same number of molecules”

For example,

2H2(g) + O

2(g) → 2H

2O(g)

2 volumes 1 volume 2 volumes (Gay Lussac’s law)2 mol of H

21 mol of O

22 mol of H

2O (Avogadro’s law)

Multiplying both sides of equation by the same number, equation does not change.Now let us multiply by 6.022 × 1023, we get


2 × 6.022 × 1023 1 × 6.022 × 1023 2 × 6.022 × 1023

molecules of H2 molecules of O2 molecules of H2OSimilarly,

H g Cl g HCl g

volume volume volume

2 2 2

1 1 2

( ) ( ) ( )+ →

1 mol of H2 1 mol of Cl2 2 mol of HCl

or 6.022 × 1023 6.022 × 1023 2 × 6.022 × 1023

molecules of H2 molecules of Cl2 molecules of HCl

Experimentally, it has been found that at standard temperature (0 oC) and standardpressure (1 bar) volume of 1 mol of most of the gases is 22.7 litres. Since this volume is of1 mol of a gas, it is also called molar volume. Volume of liquids and solids does notchange much with temperature and pressure and same is true with its molar volume. If weknow molar mass and density of a solid or of a liquid, we can easily calculate its

molar massmolar volume by the relationship, volume = _________________

density

2.7 ISOTOPES AND ATOMIC MASS

As you might have read in your earlier classes that an atom consists of several fundamentalparticles: electrons, protons and neutrons. An electron is negatively charged and a protonis positively charged particle. Number of electrons and protons in an atom is equal. Sincecharge on an electron is equal and opposite to charge of a proton, therefore, an atom iselectrically neutral. Protons remain in the nucleus in the centre of the atom and nucleusis surrounded by negatively charged electrons.

The number of protons in the nucleus is called atomic number, and is denoted by Z.There are also neutral particles in the nucleus and they are called neutrons. Mass of aproton is nearly equal to the mass of neutron. Total mass of nucleus is equal to the sum ofmasses of protons and neutrons. The total number of protons and neutrons is called massnumber or the nucleons number denoted by A. By convention, atomic number is writtenat the bottom left corner of the symbol of the atom and mass number is written at the topleft corner. For example, we write, 4

2He, 7

3Li and 12

6Cfor helium, lithium and carbon

respectively. The symbol 6

12C indicates that there is a total of 12 particles (nucleons) in the

nucleus of carbon atom, 6 of which are protons. Thus, there must be 12 – 6 = 6 neutrons.

Similarly, 8

16O has 8 protons and 8 electrons and there are 8 neutrons. Also atomic number,

Z differentiates the atom of one element from the atoms of another. Also an element maybe defined as a substance whose atoms have the same atomic number. Thus, all atoms ofan element have nuclei containing the same number of protons and having the same charge.But the nuclei of all the atoms of a given element do not necessarily contain the samenumber of neutrons. For example, atoms of oxygen, found in nature have the same number


of protons which makes it different from other elements, but their neutrons are different.This is the reason that the masses of the atoms of the same elements are different. Forexample, one type of oxygen atom contains 8 protons and 8 neutrons in the nucleus, secondtype 8 protons and 9 neutrons and third type contains 8 proton and 10 neutrons. Werepresent them as 16

8O, 17

8O and 18

8O. Atoms of an element that have same atomic number(Z)

but different mass number(A) are called isotopes.

2.7.1. Atomic mass

The mass of an atom is related to the number of protons, electrons, and neutrons it has.Atom of an element is extremely small and therefore it is not easy to weigh it. No doubt,it is possible to determine the mass of one atom relative to another experimentally. Forthis, it is necessary to assign a value to the mass of one atom of a given element so that itcan be used as a standard. Scientists agreed to chose an atom of carbon isotope (calledcarbon-12). Carbon-12 has six protons and six neutrons and has been assigned a mass ofexactly 12 atomic mass unit (amu now known as u). Thus one atomic mass unit is definedas a mass exactly equal to one twelfth of the mass of one carbon-12 atom.

Mass of every other element is determined relative to this mass. Further, it has beenfound by experiment that hydrogen atom is only 0.0840 times heavier than C-12 atoms.Then on carbon-12 scale, atomic mass of hydrogen = 0.0840×12.00 u = 1.008 u.

Similarly, experiment shows that an oxygen atom is, on the average, 1.3333 timesheavier than C-12 atom. Therefore,

Atomic mass of oxygen = 1.3333×12.00 u = 16.0 u

Atomic mass of a few elements on C-12 scale is provided in Table 2.4.

If you see Table 2.4, you will find that atomic mass is not a whole number. Forexample, atomic mass of carbon is not 12 u but 12.01 u. This is because most naturallyoccurring elements (including carbon) have more than one isotope. Therefore when wedetermine atomic mass of an element we generally measure or calculate average mass ofthe naturally occurring mixture of isotopes. Let us take one example. Carbon has twonatural isotope C-12 and C-13 and their natural abundance is 98.90 per cent*, 1.10 percent, respectively . Atomic mass of C-13 has been determined to be 13.00335 u. Therefore,average atomic mass of carbon

= (0.9890) (12.000 u) + (0.010) (13.00335 u)

= 11.868 + 0.1430 = 12.01 u

Thus, ‘atomic mass’ of an element means average atomic mass of that element. Thesedays actual masses of atoms have been determined experimentally using massspectrometer. You will learn about this in your higher classes.

Mass of one carbon-12 atom = 12 amu or 12 u

or 112

amumassof onecarbon atom

=


Table 2.4 Atomic masses* of some common elements

* During calculation we convert per cent into fraction by dividing by 100. Thus,98.90 per cent becomes 0.9890.

*Atomic masses are average atomic masses. They are given correct up to seconddecimal places. In practice, we use round figures and for this rounding off is necessary.

**Radioactive

2.7.2 Molecular mass

You have just read that a molecule can be represented in form of a formula popularlyknown as molecular formula. Molecular formula may be of an element or of a compound.Molecular formula of a compound is normally used for determing the molecular mass ofthat substance. If the substance is composed of molecules (for example, CO

2, H

2O or

NH3), it is easy to calculate the molecular mass. Molecular mass is the sum of atomic

masses of all the atoms present in that molecule. Thus the molecular mass of CO2 is

obtained as

C 1 × 12.0 u = 12.0 u2O 2 × 16.0 u = 32.0 uCO

2Total = 44.0 u

We write molecular mass of CO2 = 44.0 u

Similarly, we obtain molecular mass of ammonia, NH3 as follows :

N 1 × 14.0 u = 14.0 u3H 3 × 1.08 u = 3.24 uNH

3Total = 17.24 u

Element Symbol Mass(u) Element Symbol Mass(u)

Aluminium Al 26.98 Magnesium Mg 24.31Argon Ar 39.95 Manganese Mn 54.94

Arsenic As 74.92 Mercury Hg 200.59Barium Ba 137.34 Neon Ne 20.18Boron B 10.81 Nickel Ni 58.71

Bromine Br 79.91 Nitrogen N 14.01Caesium Cs 132.91 Oxygen O 16.00Calcium Ca 40.08 Phosphorus P 30.97Carbon C 12.01 Platinum Pt 195.09

Chlorine Cl 35.45 Potassium K 39.10Chromium Cr 52.00 Radon Rn (222)**

Cobalt Co 58.93 Silicon Si 28.09Copper Cu 63.54 Silver Ag 107.87Fluorine F 19.00 Sodium Na 23.00

Gold Au 196.97 Sulphur S 32.06Helium He 4.00 Tin Sn 118.69

Hydrogen H 1.008 Titanium Ti 47.88Iodine I 126.90 Tungston W 183.85Iron Fe 55.85 Uranium U 238.03Lead Pb 207.19 Vanadium V 50.94

Lithium Li 6.94 Xenon Xe 131.30Zinc Zn 65.37


Molecular mass of ammonia, NH3 = 17.24 u.

For substances which are not molecular in nature, we talk of formula mass. Forexample, sodium chloride, NaCl is an ionic substance. For this, we write formula masswhich is calculated similar to molecular mass. In case of NaCl, formula mass = mass of 1Na atom + mass of 1 Cl atom = 23 u + 35.5 u = 58.5 u.

You will learn about such compounds later on in your lesson 5.


1. Silicon has three isotopes with 14, 15 and 16 neutrons respectively. What is the massnumber and symbol of these three isotopes?

2. Calculate molecular mass of the following compounds C3H

8, PCl

5, SO

3

2.8 MOLE CONCEPT

When we mix two substances, we get one or more new substance(s). For example whenwe mix hydrogen and oxygen and ignite the mixture, we get a new substance water. Thiscan be represented in the form of an equation,

2H2 (g) + O

2(g) → 2H

2O (l)

In above equation, 2 molecules (4 atoms) of hydrogen react with 1 molecule (2 atoms)of oxygen and give two molecules of water. Similarly, we always like to know how manyatoms/molecules of a particular substance would react with atoms/molecules of anothersubstance in a chemical reaction. No matter how small they are. The solution to thisproblem is to have a convenient unit of matter that contains a known number of particles(atoms /molecules). The chemical counting unit that has come into use is the mole.

The word mole was apparently introduced in about 1896 by Wilhelm Ostwald whoderived the term from the Latin word ‘moles’ meaning a ‘heap’ or ‘pile’. The mole whosesymbol is ‘mole’ is the SI base unit for measuring amount of substance. It is defined asfollows:

‘A mole is the amount of pure substance that contains as many particles (atoms,molecules, or other fundamental units) as there are atoms in exactly 0.012 kg of C-12isotope’.

In simple terms, mole is the number of atoms in exactly 0.012 kg (12 grams) of C-12.Although mole is defined in terms of carbon atoms but the unit is applicable to any substancejust as 1 dozen means 12 or one gross means 144 of any thing. Mole is scientist’s countingunit like dozen or gross. By using mole, scientists (particularly chemists) count atoms andmolecules in a given substance. Now it is experimentally found that the number of atomscontained in exactly 12 grams of C-12 is 602,200 000 000 000 000 000 000 or 6.022×1023.This number (6.022×1023) is called Avogadro constant in honour of Amedeo Avogadroan Italian lawyer and physicist and is denoted by symbol, N

A. We have seen that

Atomic mass of C = 12 uAtomic mass of He = 4 uWe can see that one atom of carbon is three times as heavy as one atom of helium. On

the same logic 100 atoms of carbon are three times as heavy as 100 atoms of helium.Similarly 6.02×1023 atoms of carbon are three times as heavy as 6.02 × 023 atoms of helium.


But 6.02×1023 atoms of carbon weigh 12 g, therefore 6.02×1023 atoms of helium willweigh 1/3× 12g = 4g. We can take a few more examples of elements and can calculate themass of one mole atoms of the element. Numerically it is equal to its atomic mass expressedin gram. Mass of one mole of a substance is called its molar mass. Mass of one moleatoms of oxygen will be 16 g. Mass of one mole of fluorine will be 19 g. Now if we takemass of one mole molecule of oxygen it would be 32 g because there are two atoms in amolecule of oxygen (O

2). When we do not mention atom or molecule before mole, we

always mean one mole of that substance in its natural form. For example, if we simply sayone mole of oxygen, it means that we are referring one mole molecule of oxygen as oxygenoccurs in nature as molecular oxygen. If we take an example of a molecule of a compound,we find that same logic is applicable. For example, mass of one mole molecule of waterwill be 18 g as molecular mass of water is 18u.

Remember molar mass is always expressed as grams per mole or g /mol or g mol-1. Forexample,

Molar mass of oxygen (O2) = 32 g mol-1

Molar mass of lead (Pb) = 207 g mol-1

We have just seen in Section 1.6 that atoms of two different elements combine withone another in the ratio of small whole number. A modern interpretation of this observationis that atoms or molecules combine with one another in the ratio of 1:1, 1:2 or 1:3 or anyother simple ratio i.e. they combine 1 mol for 1 mol or 1 mol for 2 mol or 1 mol for 3 mol,and so on. Thus mole concept is the cornerstone of quantitative science for chemicalreactions which you will study in your higher classes.

Example 2.1: How many grams are there in 3.5 mol of sulphur?Solution: For converting mass into mole and vice visa, we always need the molar mass.Molar mass of sulphur is 32.0 g mol–1. Therefore, number of grams of sulphur in 3.50 molof sulphur is

Table 2.5 Molecular and molar mass of some common substances

Formula Molecular mass(u) Molar mass (g/mol)

O2

32.0 32.0H

22.0 2.0

Cl2

71.0 71.0P

4123.9 123.9

CH4

16.0 16.0CH

3OH 32.0 32.0

NH3

17.0 17.0CO

244.0 44.0

HCl 36.5 36.5C

6H

678.0 78.0

SO2

64.0 64.0CO 28.0 28.0

C2H

5OH 46.0 46.0

3.50 mol sulphur × 32 0

1

. g

mol

= 112.0 g sulphur


Example 2.2: Calculate number of moles present in 48 g of oxygen.Solution: Molar mass of oxygen = 32 g mol-1

Oxygen in natural form will be molecular oxygen, O2


1. Sulphur is a non-metallic element. How many atoms are present in 16.3 g of S?2. Molar mass of silver is 107.9 g. What is the mass of one atom of silver?

2.9 CHEMICAL EQUATIONS

A chemical equation is a shorthand description of a reaction carried out in a laboratory orelsewhere. It gives the formulas for all the reactants and products. For example

C + O2 → CO

2 ..... (1)

2H2 + O

2 → 2H

2O .......(2)

In a chemical reaction reactants are written on the left and products are written on theright side of the arrow. Arrow (→) indicates conversion of reactant(s) into product(s). Ina chemical reaction atoms are neither created nor destroyed. This is known as law ofconservation of mass. A chemical equation, therefore, should be consistent with this law.Total number of atoms of each element must be the same in the products and in the reactants.As shown in equation (2) above two molecules (four atoms) of hydrogen react with onemolecule (two atoms) of oxygen and give two water molecules in which there are fourhydrogen atoms and two oxygen atoms. Since number of atoms of the involved elementsis equal on both side of the arrow in the equation, we say the equation is balanced. Abalanced chemical equation is quite meaningful in science (chemistry) as it gives a lots ofinformation. In order to make an equation more informative, we also indicate the physicalstates of the reactants and products. We write in parenthesis ‘s’ if the substance is solid, ‘l’if the substance is liquid and ‘g’ if the substance is a gas. Accordingly, equation (1) and(2) can be written as,

C (s) + O2 (g) → CO

2 (g)

2H2 (g) + O

2 (g) → 2H

2O (l)

2.9.1 Balancing of a chemical equation

Balancing of a chemical equation is essential as we can derive meaningful informationfrom this. Before balancing a chemical equation, please ensure that correct formulas ofreactants and products are known. Let us consider burning of methane in oxygen to givecarbon dioxide and water. First write reactants and products,

CH4 + O

2 → CO

2 + H

2O (unbalanced equation)

reactants products

In this equation, hydrogen and carbon appear in only two formulas each, while oxygenappears three times. So we begin by balancing the number of carbon and hydrogen atoms.Here if we examine both sides, carbon appears in methane on left and in carbon dioxide on

Therefore, number of moles of oxygen =48

32

g

g mol-1

= 1.5 mol


the right side. Therefore, all carbon in methane, CH4 , must be converted to carbon dioxide.

One molecule of CH4, however, contains four hydrogen atoms, and since all the hydrogen

atoms end up in water molecule, two water molecules must be produced for each methanemolecules. Therefore, we must place coefficient 2 in front of the formula for water to give

CH4 + O

2 → CO

2 + 2H

2O (unbalanced)

Now we can balance oxygen, since there are four oxygen atoms on right hand side ofequation (two in CO

2 and two in two molecules of H

2O). Therefore, we must place 2 in

front of the formula for oxygen, O2. By doing this we get equal atoms of oxygen on both

sides of equation.

CH4 + 2O

2 → CO

2 + 2H

2O (balanced)

Now number of atoms of each element is equal on both sides of the chemical equation.In order to make the chemical equation more informative, indicate states of each reactantand product.

CH4 (g) + 2O

2 (g) → CO

2 (g) + 2H

2O(l)

Balancing of equation comes only by practice and therefore let us take one example.

Example2.3: Bottled gas sold as cooking gas contains butane, C4H

10 as the major

component. Butane when burns in sufficient oxygen (present in air) gives carbon dioxideand water. Write a balanced chemical equation to describe the reaction.

Solution: Work out the balanced equation in steps

Step 1: Write an unbalanced equation showing correct formulas of reactants and products

C4H

10 + O

2 → CO

2 + H

2O (unbalanced equation)

butane oxygen Carbon water dioxide

Now balance C and H as they appear only in two places.

Step II: Balance the number of C atoms.

Since 4 carbon atoms are in the reactant, therefore, 4CO2 must be formed.

C4H

10 + O

2 → 4CO

2 + H

2O (unbalanced)

Step III: Balance the number of hydrogen atoms

There are 10 hydrogen atoms in butane and each water molecule requires 2 hydrogenatoms, therefore, 5 water molecules will be formed.

C4H

10 + O

2 → 4CO

2 + 5H

2O (unbalanced)

Step IV: Balance the number of O atoms

There are 8 oxygen atoms in the carbon dioxide and 5 oxygen atoms with H2O molecules.

Therefore, 13 atoms or 13/2 molecules of oxygen will be required.

C4H

10 + 13/2 O

2 → 4CO

2 + 5H

2O


Normally we do not write fractional coefficient in equation as one may interpret thatmolecules can also be available in fraction. Therefore, we multiply both sides by 2 and getthe final balanced equation

2C4H

10 + 13O

2 → 8CO

2 + 10H

2O (balanced)

We can also write states of the substances involved.

2C4H

10 (g) + 13O

2 (g) → 8CO

2 (g) + 10H

2O(l)

Remember :

(i) Use the simplest possible set of whole number coefficients to balance the equation.

(ii) Do not change subscript in formulas of reactants or products during balancing as thatmay change the identity of the substance. For example, 2NO

2 means two molecules of

nitrogen dioxide but if we double the subscript we have N2O

4 which is formula of

dinitrogen tetroxide, a completely different compound.

(iii) Do not try to balance an equation by arbitrarily selecting reactant(s) and product(s). Achemical equation represents a chemical reaction which is real. Thus real reactantsand products only can be taken for balancing.

2.9.2 Uses of balanced equations

A balanced chemical equation gives a lot of meaningful information. First it gives thenumber of atoms and molecules taking part in the reaction and corresponding masses inatomic mass units (amu or u). Second it gives the number of moles taking part in thereaction, with the corresponding masses in grams or in other convenient units.

Let us consider the reaction between hydrogen and oxygen once again

2H2 (g) + O

2 (g) → 2H

2O(l)

2 molecules of 1 molecule of 2 molecules ofhydrogen oxygen water4.0 u 32.0 u 36 u

But in normal course we deal with a large number of molecules, therefore, we can considerthe above reaction as follows:

2H2 + O

2 → 2H

2O

2 molecules of 1 molecule of 2 molecules of waterhydrogen oxygen

Suppose we multiply entire chemical equation by 100, we can write

2 × 100 molecules + 1 × 100 molecules of → 2 × 100of hydrogen oxygen molecules of water

If we multiply entire equation by Avogadro constant, 6.022 × 1023 , we get

2 × 6.022 × 1023 molecules + 1 × 6.022 × 1023 → 2 × 6.022 × 1023

molecules molecules of oxygen of water of hydrogen


Since 6.022 × 1023 molecules is 1 mole, therefore, we can also write

or 2 mol of + 1 mol of → 2 mol of water hydrogen oxygen

Therefore, equation can be written as

2H2

+ O2 → 2H

2O

2 mol of hydrogen 1 mol of oxygen 2 mol of water

Or 4.0 g of hydrogen + 32.0 g of oxygen → 36 g of water

Thus a chemical equation can also be interpreted in terms of masses of reactantsconsumed and product(s) formed. This relationship in chemical reaction is very importantand provides a quantitative basis for taking definite masses of reactants to get a desiredmass of a product.

Example 2.4: In the reaction

CH4 (g)

+ 2O

2 (g) → CO

2 (g) + 2H

2O(l)

How much CO2 will be formed if 80 g of methane gas (CH

4) is burnt?

Solution:

CH4 (g)

+ 2O

2 (g) → CO

2 (g) + 2H

2O(l)

1 mol 2 mol 1 mol 2 molor 16 g 64 g 44 g 36 gWe can see in the above equation that 16 g of CH

4 gives 44 g of CO

2.

Therefore, for getting 80 g of CH4, the mass of CO

2 required will be


1. Balance the following equations.(i) H

3PO

3 → H

3PO

4 + PH

3

(ii) Ca + H2O → Ca(OH)

2 + H

2

(iii) C3H

8 + O

2 → CO

2 + H

2O

2. Name the following compounds.Na

2O, Cu

2Cl

2, BaO, Na

2SO

4

LET US REVISE

Matter is anything that has mass and occupies space. It can be classified on the basisof its (i) physical state as solid, liquid or gas, and (ii) chemical composition/constitutionas element, compound or mixture.

An element is basic form of matter that cannot be broken down into simpler substanceseven by chemical reaction. A compound is a substance composed of two or moredifferent types of elements chemically combined in a definite proportion by mass. Amixture contains more than one substance (element or compound) mixed in anyproportion.

=×

= × =44 80

1644 5 220

2

g g

gg g of CO


A solution is a homogeneous mixture of two or more than two substances. Majorcomponent of the solution is called solvent.

According to law of constant proportions, a sample of a pure substance always consistsof the same elements combined in the same proportions by mass.

When an element combines with another element and forms more than one compound,then different masses of one element that combine with a fix mass of another elementare in ratio of simple whole number or integer. This is the law of Multiple proportions.

John Dalton introduced the idea of an atom as an indivisible particle of matter. Anatom is the smallest particle of an element which can exist and retains all the chemicalproperties of that element.

A molecule is the smallest particle of an element or of a compound which can existfreely under ordinary conditions and shows all properties of that substance.

A molecule can be expressed in form of a chemical formula using symbols of constituentelements.

A molecular formula shows the actual number of atoms of different elements in amolecule of an element or of compound. In other words, composition of any compoundcan be represented by its formula. For writing formula of a compound valence orvalency of the elements is used. Valency is combining capacity of an element and isrelated to its electronic configuration.

An empirical formula shows the simplest whole number ratio of the atoms of differentelements present in a compound.

Atoms of the isotope 12C are assigned a atomic mass unit of 12 and the relative massesof all other atoms are obtained by comparison with the mass of a carbon-12.

The mole is the amount of substance which contains the same number of particles(atoms, ions or molecules) as there are atoms in exactly 0.012 kg of 12C.

Avogadros constant is defined as the number of atoms in exactly 12 g of C-12 and isequal to 6.022 × 1023 mol-1.

Mass of one mole atoms or one mole molecules of a substance is its molar mass andvolume of one mole of the substance is its molar volume.

A chemical equation is a shorthand description of a reaction. A balance chemicalequation provides quantitative information about reactants consumed and productsformed in a chemical reaction. A balance chemical equation obeys law of conservationof mass and law of constant proportions.

TERMINAL EXERCISES

1. There are many examples of homogeneous and heterogeneous mixtures in the worldaround you. How would you classify: sea-water, air (unpolluted), smoke, black coffee,tea, soil, soda water and wood ash?


2. Characterize gases, liquids and solids in terms of compressibility, fluidity and density.

3. What is atomic theory proposed by Dalton? Describe how it explains the great varietyof different substances.

4. Give normal state (solid, liquid or gas) of each of the following:

(i) Nitrogen (ii) Copper (iii) Bromine (iv) Oxygen

(v) ethyl alcohol (vi) hydrogen peroxide

5. Label each of the following as a substance, a heterogeneous mixture, or a solution.

(i) bromine (iv) soil (in front of your home) (vii) river water

(ii) petrol (v) stone (viii) Coal

(iii) concrete (vi) beach sand (ix) Soda water

6. Write the number of protons, neutrons and electrons in each of the following:19

9F, 18

8O, 40

20Ca

7. Give the symbol for each of the following isotopes

(i) Atomic number 19, mass number 40

(ii) Atomic number 18, mass number 40

(iii) Atomic number 7, mass number 15

8. Boron has two isotopes with masses of 10.01294 and 11.00931 u and abundance of19.77% and 80.23%. What is the average atomic mass of boron?

(Ans.10.81 u)

9. How does an element differ from a compound? How are elements and compoundsdifferent than mixture?

10. How will you define a solution based on its composition?

11. Charge of one electron is 1.6022 × 10-19 coulomb. What is the total charge on 1 mol ofelectron? If there is same amount of charge on one proton, calculate total charge on1 mol of protons.

12. How many molecules of O2 are in 8.00 g of O

2? If the O

2 molecules were completely

split into O (oxygen atom), how many moles of atoms of oxygen would be obtained?

(Ans. Number of molecules in 8 g of O2 =1.5055 × 1023 molecules

Number of atoms in 8 g of O2 = 3.0110 × 1023 atoms)

13. Assume that a human body is 80% water. Calculate the number of the molecules ofwater that are present in the body of a person who has mass of 65 kg.

(Ans. 1.7 × 1027 molecules of water)

14. Using atomic masses given in the table of this lesson calculate the molar masses ofeach of the following compounds:

CO,CH4, NaCl, NH

3 and HCl


15. Average atomic mass of carbon is 12.01 u. Find the number of moles of carbon in (i)2.00 g of carbon and (b) 3.00 × 1021 atoms of carbon.

16. Balance the following equations

(i) H2O

2 → H

2O + O

2

(ii) S + O2 → SO

3

(iii) C2H

2 + O

2 → CO + H

2O

(iv) MnO2 + HCl → MnCl

2 + Cl

2 + H

2O

17. Classify the following molecules as mono, di, tri, tetra, penta and hexatomic molecules.

H2, P

4, SF

4, SO

2, PCl

3, C

2H

2, CH

3OH, PCl

5, H

2O

2, HCl, Cl

2O

18. What is meant by molecular formula? Hydrogen peroxide has the molecularformula H

2O

2. What mass of oxygen can be formed from 17 g of H

2O

2 if

decomposition of H2O

2 takes place.

19. Write ‘true’ or ‘false’.

A balanced chemical equation shows

(i) the formulas of the products

(ii) the molar proportions in which the products are formed

(iii) that a reaction can occur

(iv) the relative number of atoms and molecules which react

(v) that a reaction is exothermic

20. What is the mass of

(i) 6.02 × 1023 atoms of O

(ii) 6.02 × 1023 atoms of P

(iii) 6.02 × 1023 molecules of P4

(iv) 6.02 × 1023 molecules of O2

[Ans. (a) 16.0 g (b) 31.0 g (c) 124.0 g (d) 32 g]

21. How many atoms are there in

(i) two moles of iron

(ii) 0.1 mol of sulphur

(iii) 18 g of water, H2O

(iv) 0.44 g of carbon dioxide, CO2

[Ans. (a) 1.204 × 1024 (b) 6.02 × 1022 (c) 1.8 × 1024 and (d) 1.8 × 1022]

22. Define the following

(i) Law of constant proportions

(ii) Law of multiple proportions


(iii) Avogadro’s Law

(iv) Gay Lussacis Law

(v) Dalton’s atomic theory

23. Convert into mole

(i) 12 g of oxygen gas (O2)

(ii) 20 g of water (H2O)

(iii) 22 g pf carbon dioxide (CO2)

(Ans. (a) 0.375 mol (b) 1.11 mol (c) 0.50 mol)


2.1 1. (i), (iii), (v) and (vi)

2. (ii)

2.2 1. refer text

2. (i) FePO4 (ii) BaCl

2 (iii) CaCO

3 (iv) PBr

3 (v) MgSO

4 (vi) Na

3PO

4 (v) SO

3

3. refer text

4. CH2, HCl, HNO

3

2.3 1. 2814

Si, 2914

Si, 3014

S

2. C3H

8= 44 u

PCl5

= 207.5 u

SO3

= 80 u

2.4 1. 3.08 × 1023 S atom

2. 1.77 × 10–22 g of Ag

2.5 1. (i) 4H3PO

3 → 3H

3PO

4 + PH

3

(ii) Ca + 2H2O → Ca(OH)

2 + H

2

(iii) C3H

8 + 5O

2 → 3CO

2 + 4H

2O

2. Sodium oxide, Cuperous chloride, Barium oxide, Sodium sulphate

GLOSSARY

Atomic mass: The average mass of an atom in a representative sample of atoms of anelement.

Compound: Matter that is composed of two or more different kinds of elementschemically combined in definite proportions.

Chemical reaction: A process in which substances are changed into other substancesthrough rearrangement/combination of atoms.

Diffusion: The gradual mixing of the molecules of two or more substances owing torandom molecular motion.


Element: Matter that is composed of one kind of atoms, each atom of a given kindhaving the same properties (Mass is one such property).

Heterogeneous mixture: A mixture which has no uniformity in composition.

Homogeneous mixture: A mixture with the same composition throughout

Isotopes: Isotopes are atoms having the same atomic number, Z but different massnumber, A.

Mass number: Number of protons plus number of neutrons in the nucleus of an atomof an element.

Matter: Anything that has mass and occupies space.

Mole: Mole is amount of substance that contains as many elementary particles asthere are atoms in 0.012 kg of C-12 isotope.

Molar mass: The mass (in gram) of one mole of a substance.

Molar volume: The volume of one mole of a substance.

Molecular mass: The sum of atomic masses (in u) of all the atoms of a molecule.

3

Atomic StructureIn the previous lesson, you have studied that the atoms are the smallest constituents ofmatter. But what is the structure of an atom? Why are atoms of different elements different?Let us try to find out the answers to some of these questions in this lesson.

We will start the study of this lesson by recapitulating the postulates of Dalton’s atomictheory .At that time, many Greek philosophers believed that the atoms cannot be furthersubdivided, i.e. they were structure less entities. But as you will study in this lesson, variousdevelopments such as the discoveries of sub-atomic particles such as electron, proton etc.led to the failure of this idea. Based on these discoveries, various atomic models wereproposed by the scientists. In this lesson, we would discuss how various models for thestructure of atom were developed and what were their main features. We would explainthe success as well as the shortcomings of these models. These models tell us about thedistribution of various sub-atomic particles in the atom. From the knowledge of structureof atom the arrangement of electrons around the nucleus can be obtained. This arrangementis known as electronic configuration. The electronic configurations of some simple elementsare discussed in this lesson These electronic configurations would be useful in explainingvarious properties of the elements. The electronic configuration of an element governs thenature of chemical bonds formed by it. This aspect is dealt in lesson 5 on chemical bonding.

OBJECTIVES

After completing this lesson, you should be able to:! state the reasons of failure of Dalton’s atomic theory;! name and list the fundamental particles present in the atom;! recall the developments of various atomic models;! list the shortcomings of Bohr’s atomic model;! compute the electronic configuration of first 18 elements.

3.1 FAILURE OF DALTON’S ATOMIC THEORY

You have read about Dalton’s atomic theory in lesson 2. Dalton’s theory explained variouslaws of chemical combination about which you have read earlier in lesson 2. At that time,the atom was considered to be indivisible. Later, certain experiments showed that an atomis made up of even smaller particles which are called subatomic particles. You will nowstudy about the discovery of these subatomic particles namely electrons, protons andneutrons.

: 52 : Atomic Structure

Metalelectrode

(Cathode)

Evaluatedglass vessel

Metalelectrode(anode)

(+)

(–)

High Voltage

3.1.1 Discovery of electron

During 1890s’ many scientists performed experiments using cathode ray tubes. A cathoderay tube is made of glass from which most of the air has been removed. Such a cathoderay tube has been shown in Fig. 3.1. You can see in the figure that there are two metalelectrodes; the negatively charged electrode is called cathode whereas the positively chargedelectrode is called anode.

Fig. 3.1 Cathode ray tube

An English physicist J.J. Thomson studied electric discharge through a cathode raytube. When high voltage was applied across the electrodes, the cathode emitted a streamof negatively charged particles, called electrons.

electron theof massunit per charge

electron theof chargemass =

18

19

g C 1076.1

C1060.1

/ −

−

××==

me

e = 9.10 ×10-28 g

Since the electrons were released from the cathode irrespective of the metal used for itor irrespective of the gas filled in the cathode ray tube, Thomson concluded that all atomsmust contain electrons. Robert Millikan (1868-1953) received the Nobel prize in Physicsin 1923 for determining the charge of the electron.

The discovery of the electron led to the conclusion that the atom was no more indivisibleas was believed by Dalton and others. Hence, the idea of indivisibility of atom as suggestedby Dalton was proved incorrect. In other words, the atom was found to be divisible.

If the atom was divisible, what were are its constituents? You have read above that onesuch particle is an electron. Now, what are the other particles present in an atom? Let usstudy the next section and find out the answer.

3.1.2 Discovery of proton

In 1886, Eugen Goldstein observed that rays flowing in a direction opposite to that of thecathode rays were positively charged. Such rays were named as canal rays because theypassed through the holes or the canals present in the perforated cathode. In 1898, Wilhelm

Atomic Structure : 53 :

+ +

+

++

+

+

+

Anode Cathode

Wien, a German physicist, measured e/m for canal rays. It was found that the particlesconstituting the canal rays are much heavier than electrons. Also unlike cathode rays, thenature and the type of these particles varied depending upon the gas present in the cathoderay tube. The canal rays had positive charges which were whole number multiples of theamount of charge present on the electron. The positive nature of the canal rays was explainedas follows:

In a cathode ray tube, the electrons emitted from the cathode collide with the atoms ofthe gas present in the tube and knock out one or more electrons present in them. Thisleaves behind positive ions which travel towards the cathode. If the cathode has holes in it,then these positive ions can pass through these holes or canals. Hence, they are called thecanal rays. The canal rays are shown in Fig. 3.2.

Fig. 3.2 Canal rays

When the cathode ray tube contained hydrogen gas, the particles of the canal raysobtained were the lightest and their e/m ratio was the highest. Rutherford showed thatthese particles were identical to the hydrogen ion (hydrogen atom from which one electronhas been removed). These particles were named as protons and were shown to be presentin all matter.

Now it is the time to check your understanding. For this, take a pause and solve thefollowing questions:


1. Name the extremely small particles which constitute matter.2. What do we call the negatively charged particles emitted from the cathode?3. What is a cathode ray tube?4. What is an anode?5. Why the canal rays obtained by using different gases have different e/m values?

3.2 EARLIER MODELS OF ATOM

Based on the experimental observations, different models were proposed for the structureof the atom. In this section, we will discuss two such models namely Thomson model andRutherford model.

3.2.1 Thomson model

All matter is made of atoms and all the atoms are electrically neutral. We have just seenthat all atoms contain the electrons. Based on these facts, Thomson concluded that theremust be an equal amount of positive charge present in the atom. He proposed that an atomcould be considered as a sphere of uniform positive charge in which electrons are embedded.This is shown below in Fig.3.3.


Electrons

Special cloudof positive charge

+

– –

–

–

–

–

– –

–

––

–

–

(a) (b)

Beam of

particlesScattered of

particles

Circularfluorescent

screenThin gold foil

Most particlesare undeflected

Fig. 3.3 Thomson model of atom

This model is similar to a water-melon accordingto which an atom can be thought of as a sphere ofpositive charge in which the electrons are embeddedlike seeds. This model is also called plum puddingmodel or raisin pudding model because theelectrons resembled the raisins dispersed in a pudding(an English dessert).

During this period only, the phenomenon ofradioactivity was also being studied by the scientists.This phenomenon of spontaneous emission of rays from atoms of certain elements alsoproved that the atom was divisible and it contained sub –atomic particles. ErnestRutherford and his coworkers were also carrying out experiments which revealed thatthe radiation could be of three types: α(alpha), β(beta) and γ(gamma). You will studymore about them in lesson 14.

In 1910, Rutherford and his co-workers performed an experiment which led to thedownfall of the Thomson model. Let us now study about the contribution of Rutherford.

3.2.2 Rutherford’s model

Rutherford who was a student of J.J Thomson was studying the effect of alpha (a) particleson matter. The alpha particles are helium nuclei. They are obtained by the removal of twoelectrons from the helium atom. Hans Geiger (Rutherford’s technician) and Ernest Marsden(Rutherford’s student) directed α particles from α radioactive source on a thin piece ofgold foil (about 0.00004 cm thick). This is shown below in Fig. 3.4.

Ernest Rutherford, (1871-1937) who received the Nobel Prize in Chemistry in 1908for proposing the nuclear model of the atom.

Fig.3.4 (a) The experimental set-up for the α particle bombardment on thin gold foil,(b)Scattering of α particles

If Thomson model was correct,then most of the a particles should pass through the goldfoil and their path should only be deflected by a small amount. They were surprised to findout that although the majority of the a particles passed through the gold foil undeflected(or were deflected with minor angles), some of them were deflected by a large angles anda few even bounced back. This is shown in Fig. 3.4(b). In 1911, Rutherford explained theabove observation by proposing another model of the atom. He suggested that :


(i) Most of the mass of atom and all of its positive chargereside in a very small region of space at the centre of theatom, called the nucleus.

(ii) The electrons revolve around the nucleus in circular paths.This model is also known as Rutherford’s nuclear

model of the atom and is shown in Fig. 3.5.

This model resembeled the solar system in which thenucleus was similar to the Sun and the electrons were similarto the planets. Ruthurford was able to predict the size of thenucleus by carefully measuring the fraction of α particlesdeflected. He estimated that the radius of the nucleus was atleast 1/10000 times smallerthan that of the radius of the atom. We can imagine the size of the nucleus with thefollowing similarity. If the size of the atom is that of a cricket stadium then the nucleuswould have the size of a fly at the centre of the stadium.

Thus, most of the space in the atom is empty through which the majority of the α-particles could pass. When the α- particles come close to the nucleus, they are repelled byits positive charge and hence they show a large deflection. Wherefrom this positive chargecomes in the nucleus?

The nucleus was supposed to contain positively charged particles, called protons.The positive charge on a proton was equal but opposite in nature to that on an electron.This quantity of charge, i.e. 1.602 x 10 –19 C is called the electronic charge and is expressedas a unit charge, i.e., the charge of an electron is –1 whereas that of a proton is +1.


1. Who proposed the nuclear model for the structure of atom?2. Define nucleus.3. What is a proton?

3.3 DISCOVERY OF NEUTRON

Although Rutherford’s model of the atom could explain the electrical neutrality and theresults of scattering experiment but a major problem regarding the atomic masses remainedunsolved.

The mass of helium atom (which contains 2 protons) should be double than that of ahydrogen atom (which contains only one proton). [The electron being very light weightparticle as compared to that of a proton, its contribution to the atomic mass can be ignored].Actual ratio of helium and hydrogen masses is 4:1. Rutherford and others, thus, suggestedthat there must be one more type of subatomic particle present in the nucleus which maybe neutral but must have mass. Later in 1932, James Chadwick showed the existence ofthis third type of subatomic particle. This was named as neutron. The neutron was foundto have a mass slightly higher than that of a proton electrically neutral. Thus, if the heliumatom contained 2 protons and 2 neutrons in the nucleus, its mass ratio to hydrogen as 4:1could be explained. The characteristics of these three particles, called as fundamentalparticles are given in Table 3.1.

+

–

Nucleus

Electron

Fig. 3.5 Rutherford’s nuclearmodel of atom


James Chadwick (1891-1972) was a British physicist. He received the Nobel prize in1935 for showing the existence of neutron in the nucleus of an atom.

Table 3:1 Characteristics of the subatomic particles.

Particle Symbol Mass(kg) Charge Coulomb (C)in multiple

units

Electron e 9.10939 x 10-31 –1.6022x10-19 -1Proton p 1.67262 x 10-27 +1.6022 x 10-19 +1Neutron n 1.67493 x 10-27 0 0


1. What is a neutron?2. How many neutrons are present in the α-particle?3. How will you distinguish between an electron and a proton?

3.4 ATOMIC NUMBER, MASS NUMBER AND ISOTOPES

Why do the atoms of different elements differ from each other? The numbers of protonspresent in the atom of an element are different from those present in the atom of anotherelement. Thus, the number of protons present in the atom of each element is fixed and is acharacteristic property of that element as you have already learnt in lesson 2. This numberis called the atomic number and is denoted by Z .Hydrogen has one proton in its nucleusand therefore, its atomic number is 1. Similarly, two protons are present in the nucleus ofhelium atom and hence its atomic number is 2. What about the number of electrons presentin hydrogen and helium? Since the atom is electrically neutral, the number of electronspresent in these atoms is 1 and 2 respectively.In addition to the protons, the helium atomalso has neutrons present in its nucleus. The total number of protons and neutrons presentin the nucleus of an atom of an element is called its mass number. It is denoted by A.Helium nucleus contains 2 protons and 2 neutrons; hence, its mass number is 4.The atomicnumber and the mass number of an element (X) can be denoted as follows:

AZX

Thus, helium can be represented as 42H

Similarly, 126C means that the carbon atom has 6 protons and hence 12–6 = 6 neutrons.

But some carbon atoms can have 7 or 8 neutrons also. The mass number of these carbonatoms would be 6+7=13 or 6+8=14.Such atoms which have the same atomic number buthave different mass number are called isotopes. Thus, carbon has three isotopes.

These isotopes can be represented as shown below:126C, 13

6C, 14

6C


1. How is atomic number related to the number of protons present in the atom?2. What is the mass number of an atom which has 7 protons and 8 neutrons?3. Calculate the number of neutrons present in the following isotopes of hydrogen.

11H, 2

1H, 3

1H


3.5 DRAWBACKS OF RUTHERFORD’S MODEL

As you have studied in section 3.3, Rutherford’s model could notsolve the problem of atomic mass. The existence of the neutronthus accounted for the mass of the atom. Another drawback whichRutherford’s model suffered was that it could not explain thestability of the atom. According to the electromagnetic theory ofradiation, a moving charged particle, such as the electron whichis constantly accelerating because of change in directions ofmotion, should emit radiation. The energy of the radiation wouldcome from the motion of the electron. Thus, the electron wouldemit radiation and follow a spiral path as shown in Fig.3.6.

The energy of the electron would keep on decreasing (as the electron would keep onemitting radiation) till the electron finally falls into the nucleus. But actually it does nothappen. The electron does not collapse into the nucleus. Thus, Rutherford’s model neededthe improvements which were later on suggested by Bohr. Bohr’s model now will bediscussed in the next section.


1. What were the two drawbacks of Rutherford’s model?

3.6 BOHR’S MODEL OF ATOM

In 1913, Niels Bohr proposed a model which was an improvement over Rutherford’s nuclearmodel. Bohr proposed that an electron moves around the nucleus in a well defined circularpath. He set down following two main postulates to explain the stability of atom particularlyhydrogen atom

(i) An electron can have only a definite circular path around the nucleus with specificenergy values. This circular path he called orbit or energy level

(ii) Electron may go to next higher energy level (orbit) when given a definite amountof energy. In other words, an electron absorbs energy when it goes to higher energy levelfrom a lower energy level.

Contrary to this, electron will emits out a definite amount of energy when it comesfrom a higher energy level to lower energy level. If E

2 is energy of an electron in higher

energy level and E1 is energy of electron in lower energy level, then energy released ∆E

will be expressed as,

∆E = E2 – E

1

If the electron remians in the same orbit, the energy would neither be released nor absorbed.These orbits will, therefore, were called stationary orbits or stationary states.

Niels Bohr (1885-1962). He was a Danish physicist He was awarded the NobelPrize in Physics in 1922.

Although Bohr model could explain a number of aspects related to hydrogen atom butit could not explain stability of atoms having more than one electron. After the nature ofelectron was studied in detail, it was found that an electron cannot remain in a fixed circularorbit as envisaged by Bohr. Bohr model was rejected on this ground.

Fig. 3.6 Spiral path ofan electron


Based on the nature of electron, concept of circular orbit was modified and a threedimensional shell with definite energy came into existence. These shells are similar tocircular path/energy levels given by Bohr. These shells are represented by letters K, L, M,N etc. Each shell is associated with a definite energy. The energies of these shells go onincreasing as we move away from the nucleus. The maximum number of electrons whichcan be accommodated in each shell is given by 2n2 where n can take values 1, 2, 3….etc.Thus, the first shell can have two electrons whereas the second shell can have 8 electrons.Similarly the maximum number of electrons present in third and fourth shells would be 18and 32, respectively. Each shell could be further sub-divided into various sublevels ofenergy called subshells. These subshells are denoted by letters s, p, d, f, etc about whichyou would study in your higher classes.


1. What are stationary states?2. What will happen to the energy of electron when it goes from an orbit of higher energy

to that of a lower energy?3. What is a shell?4. How many electrons can be present in a L-shell?

3.7 ELECTRONIC CONFIGURATION OF ELEMENTS

From the above discussions, you are aware that shells of different energies exist in anatom. The electrons occupy these shells according to the increasing order of their energy.You also know that the first shell can have two electrons whereas the second shell canaccommodate eight electrons. Keeping these points in mind, let us now study the filling ofelectrons in various shells of atoms of different elements.

Hydrogen atom has only one electron. Thus electronic configuration of hydrogen canbe represented as 1.

The next element helium (He) has two electrons in its atom. Since the first shell canaccommodate two electrons; hence, this second electron can also be placed in first shell. Theelectronic configuration of helium can be represented as 2.

The third element, Lithium (Li) has three electrons. Now the two electrons occupy thefirst shell whereas the third electron goes to the next shell of higher energy level, i.e.second shell. Thus, the electronic configuration of Li is 2, 1.

Similarly, the electronic configurations of beryllium (Be) and boron (B) having fourand five electrons respectively can be written as follows:

Be - 4 electrons Electronic configuration - 2, 2.

B - 5 electrons Electronic configuration - 2, 3.

The next element carbon (C) has 6 electrons. Now the sixth electron also goes to thesecond shell which can accommodate eight electrons. Hence, the electronic configurationof carbon can be represented as 2, 4. Similarly, the next element nitrogen having 7 electronshas the electronic configuration 2, 5.

The electronic configuration of other elements can be given on the same lines. Theelectronic configuration of first twenty elements is given in Table 3.2 and depicted in Fig.3.7.


Element/symbol No. of Arrangement of electrons in shells Electrons Commonelectrons distribution valency

in shells

Hydrogen, H 1 1 in first shell 1 1Helium, He 2 2 in first shell 2 0Lithium, Li 3 2 in first shell + 1 in second shell 2,1 1Beryllium, Be 4 2 in first shell + 2 in second shell 2,2 2Boron, B 5 2 in first shell + 3 in second shell 2,3 3Carbon, C 6 2 in first shell + 4 in second shell 2,4 4Nitrogen, N 7 2 in first shell + 5 in second shell 2,5 3Oxygen, O 8 2 in first shell + 6 in second shell 2,6 2Fluorine, F 9 2 in first shell + 7 in second shell 2,7 1Neon, Ne 10 2 in first shell + 8 in second shell 2,8 0Sodium, Na 11 2 in first shell + 8 in second shell + 2,8,1 1

1 in third shellMagnesium, Mg 12 2 in first shell + 8 in second shell + 2,8,2 2

2 in third shellAluminium, Al 13 2 in first shell + 8 in second shell + 2,8,3 3

3 in third shellSilicon, Si 14 2 in first shell + 8 in second shell + 2,8,4 4

4 in third shellPhosphorus, P 15 2 in first shell + 8 in second shell + 2,8,5 3,5

5 in third shellSulphur, S 16 2 in first shell + 8 in second shell + 2,8,6 2

6 in third shellChlorine, Cl 17 2 in first shell + 8 in second shell + 2,8,7 1

7 in third shellArgon, Ar 18 2 in first shell + 8 in second shell + 2,8,8 0

8 in third shellPotassium, K 19 2 in first shell + 8 in second shell + 2,8,8,1 1

8 in third shell + 1 in fourth shellCalcium, Ca 20 2 in first shell + 8 in second shell + 2,8,8,2 2

8 in third shell + 2 in fourth shell

Table 3.2: Electronic distribution in shells of first twenty elementsFig. 3.7 Electronic configuration of some elements


3.7.1 Valence electron and valency

We have just discussed the electronic configuration of first 20 elements. We can see fromthe table 3.2 that electrons are located in different shells around the nucleus. The electronsin the last shell (popularly known as valence shell) govern the chemical properties of theatoms. These electrons are known as valence electrons. Valency or combining capacity ofan atom of an element depends on the number of these electrons as mentioned in lesson 2.Valency of 20 elements along with their electronic configuration is also provided in Table3.2.

In next lesson, you would study how these electronic configurations are useful inunderstanding the periodic arrangement of elements. These electronic configurations arealso helpful in studying the nature of bonding between various elements which will bedealt in lesson 5.


1. How many shells are present in the nitrogen atom?2. Name the element which has the completely filled first shell.3. The electronic configuration of an element having atomic number11 is_____________

LET US REVISE

! Electrons are present in all the atoms.! Thomson proposed the plum-pudding model of the structure of atom.! Rutherford’s model of the structure of atom suggested that most of the mass and all of

positive charge of an atom is concentrated in its nucleus and the electrons revolvearound it in.

! The neutrons are neutral particles present in the nucleus.! Atomic number is the number of protons present in the nucleus of an atom.! Mass number gives the number of protons and neutrons present in an atom! Isotopes have same atomic number but different mass numbers.! Bohr’s model gave the idea of definite orbits or stationary states.! The electrons occupy various shells in an atom in the increasing order of their energy.

The maximum number of electrons which can be accommodated in a shell is 2n2.

TERMINAL EXERCISES

A. Fill in the blanks.1. The nucleus consists of ———————and ————————2. The model which resembled the solar system was proposed by——3. Anode rays travel towards——————————4. An electron has ——————— charge.

B. Classify the following statements as true or false.1. The plum pudding model was proposed by Rutherford.2. Cathode is the negatively charged electrode.3. Neutrons are constituents of atoms of all elements.4. The number of electrons present in a neutral atom is always equal to the number of

protons.


C. Multiple choice type questions.1. An α-particle has

(a) 2 protons only. (b) 2 neutrons only(c) 2 protons and 2 neutrons (d) 2 neutrons

2. Isotopes have(a) same mass number (b) same atomic number(c) different atomic number (d) same mass as well as atomic number

3. The mass of a neutron(a) is less than that of a proton. (b) is greater than that of a proton.(c) is equal to that of a proton. (d) zero

4. The filling of second shell starts with(a) He (b) Li(c) C (d) N

5. The electronic configuration of Cl is(a) 2, 8 (b) 2, 8, 4(c) 2, 8, 6 (d) 2, 8, 7

6. Which of the following elements has completely filled shells?(a) H (b) O(c) Ne (d) Mg

D. Descriptive type questions.1. How can you say that electrons are present in all types of matter?2. Define an orbit.3. Calculate the number of neutrons present in 16

8O and 19

9F

4. The mass number of iron is 56. If 30 neutrons present in its atom, what is its atomicnumber?

5. Which of the following are isotopes? 126C, 14

6C, 14

7N


3.1

1. atoms2. electrons3. A glass tube from which most of the air has been removed. It has two electrodes.4. It is a positively charged electrode.5. because the positive ions resulting from the different gases have different masses.3.21. J.J. Thompson2. The small region of space at the centre of the atom where most of the mass and all of

the positive charge is located.3. An alpha particle is the helium nucleus which is obtained by the removal of two electrons

from the helium atom.3.31. A neutron is a neutral subatomic particle having mass slightly higher than proton.2. 23. (i) An electron has negative charge whereas a proton has a positive charge.


(ii) An electron is present outside the nucleus whereas a proton is present in thenucleus.(iii) The electron has very less mass as compared to a proton.

3.41. Atomic number is equal to the number of protons present in the nucleus of the atom.2. 153. 0, 1, 2.3.51. It could not explain the correct atomic masses and the stability of atoms.3.61. Stationary states are energy levels of definite energy. When an electron is present in a

stationary state, its energy does not change.2. Its energy would decrease.3. A shell is a group of energy levels having similar energy.4. 8 electrons.3.71. 22. He3. 2, 8, 1

GLOSSARY

Alpha particles: Positively charged particles ejected at high speeds from certainradioactive substances;

Atom: The smallest particle of an element that retains the chemical properties of thatelemen.

Atomic nucleus: The tiny central core of an atom that contains neutrons and protons.

Atomic number: The number of protons in the nucleus of an atom of an element.

Electron: A negatively charged subatomic particle found in the space about the nucleus.

Electron shell: The collectio of orbitals with same principal quantum number.

Electronic configuration: The complete description of the orbitals occupied by allthe electrons in an atom on ion.

Isotopes: Forms of an element composed of atoms with same atomic number butdifferent mass number owing to a difference in a number of neutrons.

Mass number: The number of proton plus neutrons in the nucleus of an atom of anelement.

Neutrons: An electrically neutral subatomic particle found in the nucleus.

Orbital: Regions occupied by electrons in S, P, d, f, subshells, represened by threedimensional boundary surface diagram..

Proton: A positively charged subatomic particle found in the nucleus.

4

Periodic Classification of ElementsYou must have visited a library. There are thousands of books in a large library. In spite ofthis if you ask for a particular book, the library staff can locate it easily. How is it possible?In library the books are classified into various categories and sub-categories. They arearranged on shelves accordingly. Therefore location of books becomes easy.

In the last two lessons you have studied about the structure of atoms and their electronicconfigurations. You have also studied that elements with similar electronic configurationsshow similar chemical properties. Electrons are filled in various shells and subshells in afairly regular fashion. Therefore, properties of elements are repeated periodically. Suchtrends in their physical and chemical properties were noticed by chemists in the nineteenthcentury and attempts were made to classify elements on their basis long before structure ofatom was known.

In this lesson we shall study about the earlier attempts for classification, the firstsuccessful classification which included all the known elements at that time namelyMendeleev’s periodic table, and about the long form of modern periodic table which is animprovement over Mendeleev’s work. Finally we shall learn about some properties ofelements and their variations in the periodic table.

OBJECTIVES

After completing this lesson, you will be able to:! state different historical classifications of elements in brief;! state main features of Mendeleev’s periodic table;! explain the defects of Mendeleev’s periodic table;! state modern periodic law;! describe the features of the long form of periodic table;! define various periodic properties;! discuss the trends in various periodic properties in the periodic table.

4.1 EARLIER ATTEMPTS OF CLASSIFICATION OF ELEMENTS

The first classification of elements was as metals and non-metals. This served only limitedpurpose mainly because of two reasons:

1. All the elements were grouped in to these two classes only. Moreover the groupcontaining metals was very big.

: 64 : Periodic Classification of Elements

2. Some elements showed properties of both-metals and non-metals and they could notbe placed in any of the two classes.After this, scientists made attempts to recognize some pattern or regularity in variation

of properties of elements and to classify them accordingly. Now we shall learn about someof them.

4.1.1 Dobereiner’s triads

In 1829, Dobereiner, a German scientist madesome groups of three elements each and calledthem triads. All three elements of a triad weresimilar in their properties. He observed that theatomic mass* of the middle element of a triadwas nearly equal to the arithmetic mean ofatomic masses of other two elements. Also, same was the case with their other properties.

Let us take the example of three elements lithium, sodium and potassium. They forma Dobereiner’s triad.

Mean of the atomic masses of the first (Li) and the third (K) elements: u

The atomic mass of the middle element, sodium, Na is equal to 23 u. Two moreexamples of Dobereneir’s triads are given below.

Element Atomic mass Element Atomic mass

Calcium, Ca 40 Chlorine, Cl 35.5Strontium, Sr 88 Bromine, Br 80Barium, Ba 137 Iodine, I 127

Mean of the atomic masses of the first and third elements =2

13740 + = 88.5 u

Mean of the first atomic masses of the and third elements = 2

1275.35 + = 81.5 u

Actual atomic mass of the second element = 88 u

Actual atomic mass of the second element = 80 u

Dobereneir’s idea of classification of elements into triads did not receive wideacceptance as he could arrange only a few elements in this manner.

4.1.2 Newland’s law of Octaves

In 1864 John Alexander Newland, an English chemist noticed that “when elements arearranged in the increasing order of their atomic masses* every eighth element had propertiessimilar to the first element.” Newland called it the Law of Octaves. It was due to itssimilarity with musical notes where, in every octave, after seven different notes the eighthnote is repetition of the first one as shown below.

1 2 3 4 5 6 7 8lk js xk e i /k uh lk

232

397 =+

*Then known as atomic weight

Element Atomic mass

Lithium, Li 7Sodium, Na 23Potassium, K 39

Periodic Classification of Elements : 65 :

Look carefully at the Newland’s arrangement of elements shown below:

Li Be B C N O F

(6.9) (9.0) (10.8) (12.0) (14.0) (16.0) (19.0)

Na Mg Al Si P S Cl

(23.0) (24.3) (27.0) (28.1) (31.0) (32.1) (35.5)

K Ca(39.1) (40.1)

With the help of the arrangement given above, can you tell starting from lithium whichis the eighth element? Sodium. And starting from sodium? It is potassium. Properties ofall three are similar. Similarly, aluminnium is the eighth element from boron it showsproperties similar to it.

However, Newland could arrange elements in this manner only up to calcium out of atotal of over sixty elements known at his time. Because of this shortcoming his work wasnot received well by the scientific community. The next break through in classification ofelements came in the form of Mendeleev’s work.

4.1.3 MENDELEEV’S PERIODIC LAW AND PERIODIC TABLE

4.3.1a Mendeleev’s periodic law

Dmitry Mendeleev** a Russian chemist while trying to classify elements discovered thaton arranging in the increasing order of atomic mass*, elements with similar chemicalproperties occurred periodically. In1869, he stated this observation in the following formwhich is known as Mendeleev’s Periodic Law.

A periodic function is the one which repeats itself after a certain interval. Thus,according to the periodic law the chemical and physical properties of elements repeatthemselves after certain intervals when they are arranged in the increasing order of theiratomic mass. Now we shall learn about the arrangement of elements on the basis of theperiodic law.

The chemical and physical properties of elements are a periodic function of their atomicmasses*.

A tabular arrangement of the elements based on the periodic law is called periodictable. Mendeleev believed that atomic mass of elements was the most fundamental propertyand arranged them in its increasing order in horizontal rows till he encountered an elementwhich had properties similar to the first element. He placed this element below the firstelement and thus started the second row of elements. Proceeding in this manner he couldarrange all the known elements according to their properties and thus created the firstperiodic table.

∗ Then known as atomic weight** Also spelled as Mendeleef or Mendeleyev


Fig. 4.1 Mendeleev’s periodic table

4.1.3b Main features of Mendeleev’s periodic table

Look at the Mendeleev’s periodic table shown in fig.4.2 carefully. What do you observe?Here, elements are arranged in tabular form in rows and columns. Now let us learn moreabout these rows and columns and the elements present in them.

1. The horizontal rows present in the periodic table are called periods. You can see thatthere are seven periods in the periodic table. These are numbered from 1 to 7 (Arabicnumerals).

2. Properties of elements in a particular period show regular gradation (i.e. increase ordecrease) from left to right.

3. The vertical columns present in it are called groups. You must have noticed that theseare nine in number and are numbered from I to VIII and Zero (Roman numerals).

4. Groups I to VII are subdivided into A and B subgroups. Groups Zero and VIII don’thave any subgroups.

5. All the elements in a particular group are chemically similar in nature. They showregular gradation in their physical properties and chemical reactivities.After learning about the main features we shall now learn about the main merits of

Mendeleev’s periodic table.

4.1.3c Merits of Mendeleev’s periodic classification1. Classification of all elements

Mendeleev’s was the first classification which successfully included all the elements.2. Prediction of new elementsMendeleev’s periodic table had some blank spaces in it. These vacant spaces were forelements that were yet to be discovered. For example, he proposed the existence of an

PERIODIC TABLE ( )Modified form of Mendleeff’s Table

PERIODICS

Group :

Oxide:Hydride:

I

R O2

RH

A B

II

RO

RH2

A B

III

R O2 3

RH3

A B

IV

R O2 5

RH4

A B

V

R O2 5

RH3

A B

VI

RO3

RH2

A B

VII

R O2 7

RH

A B

VII

Ro4

Transition Traids

Zero

Noblegases

1

2

3

4

5

6

7

Firstseriessecondseries

Firstseriessecondseries

Firstseries

secondseries

H 1 (At. No.)1.008(At.Wt.)

Li 36.939

Na 1122.99

K 1939.102

Cu 2963.54

Rb 3785.47

Ag 47107.87

Cs 55132.90

Au 79196.97

Fr 87(223)

Be 49.012

Mg 1224.312

Ca 2040.08

Zn 3065.37

Sr 3887.62

Cd 48112.40

Ba 56137.34

Hg 80200.59

Ra 88(226)

B 510.811Al 1326.981

Sc 2144.96

Ga 3169.72

Y 3988.905

In 49114.82

*RareEarths57-71

Tl 81204.37

ActinideElements89-103

C 612.011

N714.007

Si 1428.086

Ti 2247.90

Ge 3272.59

Zr 4091.22

Sn 50118.69

Hf 72178.49

Pb 82207.19

Ku 104 Ha 105

P 1530.974

V 2350.94

As 3374.92

Nb 4192.906

Sb 51121.75

Ta 73180.948

Bi 83208.98

O 815.999S 1632.06

Cr 2451.99

Se 3478.96

Mo 4295. 94

Te 52127.60

Po 84(210)

W 74183.85

F 918.998

Cl 1735.453

Mn 2554.939

Br 3579.909

Tc 43(99)

I 53124.9014

Re 75186.2

At 85(210)

Fe 26 Co 27 Ni 2855.85 58.93 58.71

Ru 44 Rh 45 Pd 46101.07 102.91 106.4

O s 76 Ir 77 Pt 78190.2 192.2 195.09

He 24.0026

Ne 1020.183

Ar 1839.948

Kr 3683.80

Xe 54131.30

Rn 86(222)

* Lanthaandie

Elements

( La 57 Ce 58 Pr 59 Nd 60 Pm 61 Sm 62 Eu 63 Gd 64 Tb 65 Dy 66 Ho 67 Er 68 Tm 69 Yb 70 Lu 71( 138.91 140.12 140.91 144.24 (147) 150.35 151.96 157.25 158.92 162.50 164.93 167.26 168.93 173.04 174.97

(Rare Earth Series)

Actinide Series ( Ac 89 Th 90 Pa 91 U 92 Np 93 Pu 94 Am 95 Cm 96 Bk 97 Cf 98 Es 99 Fm 100 Md 101 No 102 Lr 103( (227) 232.04 (231) 238.3 (237) (244) (243) (245) (247) (249) (254) (253) (256) (253) (257)


unknown element that he called eka-aluminium. The element gallium was discovered fouryears later and its properties matched very closely with the predicted properties of eka-aluminium.

In this section we have learnt about the success of Mendeleev’s periodic classificationand also about its merits. Does it mean that this periodic table was perfect? No. Althoughit was a very successful attempt but it also had some defects in it. Now we shall discuss thedefects in this classification.4.3.1d Defects in Mendeleev’s periodic classificationIn spite of being a historic achievement Mendeleev’s periodic table had some defects in it.The following were the main defects in it:1. Position of hydrogen

Hydrogen resembles alkali metals (forms H+ ion just like Na+ ions) as well as halogens( forms H- ion similar to Cl- ion).Therefore, it could neither be placed with alkali metals(group I ) nor with halogens (group VII ).

2. Position of isotopesDifferent isotopes of same elements have different atomic masses, therefore, each oneof them should be given a different position in the periodic table. On the other hand,because they are chemically similar, they had to be given same position.

3. Anomalous pairs of elementsAt certain places, an element of higher atomic mass has been placed before an elementof lower atomic mass. For example, Argon (39.91) is placed before potassium (39.1)

CHECK YOUR PROGRESS 4.11. Elements A, B and C constitute a Dobereiner’s triad. What is the relationship in their

atomic masses?2. How many elements were included in the arrangement given by Newland?3. Which property of atoms was used by Mendeleev to classify the elements?4. How many groups were originally proposed by Mendeleev in his periodic table?5. Where in the periodic table are chemically similar elements placed, in a group or in a

period?6. Mendeleev’s periodic table had some blank spaces in it. What do they signify?7. What name was given to the element whose properties were similar to the element

eka-aluminium predicted by Mendeleev?

4.2 MODERN CLASSIFICATION

Henry Moseley, an English physicist discovered in the year 1913 that atomic number, isthe most fundamental property of an element and not its atomic mass. Atomic number,(Z), of an element is the number of protons in the nucleus of its atom. The number ofelectrons in the neutral atom is also equal to its atomic number. This discovery changedthe whole perspective about elements and their properties to such an extent that a needwas felt to change the periodic law also. Now we shall learn about the changes made in theperiodic law.

4.2.1 Modern periodic law

After discovery of atomic number the periodic law was modified and the new law wasbased upon atomic numbers in place of atomic masses of elements.


The Modern Periodic Law states “The chemical and physical properties of elementsare a periodic function of their atomic numbers”

After the change in the periodic law many changes were suggested in the periodictable. Now we shall learn about the modern periodic table which finally emerged.

4.2.2 Modern periodic table

The periodic table based on the modern periodic law is called the Modern Periodic Table.Many versions of this periodic table are in use but the one which is most commonly usedis the Long Form of Modern Periodic Table. It is shown in figure 4.3.

Fig. 4.3 Modern periodic table

If you look at the modern periodic table shown in the fig.4.3 you will observe that it isnot much different from Mendeleev’s periodic table. Now let us learn the main features ofthis periodic table.

4.2.2a GroupsThere are 18 vertical columns in the periodic table. Each column is called a group. Thegroups have been numbered from 1 to 18 (in Arabic numerals) from left to right. Group 1on extreme left position contains alkali metals (Li, Na, K, Rb, Cs and Fr) and group 18 onextreme right side position contains noble gases (He, Ne, Ar, Kr, Xe and Rn).

All elements present in a group have similar electronic configurations and have samenumber of valence electrons. You can see in case of group 1 (alkali metals) and group 17elements (halogens) that as one moves down a group, more and more shells are added.

Group 1 Group 17

Element Electronic configuration Element Electronic configuration

Li 2,1 F 2,7Na 2,8,1 Cl 2,8,7K 2,8,8,1 Br 2,8,8,7Rb 2,8,8,8,1 I 2,8,18,8,7

All elements of group 1 have only one valence electron. Li has electrons in two shells,Na in three, K in four while Rb has electrons in five shells. Similarly all the elements ofgroup 17 have seven valence electrons however the number of shells is increasing fromtwo in F to five in I.

Elements present in groups 1 and 2 on left side and groups 13 to 17 on the right side ofthe periodic table are called normal elements or representative elements. Theiroutermost shells are incomplete. They are also called typical or main group elements

H

1

Li

3

Na

11

K19

Rb37

Cs

55

Fr

87

Be

4

Mg

12

Ca20

Sr

Ba

56

Ra88

Sc21

38

Y39

La

57

Ce

58

Pr

59

Nd

60

Pm

61

Sm

62

Eu

63

Gd

64

Tb

65

Dy

66

Ho

67

Er

68

Tm

69

Yb

70

Lu

71

Hf

72

Ta

73

Ru

74

Re

75

Os

76

Ir

77

Pt

78

Au

79

Hg

80

Tl

81

Pb

82

Bi

83

Po

84

At

85

Rn

86

Ac

89

Th90

Pa

91

U92

Np

93

Pu94

Am

95

Cm96

Bk

97

Cf98

Es

99

Fm100

Md

101

No102

Lr

103

Unq104

Unp

105

Xe

54

Kr

36

Ar

18

Ne

10

He

2

I

53

Br

35

Cl

17

F

9

Te

52

Se

34

S

16

O

8

Sb

51

As

33

P

15

N

7

Sn

50

Ge

32

Si

14

C

6

In

49

Ga

31

Al

13

B

5

Cd

48

Zn

30

Ag

47

Pd

46

Rh

45

Ru

44

Tc

43

Mo

42

Nb

41

Zr

40

Cu

29

Ni

28

Co

27

Fe

26

Mn

25

Cr

24

V

23

Ti

22

Unh

106


Elements present in groups 3 to 12 in the middle of the periodic table are calledtransition elements. (Although groups 11 and 12 elements are, strictly speaking, nottransition elements). Their two outermost shells are incomplete.However, it should be noted here that more and more electrons are added to valenceshell only in case of normal elements. In transitions elements, the electrons are addedto incomplete inner shells.Elements 113, 115 and 117 are not known but included at their expected positions.

Group 18 on extreme right side of the periodic table contains noble gases. Theiroutermost shells contain 8 electrons.

Inner transition elements:14 elements with atomic numbers 58 to 71 (Ce to Lu) arecalled lanthanides# and they are placed along with the element lanthanum (La), atomicnumber 57 in the same position (group 3 in period 6) because of very close resemblancebetween them. However, for convenience sake they are shown separately below themain periodic table

14 elements with atomic numbers 90 to103 (Th to Lr) are called actinides* and theyare placed along with the element actinium (Ac), atomic number 89 in the same position(group 3 in period 7) because of very close resemblance between them. They are shownalso separately below the main periodic table along with lanthanides.

4.2.2b PeriodsThere are seven rows in the periodic table. Each row is called a period. The periods havebeen numbered from 1 to 7 (Arabic numerals).

In each period a new shell starts filling up. The period number is also the number ofshell which starts filling up in it. For example, in elements of 3rd period, the third shell(M shell) starts filling up as we move from left to right@ . The first element of thisperiod sodium Na (2,8,1) has only one electron in its valence shell (third shell) whilethe last element of this period, argon Ar (2,8,8) has eight electrons in its valence shell.The gradual filing of the third shell can be seen below.

Element Na Mg Al Si P S Cl Ar

Electronic 2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8configuration

! The first period is the shortest period of all and contains only 2 elements, H and He.! The second and third periods are called short periods and contain 8 elements each.! Fourth and fifth periods are long periods and contain 18 elements each.! Sixth and seventh periods are very long periods containing 32 elements* * each.

@ However, it should be noted here that more and more electrons are added to valence shell only in case ofnormal elements. In transitions elements, the electrons are added to incomplete inner shells.

# These elements have been named after the 1st elements lanthanum present in their position in the periodictable.

* These elements have been named after the 1st elements actinium present in their position in the periodictable.

** Including elements up to atomic number 118. Elements 114, 116 and 118 have been reported onlyrecently.


4.2.2c Merits of modern periodic table over Mendeleev’s periodic table

The modern periodic table is based on atomic number which is more fundamental propertyof an atom than atomic mass. The long form of modern periodic table is therefore free ofmain defects of Mendeleev’s periodic table.

1. Position of isotopes

All isotopes of the same elements have different atomic masses but same atomic number.Therefore, they occupy the same position in the modern periodic table which they shouldhave because all of them are chemically similar.

2. Anomalous pairs of elements

When elements are arranged in the periodic table according to their atomic numbers theanomaly regarding certain pairs of elements in Mendeleev’s periodic table disappears. Forexample, atomic numbers of argon and potassium are 18 and 19 respectively. Therefore,argon with smaller atomic number comes before potassium although its atomic mass isgreater and properties of both the elements match with other elements of their respectivegroups.


1. According to the modern periodic law the properties of elements are periodic functionof which property of theirs?

2. List any two defects of Mendeleev’s periodic table which have been corrected in themodern periodic table?

3. How many group and periods are present in the long form of periodic table?4. What is the name of the family of elements present in group 2 of the modern periodic

table?5. The elements that are present in the right hand portion of the periodic table are metals

or non-metals?6. How many elements are present in 6th period of the periodic table?

4.3 PERIODIC PROPERTIES

In the previous section we have learnt about the main features of the Modern PeriodicTable. We have also learnt that in a period the number of valence electrons and the nuclearcharge increases from left to right. It increases the force of attraction between them. In agroup the number of filled shells increases and valence electrons are present in highershells. This decreases the force of attraction between them and the nucleus of the atom.These changes affect various properties of elements and they show gradual variation in agroup and in a period and they repeat themselves after a certain interval of atomic number.Such properties are called periodic properties. In this section we shall learn about someperiodic properties and their variation in the periodic table.

4.3.1 VALENCY

(a) Valency in a period : You have already learnt in the previous section that the numberof valence electrons increases in a period. In normal elements it increases from 1 to 8


in a period from left to right. It reaches 8 in group 18 elements (noble gases) whichshow practically no chemical activity under ordinary conditions and their valency istaken as zero. Carefully look at the table given below. What do you observe? Valencyof normal elements with respect oxygen increases from 1 to 7 as shown below forelements of third period. This valency is equal to the number of valence electrons orgroup number for groups 1 and 2, or (group number-10) for groups 13 to 17.

Group 1 2 13 14 15 16 17

Element Na Mg Al Si P S Cl

No. of valence electrons 1 2 3 4 5 6 7

Valency with respect 1 2 3 4 5 6 7to oxygen

Formula of oxide Na2O MgO Al

2O

3SiO

2P

4O

10SO

3Cl

2O

7

In the following table for elements of second period you will observe that valency ofelements of with respect to hydrogen and chlorine increases from 1 to 4 and then decreasesto 1 again.

Group 1 2 13 14 15 16 17Element Li Be B C N O FNo. of valence electrons 1 2 3 4 5 6 7Valency with respect to 1 2 3 4 3 2 1hydrogen and chlorineFormula of hydride LiH BeH

2BH

3CH

4NH

3H

2O HF

Formula of chloride LiCl BeCl2

BCl3

CCl4

NCl3

Cl2O ClF

(b) Valency in a group : All the elements of a group have the same number of valenceelectrons. Therefore, they all have the same valency. Thus valency of all group 1elements, alkali metals, is 1. Similarly valency of all group 17 elements, halogens, is1 with respect to hydrogen and 7 with respect to oxygen.

4.3.2 Atomic radiiA number of physical properties like density and melting and boiling points are related tothe sizes of atoms. Atomic size is difficult to define. Atomic radius determines the size ofan atom. For an isolated atom it may be taken as the distance between the centre of atomand the outermost shell. Practically, measurement of size of an isolated atom is difficult;therefore, it is measured when an atom is in company of another atom of same element. Itis defined as one-half the distance between the nuclei of two atoms when they are linked toeach other by a single covalent bond.

4.3.2a Variation of atomic radii in a periodAtomic radii (in picometer) of 2nd and 3rd period elements are given in the table givenbelow. What do you observe? In a period, atomic radius generally decreases from left toright.

2nd Period Li Be B C N O F155 112 98 91 92 73 72

3rd Period Na Mg Al Si P S Cl

190 160 143 132 128 127 99


Can you explain this trend? You have learnt in the beginning of this section that in aperiod there is a gradual increase in the nuclear charge. Since valence electrons are addedin the same shell, they are more and more strongly attracted towards nucleus. This graduallydecreases atomic radii.

4.3.2b Variation of atomic radii in a groupWhat happens to atomic radii in a group? Atomic radii increase in a group from top tobottom. This can be seen from the data of atomic radii in picometers given for groups 1and 17 elements below.

Element Atomic radius Element Atomic radiusLi 155 F 72

Na 190 Cl 99K 235 Br 114

Rb 248 I 133

As we go down a group the number of shells increases and valence electrons arepresent in higher shell and the distance of valence electrons from nucleus increases. Forexample, in lithium the valence electron is present in 2nd shell while in sodium it is presentin 3rd shell. Also, the number of filled shells between valence electrons and nucleus increases.Thus in group 1 Li (2,1) has one filled shell between its nucleus and valence electron whileNa (2,8,1) has two filled shells between them. Both the factors decrease the force of attractionbetween nucleus and valence electron. Therefore, atomic size increases on moving downa group.

4.3.3 Ionic radii

Ionic radius is the radius of an ion. On converting into an ion the size of a neutral atomchanges. Anion is bigger than the neutral atom. This is because addition of one or moreelectrons increases repulsions among electrons and they move away from each other. Onthe other hand a cation is smaller than the neutral atom. When one or more electrons areremoved, the repulsive force between the remaining electrons decreases and they come alittle closer.

4.3.3a Variation of ionic radii in periods and groups

Ionic radii show variations similar to those of atomic radii. Thus, ionic radii increase in agroup. You can see such increases in groups 1 and 16 elements from the data given below.

Group 1 Group 16Element Electron radius Element Ionic radiusLi+ 60 O2- 140

Na+ 95 S2- 184K+ 133 Se2- 198

Rb+ 148 Te2- 221

Ionic radii decrease in a period . It can be seen from the data of ionic radii in picometerfor 2nd period elements given below.

Element Li+ Be2+ B C N3- O2- F- I o n i cradii 60 31 - - 171 140 136


In the data given above, the positions of boron and carbon have been left vacant asthey do not form ions. Also, the trend in radii of cations is seen in Li+ and Be2+and in radiiof anions is seen in N3–, O2– and F–.

4.3.4 Ionization energy

Negatively charged electrons in an atom are attracted by the positively charged nucleus.For removing an electron this attractive force must be overcome by spending some energy.The minimum amount of energy required to remove an electron from a gaseous atom in itsground state to form a gaseous ion is called ionization energy. It is measured in unit of kJmol-1. It is a measure of the force of attraction between the nucleus and the outermostelectron. Stronger the force of attraction, greater is the value of ionization energy. Itcorresponds to the following process:

If only one electron is removed, the ionization energy is known as the first ionizationenergy. If second electron is removed the ionization energy is called the second ionizationenergy. Now we shall study the variation of ionization energy in the periodic table.

4.3.3a Variation of ionization energy in a group

We have already seen earlier, that the force of attraction between valence electrons andnucleus decreases in a group from top to bottom. What should happen to their ionizationenergy values? Ionization energy decreases in a group from top to bottom. This can beseen from ionization energy values (in kJ mol-1) of groups 1 and 17 elements given below.

Group 1 Group 17Element Ionization Energy Element Ionization EnergyLi 520 F 1680

Na 496 Cl 1251K 419 Br 1143

Rb 403 I- 1009

4.3.4b Variation of ionization energy in a period

We know that the force of attraction between valence electron and nucleus increases in aperiod from left to right. As a consequence of this, the ionization energy increases in aperiod from left to right. This trend is can be seen in ionization energies (in kJ mol-1) ofelements belonging to 2nd and 3rd periods.

2nd Period ElementsElement Li Be B C N O F Ne

IonizationEnergy 520 899 801 1086 1400 1314 1680 2080

3rd Period ElementsElement Na Mg Al Si P S Cl Ar

IonizationEnergy 496 738 578 786 1021 1000 1251 1521

4.3.5 Electron affinity

Another important property that determines the chemical properties of an element is thetendency to gain an additional electron. This ability is measured by electron affinity. It is


the energy change when an electron is accepted by an atom in the gaseous state. Itcorresponds to the process

X(g) + e– → X–(g) + E

Here, X is an atom of an element. The energy change is measured in the unit kJ mol-1.By convention, electron affinity is assigned a positive value when energy is released duringthe process. Greater the value of electron affinity, more energy is released during the processand greater is the tendency of the atom to gain electron. Let us now learn about its variationin the periodic table.

4.3.5a Variation of electron affinity in a group

In a group, the electron affinity decreases on moving from top to bottom, that is, less andless amount of energy is released. Such trends in its values (in kJ mol-1) for group 1 andgroup 17 elements are given below.

Group 1 Group 17Element Electron affinity Element Electron affinityLi 58 F 333

Na 53 Cl 348K 48 Br 324

Rb 45 I- 295

4.3.5b Variation of electron affinity in a period

In a period, the electron affinity increases from left to right, that is, more and more amountof energy is released. You can see this increase in electron affinity values (in kJ mol-1)below for elements of 2nd and 3rd periods.

2nd Period elementsElement Li Be B C N O FElectron affinity 58 - 23 123 0 142 333

3rd Period elementsElement Na Mg Al Si P S ClElectron affinity 53 - 44 120 74 200 348

4.3.6 Electronegativity

You have learnt in the previous section that electron affinity of an element is a measure ofan isolated atom to attract electrons towards it self. We normally do not deal with isolatedatoms. Mostly we come across atoms which are bonded to other atoms. There is anotherproperty which deals with the power of bonded atoms to attract electrons. This property isknown as electronegativity. Electronegativity is relative tendency of a bonded atom toattract the bond-electrons towards itself. Electronegativity is a dimensionless quantityand does not have any units. It just compares the tendency of various elements to attractthe bond-electrons towards themselves. The most widely used scale of electronegativitywas devised by Linus Pauling. Electronegativity is a useful property. You will learn in thenext chapter how it helps to understand the nature of chemical bond formed between twoatoms. Now let us learn about its variation in groups 1 and 17.


Group 1 Group 17Element Electronegativity Element ElectronegativityLi 1.0 F 4.0

Na 0.9 Cl 3.0K 0.8 Br 2.8

Rb 0.8 I- 2.5

What do you observe? Electronegativity decreases in a group from top to bottom.

Now let us see its variation in 2nd and 3rd period elements.

2nd Period ElementsElement Li Be B C N O FElectronegativity 1.0 1.5 2.0 2.5 3.0 3.5 4.0

3rd Period ElementsElement Na Mg Al Si P S Cl

Electronegativity 0.9 1.2 1.5 1.8 2.1 2.5 3.0

Now what do you observe? Electronegativity increases in a period from left to right.

4.3.7 Metallic and non-metallic character

You know what are characteristic properties of a metal? They are its electropositive character(the tendency to lose electrons), metallic luster, ductility, malleability and electricalconductance. Metallic character of an element largely depends upon its ionization energy.Smaller the value of ionization energy, more electropositive and hence more metallic theelement would be.

4.3.7a Variation of metallic character in a group

You know the variation of ionization energy in a group. Can you predict the variation ofmetallic character on its basis? Metallic character of elements increases from top to bottom.This can best be seen in elements of group 14. Its first element, carbon is a typical non-metal, next two elements Si and Ge are metalloids and the remaining elements Sn and Pb,are typical metals as shown below.

Group 14

Element Nature

C Non-metal

Si Metalloid

Ge Metalloid

Sn Metal

Pb Metal

4.3.7b Variation of metallic character in a period

How does metallic character change in a period? Metallic character of elements decreasesin a period from left to right as shown below for 3rd period elementsElement Na Mg Al Si P S Cl

Character Metal Metal Metal Metalloid Non-metal Non-metal Non-metal



Fill in the blanks with appropriate words.

1. The force of attraction between nucleus and valence electrons _______________ in aperiod.

2. Atomic radii of elements _______________ in a period from left to right.3. Radius of cation is _______________ than that of the neutral atom of the same element4. Electronegativity _______________ in a period from left to right and

_______________ in a group from top to bottom.5. Metallic character of elements _______________ from top to bottom in a group.6. Ionization energy of the 1st element in a period is _______________ in the entire

period.

LET US REVISE

! The first classification of elements was s metals and non-metals. It served only limitedpurpose.

! After atomic masses (old term, atomic weight) of elements had been determined, itwas thought to be their most fundamental property and attempts were made to correlateit to their other properties.

! Dobereiner grouped elements into triads. The atomic mass and properties of the middleelement were mean of the other two. He could group only a few elements into triads.For example (i) Li, Na and K (ii) Ca, Sr and Ba (iii) Cl, Br and I.

! Newland tried to see the periodicity of properties and stated his law of octaves that,“When elements are arranged in the increasing order of their atomic weights everyeighth element has properties similar to the first”. He could arrange elements up tocalcium only out of more than sixty elements known then.

• Mendeleev observed correlation between atomic masses and other properties and statedhis periodic law as, “The chemical and physical properties of elements are a periodicfunction of their atomic weights”.

• Mendeleev gave the first periodic table which is named after him which included allthe known elements. It consists of seven horizontal rows called periods and numberedfrom 1 to 7. It has nine vertical columns called groups and numbered from zero toVIII.

• Main achievements of Mendeleev’s periodic table were (i) inclusion of all the knownelements and (ii) prediction of new elements.

• Main defects of Mendeleev’s periodic table were (i) position of isotopes, (ii) anomalouspairs of elements like Ar and K and (iii) grouping of dissimilar elements and separationof similar elements.

• Moseley discovered that atomic number and not atomic mass is the most fundamentalproperty of elements. In the light of this the periodic law was modified to “ The chemicaland physical properties of elements are a periodic function of their atomic numbers”.This is the modern periodic table.

• Modern periodic table is based upon atomic number. Its long form has been acceptedby IUPAC. It has seven periods (1 to 7) and 18 groups (1 to 18). It is free of main


defects of Mendeleev’s periodic table. Elements belonging to same group have samenumber of valence electrons and thus show same valency and similar chemicalproperties.

• Arrangement of elements in the periodic table shows periodicity. Atomic and ionicradii and metallic character increase while ionization energy , electron affinity andelectronegativity decrease in a group from top to bottom.

• Number of valence electrons, ionization energy, electron affinity and electronegativityincrease while metallic character and atomic and ionic radii decrease in a period fromleft to right.

TERMINAL EXERCISES

A. Multiple choice type questions.1 The first attempt to classify elements was made by

(a) Mendeleev(b) Moseley(c) Newland(d) Dobereiner

2. Which group has maximum number of elements in the periodic table?(a) 1(b) 2(c) 3(d) 4

3. The law of octaves applies to(a) B,C,N(b) As, K, Ca(c) Be, Mg, Ca(d) Se, Te, As

4. Representative elements belong to groups(a) 1, 2, 3,4, 5, 6, 7 and 8(b) 1 and 2(c) 1, 2, 13, 14, 15, 16, 17 and 18(d) 1, 2, 13, 14, 15, 16 and 17

5. Which of the following ions is the largest in size?(a) Al3+

(b) Ba2+

(c) Mg2+

(d) Na+

B. Mark the following statements as true or false.1. Ionization energy of an element increases with an increase in atomic number.2. Electron Affinity of fluorine is greater than that of chlorine.3. Out of P3+, S2- and Cl- ions Cl- ion is the smallest one.


4. The first member of lanthanide series of elements is lanthanum.

C. Descriptive type questions.1. Name the group and period of element having the atomic number 21.2. Give an example of Dobereiner triad.

3. State Newland’s law of Octaves.

4. State the Modern Periodic Law.

5. How many groups were present in Mendeleev’s Periodic Table and give their numbers.

6. What are periods and groups in periodic table.

7. List two main achievements of Mendeleev’s periodic table.

8. What are main defects of Mendeleev’s periodic table?

9. How is modern periodic law different from the Mendeleev’s periodic law?

10. Why argon (atomic mass 40) was placed before potassium (atomic mass 39)?

11. In each of the following pairs of ions, which one is bigger in size and why?(i) Li and Ne(ii) O and S(iii) K and K+

(iv) Br and Br-

12. Define atomic radius. How does it vary in a period and in a group?13. What is ionization energy? How does it vary in a group? Give two reasons for it.14. Which element of the following has the highest ionization energy? Na, Ba and Cl15. Explain why does ionization energy increase from left to right in a period but decrease

from top to bottom in a group?16. What do you understand by ‘periodicity’ of properties? Explain taking metallic

character of elements as an example.17. Potassium is more reactive than sodium. Explain with the help of ionization energy.18. An element has atomic mass 32 and its nucleus has 16 neutrons. To which group of

periodic table does it belong? Explain.19. The following is a portion of periodic table. Look at it and answer the following

questions.1 2 3-15 16 17 18H He

C DA

EB(i) Out of A and B which one has lower ionization energy ?

(ii) Which is bigger atom C or D?

(iii) Which is the most electropositive element of all?

(iv) Which is more metallic in nature D or E?


(v) Which is more non-metallic in nature C or D?(vi) Which is the least electronegative element of all?


4.11. Atomic mass of the middle element B must be nearly equal to the average of the other

two elements A and C.Or

Atomic mass of B = Atomic mass of A + Atomic mass of B

2

2. 163. Atomic weight4. 85. Group6. These were the positions of elements which were yet to be discovered.7. Gallium

4.2

1. Atomic number2. Any two of the following:

i. Position of isotopesii. Anomalous pairs of elementsiii. Grouping of dissimilar elementsiv. Separation of similar element.

3. Seven periods and eighteen groups4. Alkaline earths5. Non-metals6. 32

4.3

1. increases2. decreases3. smaller4. increases, decreases5. increases6. minimum

GLOSSARY

Actinides: A group of 14 elements with atomic numbers 90-103 (Th–Lr) which areplaced along with the element actinium (Ac), atomic number 89 in the some position ingroup 3 in the periodic table.

Atomic number: It is the number of protons in the nucleus of the atom of an element.


Atomic radius: It is defined as one-half the distance between the nuclei of two atomswhen they are linked to each other by a single covalent bond.

Dobereiner’s triad: A group of three chemically similar elements in which the atomicmass and properties of the middle element are mean of the other two.

Electron affinity: It is the energy change when an electron is accepted by an atom inan isolated gaseous state. By convention, it is assigned a positive value when energy isreleased during the process.

Electronegativity: It is a measure of the tendency of a bonded atom to attract thebond-electrons towards itself.

Groups: The vertical columns present in periodic table.

Ionic radius: It is the radius of an ion i.e. the distance between the centre of ion and itsoutermost shell.

Ionization energy: It is the minimum amount of energy required to remove an electronfrom an isolated gaseous atom in its ground state to form a gaseous ion.

Lanthanides: A group of 14 elements with atomic numbers 58 to 71 (Ce to Lu) whichare placed along with the element lanthanum (La), atomic number 57 in the some positionin group 3 in the periodic table

Mendeleev’s periodic law: The chemical and physical properties of elements are aperiodic function of their atomic masses.

Modern periodic law : The chemical and physical properties of elements are a periodicfunction of their atomic numbers.

Newland’s law of octaves: When elements are arranged in the increasing order oftheir atomic weights every eighth element has properties similar to the first.

Noble gases: The elements present in group 18 on extreme right side of the periodictable. Their outermost shells contain 8 electrons.

Normal elements: These are the elements present in groups 1 and 2 on left side andgroups 13 to 17 on the right side of the periodic table whose only outermost shells areincomplete.

Periodic properties: These are the properties which repeat themselves after a certaininterval of atomic number.

Periodic table: A tabular arrangement of the elements based on the periodic law.

Periods: The horizontal rows present in the periodic table.

Transition elements: These are the elements present in groups 3 to 12 in the middleof the periodic table whose two outermost shells are incomplete.

Chemical Bonding

INTRODUCTION: In lessons 3 and 4, you have read about the electronic configurations of atoms of various elements and variation in the periodic properties of elements. But every thing present around us is not just the elements. We see substances which can be either elements or compounds You know that the atoms of same or different kinds may combine. When atoms of same elements combine, we get elements. But we get compounds when atoms of different elements combine. Have you ever thought why do atoms combine at all? In this lesson, we will find an answer to this question. We will first explain what a chemical bond is and then discuss various types of chemical bonds which join the atoms together to give various types of substances. The discussion would also highlight how are these bonds form. The properties of substances depend on the nature of bonds present between their atoms. For example, in the lesson you will learn sodium chloride, the common salt and glucose dissolve in water whereas methane gas or naphthalene does not .This is because the type of bonds present between them are different. In addition to the difference in solubility, these two types of compounds differ in other properties about you will study in this lesson. We will also briefly cover the nature of bonding in metals and correlate it to various characteristic properties of metals. Finally, hydrogen bonding which is an important interaction present between molecules would be explained.

OBJECTIVES After completing this lesson, you should be able to :

- give reason for the formation of chemical bonds; - list various types of chemical bonds present in different substances; - describe the formation of an ionic bond with suitable examples; - explain the characteristic properties of ionic compounds; - describe the formation of a covalent bond with suitable examples; - explain the characteristic properties of covalent compounds; - explain bond parameters such as bond length, bond energy and

bond polarity; - distinguish between polar and non–polar molecules; - state the differences between ionic and covalent compounds; - explain the nature of bonding present in metals; - explain hydrogen bonding.

WHY DO ATOMS COMBINE ? The answer to this question is hidden in the electronic configurations of the noble gases. It was found that noble gases namely helium, neon, argon, krypton, xenon and radon did not react with other elements to form compounds, i.e. they were non-reactive. Earlier they were also called as inert gases. It was, thus, thought that these noble gases lacked reactivity because they had electronic arrangements which were quite stable. When we write the electronic configurations of the noble gases (see table 5.1 below), we find that except helium all of them have 8 electrons in their outermost shell.

Electronic Configuration of Noble gases

Name Symbol Atomic number

Electronic configuration

No. of electrons in the outermost shell

Helium He 2 2 2 Neon Ne 10 2,8 8 Argon Ar 18 2,8,8 8

Krypton Kr 36 2,8,18,8 8 Xenon Xe 54 2,8,18,18,8 8 Radon Ra 86 2,8,18,32,18,8 8

Thus, it was concluded that atoms having 8 electrons in their outermost shell are very stable and they did not form compounds. It was also observed that other atoms such as hydrogen, sodium, chlorine etc. which do not have 8 electrons in their outermost shell undergo chemical reactions. They can stabilize by combining with each other and attaining the above configurations of noble gasses, i.e. 8 electrons (or 2 electrons in case of helium) in their outermost shells. Thus, atoms tend to attain a configuration in which they have 8 electrons in their outermost shells. This is called the octet rule. The octet rule explains the chemical bonding in many compounds.

Atoms are held together in compounds by the forces of attraction which are called chemical bonds. The formation of chemical bonds results in the lowering of energy, i.e. as compared to the individual atoms the resulting compound is lower in energy and hence is more stable. Thus stability of the compound formed is an important factor in the formation of chemical bonds. In rest of the lesson; you will study about the nature of bonds present in various substances. We would explain ionic bonding and covalent bonding in detail while briefly touch upon the bonding in metals and hydrogen bonding. Before you start learning about ionic bonding in the next section, you can answer the following questions to check your understanding.

Ionic Bonding

When sodium metal and chlorine gas are brought into contact, they react violently and we obtain sodium chloride. This reaction is shown below.

2 Na (s) + Cl2 (g) ->2 NaCl (s)

The bonding in sodium chloride can be understood as follows:

Sodium (Na) has the atomic number 11 and we can write its electronic configuration as 2,8,1 i.e. it has one electron in its outermost (M) shell. If it loses this electron, it is left with 10 electrons. The resulting species is positively charged ion. Such a positively charged ion is called a cation. The cation in this case is called sodium cation Na+. This is shown below in Fig. 5.1.

Formation of Sodium Cation

Note that the sodium cation has 11 protons but 10 electrons only. It has 8 electrons in the outermost (L) shell. Thus, sodium atom has attained the noble gas configuration (that of Neon as shown in Table 5.1) by losing an electron present in its outermost shell. Thus, according to octet rule, sodium atom can acquire stability by changing to sodium cation.

The ionization of sodium atom to give sodium ion requires an energy of 496 kJ mol-1.

A chlorine atom having the atomic number 17, has the electronic configuration 2,8,7. It can complete its octet by gaining one electron. This is shown below in Fig.5.2.

Formation of chloride ion

Note that in the above process, the chlorine atom has gained an additional electron and hence it has become negatively charged ion. Such a negatively charged ion is

called an anion. This anion is called chloride ion (Cl-). The chloride ion has 8 electrons in its outermost shell and it is a stable electronic configuration according to the octet rule. The formation of chloride ion from the chlorine atom releases 349 kJ mol-1 of energy.

Thus, an ion is a species having positive or negative electrical charge. Both the cation and the anion are known by the general name ion. A cation is formed by the loss of an electron from the sodium atom whereas an anion is formed by the gain of an electron by chlorine atom.

Since the cation (Na+) and the anion (Cl-) formed above are electrically charged species, they are held together by electrostatic force of attraction. This electrostatic force of attraction which holds the cation and anion together is known as electrovalent bond or ionic bond. This is represented as follows:

. . Na + : Cl : -> Na+ + Cl-

..

Note that only outermost electrons are shown above. Such structures are also called Lewis structures.

If we compare the energy required for the formation of sodium ion and that released in the formation of chloride ion, we note that there is a net difference of 147 kJ mol-1 of energy. If only these two steps are involved, then the formation of sodium chloride is not favorable energetically. But sodium chloride exists as a crystalline solid. This is because the energy is released when the sodium ions and the chloride ions come together to form the crystalline structure. The energy so released compensates the above deficiency of energy. The crystal structure of sodium chloride thus obtained is shown below in Fig 5.3.

The crystal structure of sodium chloride

You can see that each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. The force of attraction between the sodium and chloride ions is uniformly felt in all directions. Thus, no particular sodium ion is bonded to a particular chloride ion. Hence, there is no species such as NaCl in the crystal structure shown above.

Similarly, we can explain the formation of cations resulting from lithium and potassium atoms and the formation of anions resulting from fluorine, oxygen and sulphur atoms.

Let us now study the formation of another ionic compound namely magnesium chloride. We will proceed in the same way as we had done for sodium chloride.

We will first consider magnesium (Mg) atom. Its atomic number is 12. Thus, it has 12 protons. The number of electrons present in it is also 12. Hence, the electronic configuration of Mg atom is 2, 8, 2.

Let us consider the formation of magnesium ion from magnesium atom. We see that it has 2 electrons in its outermost shell. If it loses these two electrons, then it can achieve the stable configuration of 2, 8 (that of noble gas neon). This can be represented in Fig. 5.4 as follows:

Formation of magnesium ion

You can see that the resulting magnesium ion has only 10 electrons and hence it has 2+ charge. It is a dipositive ion and can be represented as Mg2+ion. The two electrons lost by the magnesium are gained one each by two chlorine atoms to give two chloride ions.

The formation of chloride ion has already been explained above.

Thus, one magnesium ion and two chloride ion joins together to give magnesium chloride, MgCl2. Hence, we can write

Mg + 2Cl -> Mg2+ 2(Cl)

(2,8,2) 2(2,8,7) or MgCl2 (2,8) 2(2,8,8)

Let us now see what would happen if instead of chloride ion, the magnesium ion combines with another anion say oxide anion. The oxygen atom having atomic number 8 has 8 electrons. Its electronic configuration is 2,6. It can attain a stable electronic arrangement (2,8) of the noble gas neon if it gains two more electrons. The two electrons, which are lost by the magnesium atom, are gained by the oxygen atom. On gaining these two electrons, the oxygen atom gets converted to the oxide anion. This is shown below in Fig 5.5.

Formation of oxide ion

The oxide has 2 more electrons as compared to the oxygen atom. Hence, it has 2 negative charges on it. Therefore, it can be represented as O2- ion.

The magnesium ion (Mg2+) and the oxide ion (O2-) are held together by electrostatic force of attraction. This leads to the formation of magnesium oxide.

Mg2+ + O2- -> Mg2+ + O2- (2,8) (2,8) (2,8) (2,8)

Thus, magnesium oxide is an ionic compound in which a dipositive cation (Mg2+) and a dinegative anion (O2-) are held together by electrostatic force.

Similar to the case of sodium chloride the formation of magnesium oxide is also accompanied with a lowering of energy which leads to the stability of magnesium oxide as compared to individual magnesium and oxygen atoms.

Similarly, the ionic bonding present in many other ionic compounds can be explained. The ionic compounds show many characteristic properties which are discussed below:

Let us consider the formation of magnesium ion from magnesium atom. We see that it has 2 electrons in its outermost shell. If it loses these two electrons, then it can achieve the stable configuration of 2, 8 (that of noble gas neon). This can be represented in Fig. 5.4 as follows:

COVALENT BONDING

In this section, we will study about another kind of bonding called covalent bonding. Covalent bonding is helpful in understanding the formation of molecules. In lesson 2, you studied that molecules having similar atoms such as H2, Cl2, O2, N2 etc. are constituents of elements whereas those containing different atoms like HCl, CO2 etc. are constituents of compounds. Let us now see how are these molecules formed?

We will begin with the formation of hydrogen molecule (H2). The hydrogen atom has one electron. It can attain the electronic configuration of the noble gas helium by sharing one electron of another hydrogen atom. When the two hydrogen atoms come closer, there is an attraction between the electrons of one atom and the proton of another and there are repulsions between the electrons as well as the protons of the two hydrogen atoms. In the beginning, when the two hydrogen atoms approach each other, the potential energy of the system decreases due to the force of attraction. The value of potential energy reaches a minimum at some particular distance between the two atoms. If the distance between the two atoms further decreases, the potential energy increases because of the forces of repulsion. The covalent bond forms when the forces of attraction and repulsion balance each other and the potential energy is minimum. It is this lowering of energy which leads to the formation of the covalent bond.

Potential energy diagram

We will next consider the formation of chlorine molecule (Cl2). A molecule of chlorine contains two atoms of chlorine. Now how are these two chlorine atoms held together in a chlorine molecule?

You know that the electronic configuration of Cl atom is 2,8,7. Each chlorine atom needs one more electron to complete its octet. If the two chlorine atoms share one of their electrons as shown below, then both of them can attain the stable noble gas configuration of argon as shown below.

Note that the shared pair of electrons is shown to be present between the two chlorine atoms. Each chlorine atom thus acquires 8 electrons. The shared pair of electrons keeps the two chlorine atoms bonded together. Such a bond, which is formed by sharing of electrons between the atoms is called a covalent bond. Thus,

we can say that a covalent bond is present between two chlorine atoms. This bond is represented by drawing a line between the two chlorine atoms as follows:

Sometimes the electrons shown above on the chlorine atoms are omitted and the chlorine-chlorine bond is shown as follows:

Cl - Cl

Similarly, we can understand the formation of oxygen molecule (O2) from the oxygen atoms. The oxygen atom has atomic number 8. It has 8 protons and also 8 electrons. The electronic configuration of oxygen atom is 2,6. Now each oxygen atom needs two electrons to complete its octet. The two oxygen atoms share two electrons and complete their octet as is shown below:

The 4 electrons (or 2 pairs of electrons) which are shared between two atoms of oxygen are present between them. Hence, these two pairs of shared electrons can be represented by two bonds between the oxygen atoms. Thus, an oxygen molecule can be represented as follows:

The two oxygen atoms are said to be bonded together by two covalent bonds. Such a bond consisting of two covalent bonds is also known as a double bond.

.. .. : O : = O :

Let us next take the example of nitrogen molecule (N2) and understand how the two nitrogen atoms are bonded together. The atomic number of nitrogen is 7. Thus, it has 7 protons and 7 electrons present in its atom. The electronic configuration can be written as 2,5. To have 8 electrons in the outermost shell, each nitrogen atom requires 3 more electrons. Thus, a sharing of 3 electrons each between the two nitrogen atoms is required. This is shown below:

Each nitrogen atom provides 3 electrons for sharing. Thus, 6 electrons or 3 pairs of electrons are shared between two nitrogen atoms. Hence, each nitrogen atom is able to complete its octet.

Since 6 electrons (or 3 pairs of electrons) are shared between the nitrogen atoms, we say that three covalent bonds are formed between them. These three bonds are represented by drawing three lines between the two nitrogen atoms as shown below:

Such a bond which consists three covalent bonds is known as a triple bond. So far, we were discussing covalent bond formation between atoms of the same elements. But covalent bond can be formed by sharing of electrons between atoms of different elements also. Let us take the example of HCl to understand it.

.. .. : Cl - Cl :

.. ..

Hydrogen atom has one electron in its outermost shell and chlorine atom has seven electrons in its outermost shell. Each of these atoms has one electron less than the electronic configuration of the nearest noble gas. If they share one electron pair, then hydrogen can acquire two electrons in its outer most shell whereas chlorine will have eight electrons in its outermost shell. The formation of HCl molecule by sharing of one electron pair is shown below:

Similarly, we can explain bond formation in other covalent compounds.

After knowing the nature of bonding present in covalent compounds, let us know study what type of properties these covalent compounds have.

Properties of covalent compounds

The covalent compounds consist of molecules which are electrically neutral in nature. The forces of attraction present between the molecules are less strong as compared to the forces present in ionic compounds. Therefore, the properties of the covalent compounds are different from those of the ionic compounds. The characteristic properties of covalent compounds are given below:

a) Physical state

Because of the weak forces of attraction present between discrete molecules, called inter-molecular forces, the covalent compounds exist as a gas or a liquid or a solid. For example, O2, N2, CO2 are gases; water and CCl4 are liquids and iodine is a solid.

b) Melting and boiling points

As the forces of attraction between the molecules are weak in nature, a small amount of energy is sufficient to overcome them. Hence, the melting points and boiling points of covalent compounds are lower than those of ionic compounds. For example, melting point of naphthalene which is a covalent compound is 353K (80o C). Similarly, the boiling point of carbon tetrachloride which is another covalent compound is 350 K (77o C).

c) Electrical conductivity

The covalent compounds contain neutral molecules and do not have charged species such as ions or electrons which can carry charge. Therefore, these compounds do not conduct electricity and are called poor conductors of electricity.

d) Solubility

Covalent compounds are generally not soluble in water but are soluble in organic solvents such as alcohol, chloroform, benzene, ether etc.

After understanding the nature of covalent bond and properties of covalent compounds, let us now study about certain characteristic features associated with a covalent bond.

Bond parameters

A covalent bond has characteristic values associated with it. These are called bond parameters. Some of these parameters are bond length, bond angle, bond energy, bond polarity and bond angle. We will now discuss them one by one.

a) Bond length: It is the distance between the nuclei when they combine to form covalent bond.

The bond lengths for some common bonds are shown below in Table 5.2. You can note that as the number of bonds between the two atoms increases, the bond length between them decreases.

Bond lengths for some common bonds

Bond Bond length (pm) Bond Bond length (pm)

C-C 154 N-N 147 C=C 134 N=N 124 CC 120 N=N 110 C-O 143 O-O 148 C=O 123 O=O 121 C=O 113

b) Bond angle: It is the angle between two bonds of a covalent molecule. For example, in water molecule the bond angle is 104.5o.

c) Bond energy: The stability of a molecule can be related to the strength of the covalent bonds present in it. The stronger the covalent bonds present in a molecule, the more would be its stability. The strength of the covalent bond can be expressed in terms of the energy required to break the bond. For example, 242 kJ of energy is required to break the Cl-Cl bond of Cl2 molecule present in one mole of chlorine gas.

.. .. : Cl - Cl :(g) -> 2: Cl :(g) .. ..

The energy required to break one mole of a bond in isolated molecules of a substance is known as bond energy. The bond energy values for a particular bond can vary slightly from one compound to another. The bond energy values are therefore reported as average bond energies. The average bond energies of some of the bonds are listed in Table 5.3.

You can see from Table 5.3 that the bond energy increases as the number of bonds between two atoms increases. Hence, it indicates that the bonds become stronger and stronger as the number of bonds increases.

d) Bond polarity: When a bond is formed between the atoms of the same element, the resulting molecule is called a homonuclear molecule. In such molecules, the electrons forming the bond are equally shared between the atoms. For example, in H2, Cl2, O2 molecules, the bonded electrons are equally shared between the atoms of these molecules. But when two atoms of different elements form a bond, the resulting molecule is known as a heteronuclear molecule. In these molecules, the shared pair of electrons is pulled more by the more electronegative atom towards itself. For example, in HCl molecule the shared pair of electrons is pulled more by the more electronegative chlorine atom. This leads to partial separation of charges which are represented by d+ and d– as shown below:

Thus, two poles – one negative (Cl atom) and the other positive (H atom), are formed in the HCl molecule. The dipole in HCl molecule can also be represented by H – Cl where the foot of the arrow represents the positive end of the dipole and the arrow head represents the negative end of the dipole. The bonds such as those present between the HCl molecules are called polar bonds. Hence, such molecules are called polar molecules.

Shapes of molecules

The bond lengths and bond angles of various molecules can be determined experimentally. The values so obtained give us an idea about the shapes of molecules. The covalent molecules have definite shapes because the covalent bonds are formed along a particular direction. Thus, we can say that the covalent bond is directional in nature.

Note that this is in contrast to the ionic compounds in which the electrostatic forces of attraction are felt equally strongly in all the directions. Some examples of common covalent molecules and their shapes are given below in Table 5.4.

Name Structure Shape .. .. Oxygen(O2) :O=O: linear Nitrogen(N2) :N=N: linear

Bonding in metals

You know that some of the characteristic properties of metals are malleability, ductility, conduction of heat and electricity, high melting point etc. The high melting point indicates that bonding in metals is strong in nature. These properties of metals can be explained with the help of electron sea model. According to this model, the cations of metal are present in a sea of electrons as shown below in Fig.5.7.

Electron sea model

The electrostatic forces of attraction hold the electrons and the cations together. Since these forces are strong in nature, the melting point of metals is high. The electrons are distributed throughout the metal and they are not confined to any particular metal cation. These electrons are mobile and hence can conduct electricity when the metal is connected to a battery or two electrodes. Similarly, the metal ions can also move and no specific bonds are to be broken in this movement. Since both the electrons and the metal ions can freely move and their environment does not change by this movement, the metals exhibit the malleability and ductility.

So far we have discussed chemical bonds resulting from strong forces of attraction, but weaker forces of attraction also play an important role towards the properties of many substances. One such type of interaction present between the molecules is hydrogen bonding. Let us now study about it in detail.

Hydrogen Bonding When hydrogen is bonded to an electronegative atom such as oxygen, nitrogen or fluorine, a special or unique type of attraction is present among the molecules of such compounds. The hydrogen of one molecule is attracted by the electronegative atom of the adjacent molecule. Such type of bonding is shown by dotted lines for hydrogen fluoride and water in Fig.5.8. The strength of hydrogen bonding varies from about 4 kJ mol-1 to 25 kJ mol-1 in various substances. This energy is much less than that required breaking one mole of an ionic or a covalent substance as you can see from Table 5.3.

H_____F-----H_____F-----H_____F

The existence of water in liquid state is because of hydrogen bonding. Hydrogen bonding is also responsible for the low density of ice as compared to water. In ice, hydrogen bonding gives an ordered arrangement of water molecules which has a lot of free space in between them. Since ice is lighter than water, it floats on water and provides an insulating layer over water which is very important for the survival of aquatic life.

Hydrogen bonding also explains the miscibility of alcohol in water in all proportions. Glucose which contains six-OH groups makes hydrogen bonds with water molecules and hence is very soluble in water. More than 80 g of glucose dissolves in 100 mL of water.

In proteins, hydrogen bonding is responsible for their helical structure

In Text Questions

1. State octet rule

2. Why are noble gases non-reactive?

3. Define the term ion. Name the two types of ions?

4. How many shells are present in Na+ ion?

5. What is the number of electrons present in Cl- ion?

6. Name the type of force of attraction present in ionic compounds.

7. In sodium chloride lattice, how many Cl- ions surround each Na+ ion?

8. How many electron pairs are shared between (i) Cl2 (ii) O2 and (iii) HCl molecules?

9. Which of the following statements are true for covalent molecules?

i ) They are poor conductors of electricity.

ii ) Their boiling points are high.

iii ) They have definite shape

1. Define the terms: cation and anion.

2. Classify the following as cations or anions: Na+, O2-, Cl-, Ca2+, N3-, K+, Mg2+, -OH

3. Explain the formation of Li+ ion from Li atom.

4. How would you explain the bonding in MgCl2?

5. Which of the following statements are correct for ionic compounds: i) They are insoluble in water. ii) They are neutral in nature. iii) They have high melting points.

6. State three characteristic properties of ionic compounds.

7. How does a covalent bond form?

8. What is the number of bonds present in the following molecules? i) Cl 2 ii) N2 iii) O2

9. Classify the following statements as true or false: i) Ionic compounds contain ions which are held together by weak electrostatic forces. ii) Ionic compounds have high melting and boiling points. iii) Covalent compounds are good conductors of electricity. iv) Sodium chloride is a good conductor of electricity.

10. Classify the following compounds as ionic or covalent: i) sodium chloride ii) calcium chloride iii) oxygen iv) hydrogen chloride v) magnesium oxide vi) nitrogen

11. Classify the following molecules as polar or non-polar: i) H2 (ii) HCl (iii) O2 (iv) H2O

12. Why is hydrogen bonding important? Give two examples.

13. Name the type of bonds present in H2O molecule.

14. Explain electron sea model of bonding in metals

What you have learnt

- Atoms combine to attain a stable arrangement of eight electrons in their outer most shell.

- Ions are held together by strong electrostatic forces. Hence, ionic compounds have high melting and boiling points.

- Ionic compounds are good conductors of electricity . They are soluble in water but insoluble in inorganic solvents.

- Covalent bonds are formed by sharing of electrons between atoms.

- Covalent compounds have low melting and boiling points. They are poor conductors of electricity.

- The covalent compounds are generally insoluble in water but are soluble in organic solvents.

- Covalent bonds are directional in nature and hence covalent compounds have definite shapes.

- Bond length decreases with the number of bonds whereas the bond energy increases with the number of bonds.

- Electrons are mobile in metals and hence the metals and hence the metals are good conductors of electricity.

- Hydrogen bonding is an important interaction and is responsible for variety of properties in various molecules. In water, it is responsible for its liquid nature whereas, it is responsible for its liquid nature whereas in proteins it is responsible for their shape and in glucose it is responsible for its solubility in water.

6

Chemical Arithmetic and ReactionsTotal number of reactions we study in chemistry is very large. They are of numerous types.In lesson 2, you have learnt how to write and balance chemical equations. In this lessonyou will learn, how chemical equations can be classified into various categories on thebasis of some of their features. You will also learn about the information that can be obtainedfrom a balanced chemical equation and how we can use this information for makingcalculations. You have learnt about acids, bases and salts in earlier classes. In this lessonyou will learn more about them.

OBJECTIVES

After completing this lesson, you will be able to:• list various types of reactions;• distinguish between various types of reactions;• classify the reactions according to their rates and energy changes;• work out simple problems based on stoichiometry;• define acids, bases and salts and give their examples;• explain the acidbase equilibrium in aqueous systems;

• define pH and solve simple problems based on pH.

6.1 TYPES OF CHEMICAL REACTIONS

Chemical reactions can be classified on the basis of some of their features. One classificationis based on the nature of chemical change that occurs in the reaction. On this basis reactionscan be classified into five types. These are:

(i) Combination reactions(ii) Decomposition reactions(iii) Displacement reactions(iv) Doubledisplacement reactions(v) Oxidationreduction reactionsLet us now learn about these reactions.

6.1.1 Combination reactions

A reaction in which two or more substances react to form a new substance is called acombination reaction.

: 98 : Chemical Arithmetic and Reactions

A special category of combination reactions is the one in which a compound is formedby combination of its constituent elements. Such a reaction is known as synthesis reaction.

Following are some examples of combination reactions:

1. Carbon (charcoal, coke) burns in presence of oxygen (or air) to form carbon dioxide(synthesis reaction).

C(s) + O2(g) CO

2(g)

carbon oxygen carbon dioxide

2. Hydrogen burns in presence of oxygen (or air) to form water (synthesis reaction).2H

2(g) + O

2(g) 2H

2O(l)

hydrogen oxygen water

3. Phosphorus combines with chlorine to form phosphorus pentachloride (synthesisreaction).

P4(s) + 10Cl

2(g) 4PCl

5 (s)

phosphorus chlorine phosphorus pentachloride

4. Ammonia combines with hydrogen chloride to form ammonium chloride.NH

3 (g) + HCl(g) NH

4Cl

(s)

ammonia hydrogen chloride ammonium chloride

6.1.2 Decomposition reactions

A reaction in which one substance breaks down into two or more simpler substances isknown as decomposition reaction.

A decomposition reaction always involves breaking of one or more chemical bondsand therefore occurs only when the required amount of energy is supplied. The energy maybe supplied in any of the following forms:

(i) Heat: Such decomposition reactions are called thermal decompositionreactions.

(ii) Electricity: Such decomposition reactions are called electro-decompositionreactions and the process is known as electrolysis.

(iii) Light: Such decomposition reactions are called photo-decomposition reactionsand the process is known as photolysis.

Following are some examples of decomposition reactions:

1. Potassium chlorate decomposes on heating into potassium chloride and oxygen.2KClO

3 (s) 2KCl(s) + 3O

2(g)

potassium chlorate potassium chloride oxygen

2. When calcium carbonate (limestone) is heated strongly it decomposes into calciumoxide (quicklime) and carbon dioxide.

CaCO3(s) CaO(s) + CO

2(g)

calcium carbonate calcium oxide carbon dioxide

3. Hydrogen peroxide decomposes into water and oxygen on heating.2H

2O

2(l) 2H

2O(l) + O

2(g)

hydrogen peroxide water oxygen

4. Water decomposes into hydrogen and oxygen on passing electricity through it(electrolysis).


2H 2O(l) 2H

2(g) + O

2(g)

water hydrogen oxygen

5. Lead nitrate decomposes on heating into lead monoxide, nitrogen dioxide and oxygen.2Pb(NO

3)

2(s) 2PbO(s) + 4NO

2(g) + O

2(g)

lead nitrate lead monoxide nitrogen dioxide oxygen

6.1.3 Displacement reactions

A reaction in which one element present in a compound is displaced by another element isknown as displacement reaction.

Following are examples of displacement reactions:

1. Displacement of a metal by a more reactive metal.a. Zinc displaces copper from a solution of copper sulphate.

Zn (s) + CuSO

4(aq) ZnSO

4(aq) + Cu(s)

zinc copper sulphate zinc sulphate copper

b. Magnesium displaces copper from a solution of copper sulphate.

Mg(s) + CuSO4(aq) MgSO

4(aq) + Cu

(s)

magnesium copper sulphate magnesium sulphate copper

2. Displacement of hydrogen from solutions of acids by more reactive metals.a. Zinc displaces hydrogen from dilute sulphuric acid.

Zn(s) + H2SO

4(aq) ZnSO

4(aq) + H

2(g)

zinc dil. sulphuric acid zinc sulphate hydrogen

b. Magnesium displaces hydrogen from dilute hydrochloric acidMg(s) + 2HCl(aq) MgCl

2(aq) + H

2(g)

magnesium dil hydrochloric acid magnesium chloride hydrogen

2. Displacement of a halogen by a more reactive halogen.Chlorine displaces bromine from a solution of potassium bromide.

Cl 2(g) + 2KBr(aq) 2KCl(aq) + Br

2(aq)

chlorine potassium bromide potassium chloride bromine

6.1.4 Double-displacement reactions

A reaction in which two ionic compounds exchange their ions is known as doubledisplacement reaction. The following are the examples of double displacement reactions:

a. Reaction between sodium chloride and silver nitrate.NaCl(aq) + AgNO

3(aq) AgCl(s) + NaNO

3(aq)

sodium chloride silver nitrate silver chloride sodium nitrate

b. Neutralization of hydrochloric acid by sodium hydroxide.HCl(aq) + NaOH(aq) NaCl(aq) + H

2O(l)

hydrochloric acid sodium hydroxide sodium chloride water

6.1.5 Oxidation–reduction or redox reactions

These are the reaction in which oxidation and reduction processes occur. Let us first learnwhat these processes are.

a) Oxidation: It is a process which involves loss of electrons. Earlier it was defined asa process involving addition of oxygen or loss of hydrogen.


b) Reduction: Reduction is a process which involves gain of electrons. Earlier it wasdefined as a process involving removal of oxygen or addition of hydrogen.

c) Redox reactions: From the above definitions, you must have noticed above thatoxidation and reduction processes are just opposite to each other. None of theseprocesses can occur alone. During a reaction if one substance gets oxidized the othergets reduced. Thus, both the processes occur simultaneously. That is why the reactionsin which oxidation and reduction processes occur are called redox reactions oroxidationreduction reactions. Now let us understand these processes with the help ofsome examples.(i) Consider burning of coke (carbon) in presence of oxygen:

C(s) + O2(g) CO

2(g)

carbon oxygen carbon dioxide

In this reaction carbon is getting oxidized as oxygen is added to it and oxygen isreduced.

(ii) When hydrogen sulphide reacts with sulphur dioxide the products are sulphur andwater.

2H2S(g) + SO

2(g) 3S(s) + 2H

2O(l)

hydrogen sulphide sulphur dioxide sulphur water

Here, hydrogen sulphide is oxidized to sulphur due to loss of hydrogen while sulphurdioxide is reduced to sulphur due to loss of oxygen.

(iii)When copper (II) oxide is treated with hydrogen, copper and water are produced.CuO(s) + H

2(g) Cu(s) + H

2O(l)

cupric oxide hydrogen copper water

Here cupric oxide is reduced to copper due to loss of oxygen while hydrogen isoxidized to water due to addition of oxygen.

(iv)When sodium metal reacts with chlorine it forms sodium chloride.2Na(s) + Cl

2(g) 2NaCl(s)

sodium chlorine sodium chloride

Sodium chloride is an ionic compound. Sodium is present in it as sodium ion (Na+)and chlorine as chloride ion (Cl). This reaction can be considered to occur in thefollowing steps:

• Each sodium atom loses one electron and forms sodium ion. Since two sodiumatoms are involved in the reaction, the process is:

2Na 2Na+ + 2e–

sodium sodium ionThus, sodium is oxidized due to loss of electron.

• Each chlorine atom gains one electron and forms chloride ion. Since one chlorinemolecule has two atoms of chlorine the process is:

Cl2

+ 2e– 2Cl–

chlorine chloride ionThus, chlorine is reduced due to gain of electrons.

(v) When zinc is added to an aqueous solution of copper sulphate, it displaces copper.Zn(s) + CuSO

4(aq) ZnSO

4(aq) + Cu(s)

zinc copper sulphate zinc sulphate copper


Here zinc is oxidized to zinc ions and copper ions are reduced to copper. Thisreaction is displacement reaction as well as a redox reaction.

(d) Oxidizing and reducing agents : Consider the reaction between zinc and coppersulphate:

Zn(s) + CuSO4(aq) ZnSO

4(aq) + Cu(s)

In this reaction zinc reduces cupric ions to copper. Such a substance which reducesanother substance is called a reducing agent. Here, zinc is the reducing agent.Also, in this reaction cupric ions oxidize zinc to zinc ions. Such a substance whichoxidizes another substance is called an oxidizing agent. Here, cupric ions are theoxidizing agent.


Match the type of reaction given in column I with the reactions given in column II.

I II

1. Displacement A. 2H2S(g) + SO

2(g) 3S(g) + 2H

2O(1)

reaction

2. Double B. NH3 + HCl NH

4Cl

displacementreaction

3. Combination C. 3CaCl2 + 2K

3PO

4 Ca

3(PO

4)

2 + 6KCl

reaction

4. Redox reaction D. Mg(s) + CuSO4(aq) MgSO

4(aq) + Cu(s)

5. Decomposition E. 2H2O

2 2H

2O + O

2

reaction

6.2 NATURE OF CHEMICAL REACTIONS

In the last section, we have learnt how chemical reactions have been classified into varioustypes on the basis of the nature of chemical change that occurs in them. In this section weshall learn about some other features of chemical reactions. These features have beendiscussed below.

6.2.1 Homogeneous–heterogeneous reactions

Chemical reactions can be classified on the basis of physical states of reactants and productsas homogeneous and heterogeneous reactions.

a) Homogeneous reactions

The reactions in which all the reactants and products are present in the same phase arecalled homogeneous reactions. Such reactions can occur in gas phase or solution phaseonly.

A. Gas phase homogeneous reactions

These are the reactions in which all reactants and products are gases.

(i) H2(g) + Cl

2(g) 2HCl(g)

hydrogen chlorine hydrogen chloride(ii) 2SO

2(g) + O

2(g) 2SO

3(g)

sulphur dioxide oxygen sulphur trioxide

H+


(iii) N2(g) + 3H

2(g) 2NH

3(g)

nitrogen hydrogen ammonia

B. Solution phase homogeneous reactions

These are the reactions in which all reactants and products are present in a solution.

(i) HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

hydrochloric acid sodium hydroxide sodium chloride water

(ii) CH3COOC

2H

5(l)+ H

2O(l) CH

3COOH(l) + C

2H

5OH(l)

b) Heterogeneous reactions

The reactions in which reactants and products are present in more than one phase arecalled heterogeneous reactions. Such reactions involve at least one solid substance alongwith one or more substances in solid, solution or gaseous phase. The following are theexamples of heterogeneous reactions.

(i) CaCO3(s) CaO(s) + CO

2(g)

calcium carbonate calcium oxide carbon dioxide

(ii) 2Mg(s) + O2(g) 2MgO(s)

magnesium oxygen magnesium oxide

(iii) BaCl2(aq) + Na

2SO

4(aq) BaSO

4(s) + 2NaCl(aq)

barium chloride sodium sulphate barium sulphate sodium chloride

6.2.2 Slow and fast reactions

Different reactions occur at different rates. Rusting of iron is a slow process and requiresfew days time. On the other hand burning of cooking gas is a fast reaction. On the basis oftheir rates chemical reactions can be classified as slow and fast reactions. Rusting of iron,curdling of milk, hydrolysis of esters at room temperature (e.g. reaction between ethylacetate and water), fading of colours of clothes, burning of coal, etc. are some examples ofslow reactions. On the other hand, neutralization reaction (e.g. reaction between hydrochloricacid and sodium hydroxide), explosion reactions (e.g. in a fire cracker bomb), action ofacids or bases on litmus, and burning of cooking gas are some examples of fast reactions.

A large number of reactions are neither slow nor fast. They may be termed as moderatereactions. Burning of candle, thermal decomposition of potassium chlorate, and reactionof zinc with dilute sulphuric acid are some examples of moderate reactions.

6.2.3 Exothermic and endothermic reactions

All chemical reactions are accompanied by some energy changes. Energy is either evolvedor absorbed during the reaction usually in the form of heat. Depending upon this, thereactions are classified as exothermic and endothermic reactions.

a) Exothermic reactions

The reactions in which heat is liberated or evolved are called exothermic reactions. Insuch reactions heat is shown as one of the products. If exact amount of heat evolved isknown then this amount is written otherwise simply the word heat is written. Followingare the examples of exothermic reactions.

(i) 2H2(g) + O

2(g) 2H

2O(l) + heat

or 2H2(g) + O

2(g) 2H

2O(l) + 571.5 kJ


(ii) C(s) + O2(g) CO

2(g) + 393.5 kJ

(iii) HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) + 57.3 kJ

b) Endothermic reactions

The reactions in which heat is absorbed are called endothermic reactions. In such reactionsheat is shown as one of the reactants. If exact amount of heat absorbed is known then thisamount is written otherwise simply the word heat is written. Following are the examplesof endothermic reactions

(i) N2(g) + 2O

2(g) + heat 2NO(g)

nitric oxide

or N2(g) + O

2(g) + 180.7 kJ 2NO(g)

(ii) 2KClO3(s) + heat 2KCl(s) + 3O

2(g)

potassium chlorate potassium chloride

(iii) 2Pb(NO3)

2(s) + heat 2PbO(s) + 4NO

2(g) + O

2(g)

lead nitrate nitrogen dioxide

6.2.4 Reversible and irreversible reactions

Chemical reactions can also be classified on the basis whether they can occur only in theforward direction or in forward as well as backward directions.

a) Irreversible reactions

Most of the reactions would occur till the reactants (or atleast one reactant) have beencompletely converted into products. For example, if a small piece of zinc metal is put in atest tube containing excess of dilute hydrochloric acid, it completely reacts with it.

Zn(s) + 2HCl(aq) H2(g) + ZnCl

2(aq)

Such reactions occur in forward direction only. The reactions which occur in forwarddirection only are called irreversible reactions.

The following are some more examples of irreversible reactions:

(i) 2Mg(s) + O2(g) 2MgO(s)

magnesium magnesium oxide

(ii) 2HgO(s) 2Hg(l) + O2(g)

mercuric oxide mercury

(iii) NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO

3(aq)

b) Reversible reactionsOn the other hand consider the reaction:

H2(g) + I

2(g) 2HI(g)

In this reaction hydrogen and iodine are not completely converted into hydrogen iodide.The reason for this is that the moment some HI is formed it starts decomposing back intoH

2 and I

2.

2HI(g) H2(g) + I

2(g)

The reactions that can occur in forward and reverse directions, simultaneously undersame set of conditions are called reversible reactions. Reversible nature of a reaction isindicated by writing two arrows (or twohalf arrows) in opposite directions between reactantsand products as shown below;


H2(g) + I

2(g) 2HI(g)

or H2(g) + I

2(g) 2HI(g)

Some more examples of reversible reactions are :(i) Synthesis of ammonia

N2(g) + 3H

2(g) 2NH

3(g)

(ii) Oxidation of sulphur dioxide to sulphur trioxide

2SO2(g) + O

2(g) 2SO

3(g)

6.2.5 Equilibrium in reversible reactions

In the last section we have learned that a reversible reaction can occur in forward as well asreverse directions simultaneously. Consider the following reaction:

2SO2(g) + O

2(g) 2SO

3(g)

When the reaction is started by taking a mixture of sulphur dioxide (SO2)

and oxygen

(O2) it would initially occur only in the forward direction and formation of sulphur trioxide

(SO3) would begin. Initially the rate of this reaction is fast. As it progresses its rate decreases.

This happens because as reactants are consumed their concentrations decrease.

ConcentrationConcentration is a measure of the amount of a substance contained per unit volume.In chemistry it is commonly measured in terms of molarity. Molarity is the numberof moles of a substance present in one litre volume. It has the unit of mol L1. Incase of gases it is their number of moles present in one litre volume. And in case ofsolutions it is the number of moles of solute present in one litre volume of solution.The molar concentration of a substance X is denoted by writing its formula/symbolwithin a square bracket [X].

As soon as SO3 is formed, it starts

decomposing and the backward reactionalso starts. Initially its rate is very slowbut as the reaction progresses theconcentration of SO

3 (which is reactant

for the reverse reaction) increases andthe rate of reverse reaction also increases.

Thus, with the progress of reaction,the rate of forward reaction decreases andthat of the reverse reaction increases withtime. These changes are depicted in thefigure 6.1.

After some time, the rate of theforward reaction becomes equal to the rate of the reverse reaction and the reaction reachesequilibrium state (Fig. 6.1). Under these conditions, there is no change in concentration ofany reactant or product. A system is said to be in a state of equilibrium if none of itsproperties change with time. In other words, when a system is in a state of equilibrium, allits properties remain constant.

Fig. 6.1 Changes in rates of forward and backward(reverse) reactions in a reversible reaction. When the two

become equal, the reaction attains equilibrium.


At equilibrium the concentrations of reactants [SO2] and [O

2] and product [SO

3] are

related by the following expression known as the law of equilibrium:

[SO3]2

Kc

= _______________

[SO2][O

2]

How to write the expression of the law of equilibrium ?To understand how to write the expression of law of equilibrium for any reaction let ustake a general reaction:

aA + bB cC + dD

For this reaction, the law of equilibrium is given by the following expression :[C]c[D]d

Kc

= _______________

[A]a[B]b

In this expression, Kc is the equilibrium constant for the reaction.

The numerator is obtained by multiplying the concentration terms for all productsafter each term has been raised to the power which is equal to the stoichiometric coefficientof that product. Here C and D are the two products and c and d are their respectivestoichiometric coefficients. Therefore, numerator would be the obtained by multiplying[C]c and [D]d terms. Also, conventionally if any pure solid or liquid is taking part in theequilibrium, its concentration is taken as 1.

Similarly, the denominator is obtained by multiplying the concentration terms of allreactants after each term has been raised to the power which is equal to the stoichiometriccoefficient of that reactant.

Static and dynamic equilibrium

The type of equilibrium attained by reversible reactions is called dynamicequilibrium. Such an equilibrium state is attained as a result of two equal butopposite changes occurring simultaneously so that no net change occurs in thesystem. Therefore, all the properties of the system acquire constant values. Youcan encounter a similar situation when a person is walking on a treadmill. Hisspeed of walking is exactly matched by the speed of the treadmill which moves inthe backward direction. The net result is that position of the person does not changeand he stays there only. Another similar situation in encountered when a personusing an escalator for climbing starts moving down on it and matches his speedwith that of the escalator.

Another type of equilibrium is attained when a system is acted upon by a set offorces that cancel out each other. Such an equilibrium state is attained when nochange occurs in it. This type of equilibrium is called static equilibrium. A booklying on a table is in state of static equilibrium because the downward actinggravitational force is balanced and cancelled by the upward acting force of reactionfrom the table (Newton’s third law of motion). Another similar situation isencountered in the game tug of war when the efforts of the two opponent teams(forces by which they pull the rope) exactly match and they remain where they are.


Kc of a reaction is its characteristic property at a given temperature and it characterizes

the equilibrium state. Its value changes only when temperature is changed. The sameequilibrium state (characterized by the value of K

c)

is reached finally whether the reaction

is started from the reactant side or from the product side or all reactants and products aremixed in arbitrary amounts.


Select the correct choice about the nature of each reaction out of the two options mentionedagainst it.

1. Burning of petrol in a car (homogeneous / heterogeneous).

2. CaCO3(s) CaO(s) + CO

2(g)

(exothermic / endothermic)

3. 2HgO(s) 2Hg(l) + O

2(g) (reversible / irreversible)

4. Bursting of crackers (slow / fast)5. A reversible reaction at a stage when concentration of reactants and products is changing

(equilibrium state /non-equilibrium state)

6.3 CHEMICAL CALCULATIONS AND STOICHIOMETRY

In lesson 2 you have learnt how to write and balance chemical equations. Stoichiometrydeals with the proportions in which elements or compounds react with one another. In thissection we shall learn how to use the stoichiometric information in a balanced chemicalequation for making some calculations.

6.3.1 Significance of balanced chemical equation

Balanced chemical equation carries the following information:

a) Qualitative information carried by a balanced chemical equation

• Reactants taking part in the reaction

• Products formed in the reaction

• Physical states of different reactants and products (if given)

b) Quantitative information carried by a balanced chemical equation

• Number of molecules of different reactants and products taking part in the reaction

• Number of moles of different reactants and products taking part in the reaction

• Masses of different reactants and products taking part in the reaction

• Relationship between moles of different reactants and products taking part in thereaction

• Relationship between masses of different reactants and products taking part in thereaction

• Relationship between volumes of different gaseous reactants and products takingpart in the reaction


Let us understand how to get this information from a chemical equation with the helpof an example.

Information carried by a chemical equation2Na(s) + 2H

2O(l) 2NaOH(aq) + H

2(g)

Names sodium water sodium hydroxide hydrogenPhysical states solid liquid aqueous solution gasMoles 2 moles 2 moles 2 moles 1mole(Molar masses) ( Na= 23 ) (H

2O = 2 +16 = 18) (NaOH = 23 + 16 + 1 = 40) (H

2 = 2)

Masses 2 x 23 = 46g 2 x 18 = 36g 2 x 40 = 80g1 x 2 = 2g

Volume* of 1 x 22.7L=22.7Lgaseous substance

From the information listed above we can conclude that:

(i) Sodium metal (solid) reacts with water (liquid) and produces sodium hydroxide(aqueous solution) and hydrogen (gas).

(ii) 2 moles of sodium react with 2 moles of water and produce 2 moles of sodium hydroxideand 1 mole of hydrogen. Thus the ratio of number of moles of these substances is2:2:2:1.

(iii) 46 g sodium reacts with 36 g water and produces 80 g of NaOH and 2 g of hydrogen.(iv) 2 moles or 46 g sodium produces 22.7 L of hydrogen gas when it reacts with water.(v) 2 moles or 36 g water produces 22.7 L of hydrogen gas when it reacts with sodium.

c) Limitations or information not carried by a chemical equation

• Conditions under which the reaction takes place• Rate of the reaction whether it is fast, slow or moderate• The extent up to which the reaction takes place before equilibrium state is reached

in case of a reversible reaction

6.3.2 Calculations based on chemical equations

The information that can be obtained from a chemical equation can be used to make severaltypes of calculations. Let us carry out few such calculations.

a) Mole-mole relationship


2KClO3(s) 2KCl(s) + 3O

2(g)

calculate the following:(i) How many moles of oxygen will be produced if 10 moles of KClO

3 are decomposed?

(ii) How many moles of KCl would be produced with 0.6 moles of O2?

Solution: The given reaction is2KClO

3(s) 2KCl(s) + 3O

2(g)

2 moles 2 moles 3 moles

*Volume of a gaseous substance can be calculated by making use of the fact that one mole of a gasoccupies a volume of 22.7 L at STP (standard temperature and pressure) i.e. at 273 K temperature and 1 barpressure.


(i) 2 moles of KClO3 produce 3 moles of oxygen.

Therefore, 10 moles of KClO3 would

produce

3 × 10= _________ = 15 moles of oxygen.

2(ii) With 3 moles of oxygen the number of moles of KCl produced = 2 moles

With 0.6 moles of oxygen the number of moles of KCl produced2 × 0.6

= ___________ = 0.4 moles 3

b) Mass-mass relationship

Example 6.2: For the reaction

N2(g) + 3H

2(g) 2NH

3(g)

Calculate the masses of nitrogen and hydrogen required to produce 680 g of ammonia?

Solution: The given reaction is:

N2(g) + 3H

2(g) 2NH

3(g)

1mole 3 moles 2 moles1 x 28 3 x 2 2 x (14+3)28 g 6 g 34 g

Thus to produce 34 g ammonia the mass of nitrogen required = 28 g

Therefore to produce 680 g ammonia the mass of nitrogen required28 x 680

= _____________ = 560 g 34

Similarly, to produce 34 g ammonia the mass of hydrogen required = 6 g

Therefore to produce 680 g ammonia the mass of hydrogen required6 x 680

= _____________ = 120 g 34

c) Volume-volume relationship

Example 6.3 : The following reaction is used industrially for manufacture of sulphuricacid.

2SO2(g) + O

2(g) 2SO

3(g)

How much volume of oxygen at STP (Standard Temperature and Pressure) would berequired for producing 100 L of SO

3 (at STP)?

Solution: In the reaction

2SO2(g) + O

2(g) 2SO

3(g)

2 mole 1 mole 2 mole2 x 22.7 L 22.7 L 2 x 22.7 L2 volumes 1 volume 2 volumes

To produce 2 volumes or 2 L of SO3 the oxygen required is 1 volume or 1 L.


To produce 1L of SO3 the oxygen required is 0.5 L

Therefore to produce 100 L of SO3 the volume of oxygen required is 0.5 x 100 = 50 L

d) Mixed calculationsExample 6.4: Calculate the mass of hydrochloric acid required for neutralizing 1 kg ofNaOH

Solution: The neutralization reaction involved between hydrochloric acid and sodiumhydroxide is as follows :

HCl(aq) + NaOH(aq) NaCl(aq) + H2O

(l)

1mole 1 mole1 + 35.5 23 + 16 + 1= 36.5 g = 40 g

Thus, for neutralizing 40 g of NaOH the mass of HCl required is 36.5 g.

For neutralizing 1 kg or 1000 g of NaOH the mass of HCl required is 36.5 x 1000_________________ = 912.5 g 40


2Na(s) + 2H2O(l) 2NaOH(aq) + H

2(g)

calculate the following:

(i) The maximum number of moles of sodium that can react with 4 moles of water.(ii) The mass of sodium hydroxide that would be produced when 4.6 g of sodium reacts

with excess of water.(iii) The mass and volume at STP of hydrogen gas that would be produced when 1.8 g of

water reacts completely with sodium metal.

Solution:

2Na(s) + 2H2O(l) 2NaOH(aq) + H

2(g)

2 moles 2 moles 2 moles 1 mole2 x 23 = 46 g 2 x 18 = 36 g 2 x 40 = 80 g 1 x 2

= 2 g 22.7 L at STP

(i) From the equation it can be seen that2 moles of water react with 2 moles of sodium4 moles of water can react with a maximum of 4 moles of sodium.

(ii) 46 g sodium reacts to produce 80 g sodium hydroxide 80 x 4.6

4.6 g sodium would produce _______________ = 8.0 g sodium hydroxide.46

(iii) 6 g of water produces 2 g or 22.7 L of hydrogen at STP 2 x 1.8 22.7 x 1.8

1.8 g of water would produce ___________ = 0.1 g of hydrogen and ______________ = 1.135 L 36 36

of hydrogen at STP.



Consider the equation for combustion of benzene (C6H

6):

2C6H

6(l) + 15O

2(g) 12CO

2(g) + 6H

2O(g) + heat

Some statements about this reaction are given below. Read them carefully and indicateagainst each statement whether it is true (T) or false (F).

1. It is an exothermic reaction.2. 0.1 mole of benzene would require 7.5 moles of oxygen for its combustion.3. 1 mole of benzene would produce 134.4 L of CO

2 at STP.

4. 10.8 g water would be produced by combustion of 15.6 g benzene.

5. 200 g of O2 is sufficient to convert 1 mole of benzene completely into CO

2 and H

2O.

6.4 ACIDS, BASES AND SALTS

You have learnt in your earlier classes about three types of substances–acids, bases andsalts. They are vital to many life processes and are valuable to industry. Let us do a quickrevision about them.

6.4.1 Acids

An acid is defined as a substance that furnishes hydrogen ions (H+) in its solution. Actually,the hydrogen ion, H+ does not exist in the aqueous solution as such. Instead, it attachesitself to a water molecule to form the hydronium ion (H

3O+). It is customary, however, to

simplify equations by using the symbol for the hydrogen ion (H+).

The strongest acids are the mineral or inorganic acids. These include sulphuric acid,nitric acid, and hydrochloric acid. More important to life are hundreds of weaker organicacids. These include acetic acid (in vinegar), citric acid (in lemons), lactic acid (in sourmilk), and the amino acids (in proteins).

Acids have sour taste and turn blue litmus red. They react with metals (which are morereactive than hydrogen) to liberate hydrogen.

Zn(s) + H2SO

4(aq) ZnSO

4(aq) + H

2(g)

6.4.2 Bases

Bases are the substances which furnish hydroxyl ions OH– in their solutions. The hydroxides ofmetals are the compounds that have the hydroxyl group. They are called bases.Hydroxides of alkali metals–lithium, sodium, potassium, rubidium, and caesium have thespecial name of alkalies. A basic solution is also called an alkaline solution. Bases have bittertaste and turn red litmus blue.

Taste of acids and basesAlthough you will find mention of taste of acids being sour and that of bases beingbitter in books, never attempt to taste them yourself. Many of them can causeserious damage if swallowed or even on their contact with tongue.

6.4.3 Salts

A salt is a substance produced by the reaction of an acid with a base. It consists of thecation (positive ion) of a base and the anion (negative ion) of an acid. The reaction betweenan acid and a base is called a neutralization reaction. In solution or in the molten state,


most salts are completely dissociated into cation and anion and are good conductors ofelectricity.

2NaOH(s) + H2SO

4(l) Na

2SO

4(aq) + 2H

2O(l)

sodium hydroxide sulphuric acid sodium sulphateAnother typical acidbase reaction is between calcium hydroxide and phosphoric acid

to produce calcium phosphate and water:

3Ca(OH)2(s) + 2H

3PO

4(l) Ca

3(PO

4)

2(aq) + 6H

2O(l)

calcium hydroxide phosphoric acid calcium phosphate


A substance AB is formed by reaction between an acid X and a base Y along with water.The cation and anion of the compound AB are monovalent.

1. What type of substance is AB?2. Which one out of AB, X and Y would turn red litmus blue?

3. Which one out of AB, X and Y would have sour taste?

6.5 ACID-BASE EQUILIBRIA IN AQUEOUS SYSTEMS

In the last section we discussed the nature of three important types of substances–acids,bases and salts. They show their typical properties in aqueous solutions. In this section weshall learn about their behaviour in such solutions.

6.5.1 Electrolytes and non-electrolytes

An electrolyte is a substance that conducts electric current through it in the molten stateor through its solution. The most familiar electrolytes are acids, bases, and salts, whichdissociate in their molten state when dissolved in such solvents as water or alcohol. Whencommon salt (sodium chloride, NaCl) is dissolved in water, it forms an electrolytic solution,dissociating into positive sodium ions (Na+) and negative chloride ions (Cl–).

A non-electrolyte is a substance that does not conduct electric current through it inthe molten state or through its solution. Nonelectrolytes consist of molecules that bear nonet electric charge and they do not dissociate in their molten state or in their solutions.Sugar dissolved in water maintains its molecular integrity and does not dissociate and it isa nonelectrolyte.

6.5.2 Strong and weak electrolytes

In the last section we learned that electrolytes dissociate into ions in their solutions. Someelectrolytes are completely dissociated into ions. They are called strong electrolytes.Sodium chloride, potassium hydroxide and hydrochloric acid are strong electrolytes. Onthe other hand some other electrolytes are dissociated only partially into ions. They arecalled weak electrolytes. Acetic acid and ammonium hydroxide are weak electrolytes.

6.5.3 Dissociation of acids and bases in water

In the last section we learned that some electrolytes are strong while others are weak. Inthis section we shall study more about dissociation processes that occur in aqueous solutionsof acids and bases.


6.5.3a Dissociation of acids

(i) Dissociation of strong acids

Strong acids are completely dissociated into ions in their aqueous solutions. Considerdissociation of hydrochloric acid:

HCl(aq) H+(aq) + Cl–(aq)From the above equation it can be seen that

• HCl is completely converted into its ions and no amount of it remains in theundissociated form.

• One mole of HCl forms one mole each of hydrogen ions and chloride ions. Thus,concentration (molarity) of H+ ions is same as that of HCl in the solution.

(ii) Dissociation of weak acids

Weak acids are only partially dissociated into ions in their aqueous solutions. Considerdissociation of acetic acid.

CH3COOH(aq) H+(aq) + CH

3COO–(aq)

From the process depicted above it can be seen that:

• CH3COOH is only partially dissociated into ions.

• The process of dissociation is reversible and an equilibrium is established betweendissociated and undissociated CH

3COOH.

• The amount of hydrogen ions and acetate ions formed is less than the total amount ofacetic acid taken initially. Thus, if one mole of acetic acid was dissolved in one litre ofsolution (concentration = 1 mol L1) the concentration of hydrogen ions H+ formed inthe solution would be less than 1 mol L1. In fact acetic acid is such a weak electrolytethat less than 1% of it would dissociate in this solution.

• We can write expression of the law of equilibrium for the above equilibrium as[H+][CH3COO–]

Ka = ________________________

[CH3COOH]

Here the symbol used for equilibrium constant is Ka in place of K

c. Here K

a is

dissociation constant of acetic acid.

6.5.3b Dissociation of bases

(i) Dissociation of strong bases

Strong bases like sodium hydroxide are completely dissociated in their solutions.

NaOH(aq) Na+(aq) + OH−(aq)

From the above equation it can be seen that

• NaOH is completely converted into its ions and no amount of it remains in theundissociated form.

• One mole of NaOH forms one mole each of sodium ions and hydroxyl ions. Thusconcentration (molarity) of OH– ions is same as that NaOH of in the solution.


(ii) Dssociation of weak bases

Weak bases like ammonium hydroxide are only partially dissociated in their solutions.

NH4OH(aq) NH

4+(aq) + OH–(aq)

NH4OH(aq) NH

4+(aq) + OH–(aq)

From the process shown above it can be seen that

• NH4OH is only partially dissociated.

• The dissociation process is a reversible process and in the solution equilibrium isestablished between dissociated and undissociated NH

4OH.

• The amount of OH– ions and NH4

+ ions formed is less than the total amount ofammonium hydroxide taken initially.

• We can write expression of the law of equilibrium as

[NH4

+][OH–]Ka = __________________

[NH4OH]

Here the symbol used for equilibrium constant is Kb in place of K

c. K

b is the dissociation

constant of ammonium hydroxide.

6.5.4 Self-dissociation of water

Pure water is neutral in nature. It ionizes to a small extent and releases an equal number ofhydrogen and hydroxide ions.

H2O(1) H+(aq) + OH–(aq)

It can be seen from the above equation that in pure water

[H+] = [OH−]

Also, for this equilibrium

Kw = [H+]. [OH–]

where Kw

is known as ionic product of water. This is in fact the equilibrium constant for selfdissociation process of water. The term in the denominator is [H

2O] which by convention is taken

as 1 for any pure solid or liquid (see section 6.2.5).

The concentration of H+ and OH– ions in water has been measured and found to be 1 ×10–7 mol L1 each at 25 0C. Instead of saying that the hydrogen ion concentration in purewater is 1 × 10–7 mol L1, it is customary to say that the pH of water is 7.0 . The pH is thelogarithm (see box) of the reciprocal of the hydrogen ion concentration. It is written:

lpH = log _________

[H+]

Alternately, the pH is the negative logarithm of the hydrogen ion concentration i.e.

pH = log [H+]

Because of the negative sign in the expression, if [H+] increases pH would decrease andif it decreases the pH would increase.


LOGARITHM

Logarithm is a mathematical function.

If, x = 10y

Then y = log x

You will study more about logarithm in your higher classes.

Similarly, we may define pOH and pKw as:

pOH = –log [OH–]

and pKw = –log K

w

Since the concentration of OH ions, [OH] is

1 × 10–7 mol L–1 ; pOH = 7

The relationship between pKw,

pH and pOH is

pKw

= pH + pOH

= 7+7 = 14

The following points should be noted regarding selfdissociation of water:

(i) Water produces H+ and OH ions in equal amounts therefore: [H+] = [OH–]

(ii) Water is a neutral liquid.(iii) pH of water is 7.0 at 25 0C temperature.(iv) The sum of pH and pOH of any aqueous solution is always 14 at 25 0C.

6.5.5 Neutral, acidic and basic solutions and their pH

In the light of discussion on selfdissociation of water in the last section, we can nowdiscuss the characteristics of neutral, acidic and basic aqueous solutions.

6.5.5a Neutral aqueous solutions

Neutral solutions would be similar to water, which is also neutral in nature. Therefore, thefollowing are the characteristics of neutral aqueous solutions:

(i) [H+] = [OH–](ii) pH = 7.0 at 25 0C

6.5.5b Acidic aqueous solutions

Acidic solutions would have more [H+] than in water. Therefore the following are thecharacteristics of acidic aqueous solutions:

(i) [H+] > [OH](ii) Since hydrogen ion concentration in acidic solutions is more than in water their pH

would be less than that of water i.e.pH < 7.0 at 25 0C

(iii) For calculation of pH of acidic solutions first the concentration of H+ ions i.e. [H+] iscalculated. From it the pH is calculated by the relation pH = –log [H+]

Such calculations have been shown in the next section.


6.5.5c Basic aqueous solutions

Basic solutions would have more [OH–] than in water. Therefore they would have less[H+] than water. The following are the characteristics of basic aqueous solutions:

(i) [H+] < [OH–](ii) Since hydrogen ion concentration in basic solutions is less than in water their pHwould be more than that of water i.e.

pH > 7.0 at 25 0CThe pH of such solutions can be calculated indirectly. First pOH is calculated from theconcentration of OH ions using the relation (see next section).

pOH = − log [OH−]Then pH is calculated by the relation

pH = 14 –pOH

Thus in brief we may conclude that at 25 0C:

(i) Water has a pH of 7 and is neutral.

(ii) Solutions with pH 7 are neutral.

(iii) Solutions with pH less than 7 are acidic.

(iv) Solutions with pH more than 7 are basic.

6.5.7 Calculations based on pH concept

In the last section we learned the concept of pH and its relationship with hydrogen ion orhydroxyl ion concentration. In this section we shall use these relations to perform somecalculations.

It may be noted that the methods of calculation of pH used in this lesson are valid forsolutions of strong acids and bases only. The method is not valid for solutions, which areextremely dilute. The concentration of H+ or OH– should not be less than 10–6molar.

Example 6.6 : Calculate the pH of 0.001 molar solution of HCl.

Solution: HCl is a strong acid and is fully dissociated in its solutions according to theprocess:

HCl(aq) H+(aq) + Cl−(aq)

From the above process it is clear that one mole of HCl will give one mole of H+ ions.Therefore the concentration of H+ ion would also be 0.001 molar or 1 x 10–3 mol L–1.

Thus

[H+] = 1x 10– 3 mol L–1

pH = −log [H+] = − ( − 3) =3

Thus pH= 3

Example 6.7 : What would be the pH of an aqueous solution of sulphuric acid which is5 x 10–5 molar in concentration?

Solution : Sulphuric acid dissociates in water as:

H2SO

4(aq) 2H+(aq) + SO

42−(aq)


Thus each mole of sulphuric acid gives two moles of H+ ions in solutions. One litre of 5 x10−5 molar solution contains 5 x 10−5 moles of H

2SO

4, which would give 2 x 5 x 105 = 10

x 105 = 104 mol of H+ , therefore

[H+] = 10–4 mol L–1

Therefore, pH = –log [H+] = –log 10– 4 = – (–4) = 4

Example 6.8 : Calculate the pH of 1x10– 4 molar solution of NaOH.

Solution: NaOH is a strong base and dissociates in its solutions as:

NaOH(aq) Na+(aq) + OH– (aq)

One mole of NaOH would give one mole of OH ions. Therefore

[OH−] = 1x10− 4 molar

pOH = – log [OH−] = – (log 10−4) = − (− 4)

pOH = 4

Since pH = 14 – pOH

= 14 – 4

= 10


1. Aqueous solution of a substance does not conduct electricity through it. What type ofsubstance is it?

2. A substance completely dissociates in to ions when dissolved in water. What type ofsubstance is it?

3. X is a strong acid while Y is a weak acid. In whose aqueous solution a dynamicequilibrium will be established?

4. In an aqueous solution [H+] = [OH–]

What type of solution is it, acidic, basic or neutral?5. pH of a solution is 4. What is the hydrogen ion concentration in it?

LET US REVISE

• Based on the nature of chemical changes, reactions can be classified into five types (i)combination reactions, (ii) decomposition reactions, (iii) displacement reactions, (iv)doubledisplacement reactions, and (v) oxidationreduction reactions.

• The reactions in which all the reactants and products are present in the same phase arecalled homogeneous reactions and the reactions in which reactants and products arepresent in different phases are called heterogeneous reactions.

• The reactions in which heat is evolved are called exothermic reactions and the reactionsin which heat is absorbed are called endothermic reactions.

• The reactions that can occur in forward and reverse directions simultaneously undersame set of conditions are called reversible reactions.

• A system is said to be in a state of equilibrium if none of its properties changes withtime.


• One mole of a gas occupies a volume of 22.7 L at STP (standard temperature andpressure) i.e. at 273 K temperature and 1 bar pressure.

• An acid is a substance that furnishes hydrogen ions, H+; a base is a substance thatfurnishes hydroxyl ions, OH– in its solutions and a salt is produced when an acid anda base react with each other.

• An electrolyte conducts electric current through itself in the molten state or through itssolution. If it dissociates completely it is known as a strong electrolyte and if itdissociates only partially it is known as a weak electrolyte.

• pH of a neutral solution is 7, that of an acidic solution is less than 7 and that of a basicsolution is more than 7 at 250C

TERMINAL EXERCISES

A. Multiple choice type questions.1. The reaction given below is:

Zn(s) + CuSO4(aq)

ZnSO

4(aq) + Cu

(s)

(a) Combination reaction

(b) Displacement reaction(c) Redox reaction(d) Displacement and redox reaction.

2. The reaction given below is not a:

C(s) + O

2(g)

CO

2(g)

(a) Heterogeneous reaction(b) Displacement reaction(c) Exothermic reaction

(d) Redox reaction.3. In the reaction

2KClO3(s) 2KCl(s) + 3O

2(g)

(a) 1 mole of KClO3 produces 1.5 mole of O

2

(b) 1 mole of KClO3 produces 3 moles of O

2

(c) 2 moles of KClO3 produce 1 mole of KCl

(d) when 1 mole of KCl is produced 3 moles of O2 are produced

4. Which of the following statements about chemical equilibrium is not correct ?

(a) It is dynamic equilibrium.(b) It can be established by a reversible reaction only.(c) It is established in any aqueous solution of a strong acid or a strong base.(d) On changing the temperature the equilibrium constant’s value would also change.

5. pH of a solution is equal to(a) log [H+](b) log [H+](c) log [OH–]

(d) log [OH–]


6. In which of the following reactions H2O

2 acts as a reducing agent?

(a) H2O

2 +2KI 2KOH + I

2

(b) H2O

2 + SO

2 H

2SO

4

(c) H2O

2 + Ag

2O 2Ag + H

2O + O

2

(d) 4H2O

2 + PbS PbSO

4 + 4H

2O

B. Descriptive type questions.1. Write electronic definitions of oxidation and reduction.2. Give one example each of slow and fast reactions.3. Give any two examples of quantitative information carried by a chemical equation.4. What is an acid?5. What is pH?6. What is an exothermic reaction? Give one example.7. Differentiate between displacement reactions and double displacement reactions.8. What are weak electrolytes? Give one example.9. In the reaction

Cl2(g) + 2KBr(aq)

2KCl(aq) + Br

2(aq)

How much mass of Cl 2 is required to produce 1.5 moles of Br

2?

10. What is the pH of 5x10–4 molar solution of H2SO

4?

11. In the reaction:CuO(s) + H

2(g) Cu(s) + H

2O(l)

Identify the species that is getting (i) reduced (ii) oxidized.12. What is the difference between dynamic and static equilibrium? Give example of

each.13. NH

4OH is a weak base. Write down the equilibrium established in its aqueous solution

and the expression of its dissociation constant Kb.

14. Given the following reaction2Al(s) + Fe

2O

3(s) 2Fe + Al

2O

3(s)

calculate the mass of Fe2O

3 in grams required to produce 20.0 g of Fe. (Relative atomic

masses: Fe = 55.8; O = 16).15. Calculate the pH of (i) 10–5 mol L–1 HCl and (ii) 10–4 mol L–1 NaOH.16. What are oxidation and reduction? Give one example with equation of a redox reaction.

Identify the oxidizing agent and the reducing agent in it.17. (i) What is a homogeneous reaction? Give one example each of gas phase and solution

phase homogeneous reactions.(ii) What is a reversible reaction? Give one example.

18. In the reaction3C

3H

6 + 2KMnO

4 + 4H

2O 3C

3H

8O

2 + 2KOH + 2MnO

2

Calculate, (i) the number of moles of MnO

2 produced by 12 moles of C

3H

6.

(ii) the number of moles of KMnO4 needed to react with 0.006 moles of C

3H

6.

(iii) the number of moles of KMnO4 needed to produce 0.15 moles of C

3H

8O

2.

(iv) the mass of C3H

6 required to produce 5.6 grams of KOH. (Atomic mass of K = 39)


19. What is a neutralization reaction? A titration was started by taking 20 mL of 10–2

molar HCl. Then a solution of NaOH was gradually added from the burette. By mistakethe student missed the end point and added excess of NaOH. When he finished thetitration, the solution was 10−4 molar in NaOH. What was the pH of the solution presentin the titration flask?(i) In the beginning of the titration(ii) at the end point when NaOH had just neutralized the HCl and(iii) at the end of the titration.

20. Sodium metal reacts with excess of water according to the reaction:2Na(s) + 2H

2O(l) 2NaOH(aq)

+ H

2(g)

(i) Calculate the mass of sodium required to produce 1 kg of NaOH.(ii) Find out the volume of H

2 evolved at STP when 1.012 kg of sodium reacts with

excess of water.


6.11. D2. C3. B4. A5. E

6.21. Heterogeneous2. Endothermic3. Irreversible4. Fast5. Nonequilibrium

6.31. T2. F3. T4. T5. F

6.41. Salt2. Y3. X

6.51. Nonelectrolyte2. Strong electrolyte3. In solution of Y4. Neutral

5. 10–4 mol L1


GLOSSARY

Acid: A substance containing hydrogen that furnishes hydrogen ions (H+) in itssolutions.

Base: A substance that furnishes hydroxyl ions, OH– in its solutions.

Combination reaction: A reaction in which two or more substances react to form anew substance.

Decomposition reaction: A reaction in which one substance breaks down into two ormore substances.

Displacement reaction: A reaction in which an ion present in a compound is displacedby another ion.

Double displacement reactions: The reactions in which two ionic compoundsexchange their ions.

Electrolyte: A substance that conducts electric current through it in the molten stateor through its solution.

Endothermic reactions: The reactions in which heat is absorbed

Equilibrium state: A state in which no property of system changes with time.

Exothermic reactions: The reactions in which heat is evolved

Heterogeneous reactions: Reactions in which reactants and products are present inmore than one phase.

Homogeneous reaction: Reactions in which all the reactants and products are presentin the same phase.

Molarity: It is the number of moles of a substance present in one litre volume.

Neutralization: The reaction between an acid and a base to produce salt and water.

Non-electrolyte: A substance that does not conduct electric current through it in themolten state or through its solution.

Oxidation: A process which involves loss of electrons.

pH: The negative logarithm of the hydrogen ion concentration.

Reduction: A process which involves gain of electrons.

Reversible reactions: The reactions that can occur in forward and reverse directionssimultaneously under same set of conditions.

Salt: A substance produced by the reaction of an acid with a base along with water.

STP: Standard temperature and pressure i.e. when temperature is 273 K temperatureand pressure is 1 bar.

Strong electrolytes: The electrolytes that dissociate completely in their solutions.

Synthesis reaction:The reaction in which a compound is formed by combination ofits constituent elements.

Weak electrolytes: The electrolytes that dissociate only partially into ions in their

aqueous solutions.

7

Motion and Its DescriptionIn this world, we see many objects moving around us, for example, cars, buses, trucks, andbicycles moving on the road, aeroplanes flying in air and ships sailing on the sea, leavesfalling from the trees and water flowing in the river. All these objects are changing theirposition with time. When an object changes its position with time, it is said to be inmotion. In these examples, motion is easily visible to us. But in some cases, motion is noteasily visible to us. For example, air moves in and out of our lungs and blood flows in ourbody. The moon moves around the earth, while the two together go around the sun. Thesun itself with its planets travels through our own galaxy.

An object that does not change its position with time is said to be at rest, for examplea book lying on a table.

In this lesson, you will learn how to describe motion. For this, we will develop theconcepts of displacement, velocity and acceleration. You will also learn how thesequantities are related to each other. For an object moving along a straight line with uniformacceleration, we will obtain simple equations (known as equations of motion) connectingthese quantities with time.

OBJECTIVES


! define the terms motion, scalar and vector quantities, displacement, speed, velocityand acceleration,

! distinguish between(a) rest and motion(b) scalar and vector quantities(c) speed and velocity;

! differentiate between uniform and non-uniform motion;! plot and interpret the following graphs

(a) displacement – time graph for uniform motion,(b) velocity – time graph for uniformly accelerated motion;

! establish three equations of motion;! solve problems based on equations of motion.

: 122 : Motion and Its Description

7.1 SOME BASIC ASPECTS OF MOTION

7.1.1 Types of motionIn our daily life we see many objects moving. Some objects move in a straight line. For example,a ball rolling on a horizontal surface, a stone falling from a building and a runner on a 100m racetrack. In all these examples, objects change their positions with time along a straight line. Thistype of motion is called rectilinear motion.

Observe the motion of a second’s hand of a clock, or motion of a child sitting on amarry-go round, or the motion of the blades of an electric fan. In such a motion, an objectfollows a circular path during motion. This type of motion is called circular motion. Ifyou take a stone, tie a thread to it and whirl it with your hand, you will find that the stonemoves on a circular path. In all such cases, though an object changes its position with time,it remains at a fixed distance from a point.

Some objects move to and fro, such as a swing, a pendulum, the branches of a tree inthe wind and the needle of a sewing machine. Such type of motion is called oscillatorymotion. In such a motion, an object oscillates about a point, often called equilibriumposition.

7.1.2 Scalar and vector quantitiesEach of the physical quantities you encounter in this book can be categorized as either ascalar or a vector quantity. A scalar is a quantity that can be completely specified by itsmagnitude with appropriate units; i.e. a scalar has only magnitude and no direction. Avector is a physical quantity that requires the specification of both magnitude and direction.

Mass is an example of a scalar quantity. If someone tells you that mass of an object is2 kg, that information completely specifies the mass of the object; no direction is required.Other examples of scalar quantities are, temperature, time interval, the number of studentsin a class, the volume of water in a bucket and the number of pages in this book.

An example of a vector quantity is force. If your friend tells you that he is going toexert a force of 5N on an object, this is not enough information to let you know what willhappen to the object. The effect of a force of 5N exerted horizontally is different from theeffect of a force of 5N exerted vertically upward or downward. In other words, you needto know the direction of the force as well as its magnitude. Velocity is a vector quantity. Ifyou wish to describe the velocity of a moving vehicle, you must specify both its magnitude(say, 30 m/s) and the direction in which the vehicle is moving (say, northeast). Otherexamples of vector quantities include displacement and acceleration, which are defined inthis lesson.

We use different symbols to represent scalar and vector quantities. A scalar quantityis represented by an ordinary letter (such as a) or number (such as 5) with appropriate unit.3 cm, 6 L, and 12 kg represent scalar quantities.

A vector quantity is represented by a symbol printed in boldface, such as a or A. Sincein handwriting, this representation is not practical, a common notation is to indicate a

vector quantity by an arrow over its symbol, ra or

rA . When we are interested only in the

magnitude of a vector quantity, such as a, we write it as a scalar (that is, a) indicating thatits direction is not being considered. Graphically, a vector is represented by an arrow. The

Motion and Its Description : 123 :

length of the arrow is proportional to the magnitude of the vector and the arrow points inthe direction of the vector. Fig. 7.1 a shows vector A and vector –A, both has the samemagnitude but are in opposite directions.

Fig. 7.1 (a) Vectors in same direction Fig. 7.1 (b) Vectors in different directions

Figure 7.1 (b) shows vector A and another vector B whose magnitude is same as thatof A but direction is different.

Scalars can be added and subtracted like ordinary numbers. Vectors follow differentlaws. However, vectors having same direction can be added easily. For example, sum ofvector A and a vector C (Fig. 7.1c) is a vector D whose magnitude is the sum of themagnitudes of vector A and C and direction is the same as that of A.

Fig. 7.1 c Addition of vectorSubtraction of vector C from vector A can be seen as addition of vector – C to vector

A as shown in Fig. 7.1d. The resultant has a magnitude equal to the difference of themagnitude of A and the magnitude of B. It points in the direction of A (the bigger of thetwo vectors).

Fig. 7.1 (d) Subtraction of vectors

7.1.3 Distance and displacement

Motion occurs when an object changes position. Therefore, in order to describe the motionof an object, one must be able to specify its position at all times. In this course, we shallconsider motion of objects in which position changes along a straight line, known asrectilinear motion. Let us say that the object moves along x-axis as shown in Fig. 7.2

Fig. 7.2 Movement of an object along x-axisThen position of the object is specified with reference to a point, say O. This point is

called origin of the axis. The position is taken to be positive if it is to the right of the originand negative, if it is to the left. So, if a car is at P, its position is + 60m. If it is at R, itsposition is –40m.

A

–A

A

B

A

C

D=A+C

C

– C

A A

E (–C)

E = A–C = A+(–C)

–100 –80 –60 –40 –20 0 20 40 60 10080

ORS P Q

+x (m)–


Suppose a car starts from O, moves to Q and then comes back to P. During thismotion, the actual path length covered by the car = OQ + QP = +100m + (40m) = 140m.This is known as distance. The total path length covered by an object irrespective of itsdirection of travel is called distance. It is a scalar quantity. In SI unit, it is measured inmetres (m).

In the above example, at the end of journey, the car is at P. So, its final position is Pwhile its initial position was O. Therefore, change in position is OP +60m only. This isknown as displacement.

The displacement of an object is defined as the change in its position and is given bythe difference between its final and initial position.

Displacement of an object = final position – initial position.

Displacement is a vector quantity. In SI units, it is measured in metres (m).

In the above example, the displacement of the car is + 60m. The plus sign means thatit is along + x-axis. The magnitude of this displacement is 60m and its direction is towardsright or + x-axis.

Consider another case. Suppose a truck moves from O to R and returns to O. What isthe distance covered by the truck? What is its displacement? Though the truck is movingalong – x-direction, the length of path covered is positive. (The minus or plus sign, asexplained earlier, indicates the direction of travel).

Distance covered by the truck = Path length of OR + Path length of RO

= 40 m + 40 m = 80 m

Displacement of the truck = Final position – initial position

= O (since it returns to origin O, its initial position)

Example7.1: What is the distance covered and displacement of a car,

a) If the car moves from O to Pb) If the car moves from O to P and then back to R (see Fig. 7.2).

Solution :

a) Distance covered in = Length of path OP = 60 mmoving from O to PDisplacement = Final position – Initial position

= + 60 m – (0 m)= + 60 m

60 m is the magnitude of the displacement and + sign indicates that it is directedtowards right or towards P.

Note that in this case magnitude of displacement is equal to the distance. This is sobecause the object does not change its direction during the course of motion.

b) Distance covered in this case = Length of path OP+

Length of path PR


= 60 m + (60 m + 40 m)= 160 m

Displacement = Final position – Initial position= (–40 m ) – (0 m )= – 40 m

The minus sign shows that the direction of displacement is towards left or towards – xdirection.

Note that in this case, the magnitude of displacement (i.e. 40 m) is not equal to thedistance (160 m).

7.1.4 Speed and velocityAn object in motion travels a given distance in a certain time interval. How fast is theobject moving? This is indicated by a quantity called speed.

The speed of an object is defined as the length of the path travelled per unit time.

takenTime

covered distanceor length PathSpeed = …(7.1)

Its unit is m/s. It is also expressed in kmh-1. For example, if a car covers a distance of61 km in 2h, its speed is 61km / 2h = 30.5 kmh-1.

The velocity of an object is defined as the displacement divided by the time intervalduring which the displacement occurred:

takenTime

splacementiDVelocity = …(7.2)

Since displacement is a vector quantity, velocity is also a vector quantity. Its unit isthe same as that of speed, i.e. ms-1 or kmh-1.

ACTIVITY 7.1Aim : To calculate your average speed of walking.

What is required ?A metre stick or a measuring tape; stop watch or a wrist watch with second’s hand.

What to do?i) Take a stopwatch to a field.

ii) Using the measuring tape mark two positions (in a straight line) on the fieldthat are 50m apart.

iii) Start the clock as you walk down the marked line and stop it as you reach the50m mark. Find the time taken by you to cover this distance.

iv) Calculate your average speed of walking.

v) Measure the time it takes you to run the same distance. What is your averagespeed?

To represent displacement and velocity, we must use vector notations. But in thisclass, we shall be considering motion along a straight line. As mentioned earlier, in suchcases, direction can be represented by + or – signs. Therefore, we need not use vectornotations.


For example, consider a car moving towards + x axis (Fig. 10.1). It moves from O toA position + 900 m in 1 minute.

Then its displacement = + 900 m – (0 m ) = + 900 m.

Therefore, velocity = s 60

m 900+= +15 ms–1

The magnitude of velocity is 15 m/s and it’s direction (as indicated by + sign) is towardsright or towards + x axis.

Suppose the car travels back to origin O in 90 s.Then, speed for this motion = Distance covered/ time taken

= (900 m + 900 m)/ (60 s + 90 s)

= s 150

m 8001 = 12 ms-1

Velocity for this motion = taken Time

ntDisplaceme =

s 150

m 0 = 0 ms-1

(Displacement is zero because final position coincides with the initial position).

CHECK YOUR PROGRESS 7.11. If the average velocity of an object is zero in some time-interval, what can you say

about the displacement of the object for that time interval?2. If B is added to A, under what conditions does the resultant vector have a magnitude

equal to A+B? Under what conditions is the resultant vector equal to zero?3. Car A travelling from Delhi to Ghaziabad, has a speed of 25 ms-1. Car B, travelling

from Delhi to Gurgaon, also has a speed of 25 ms-1. Are their velocities equal? Explain.4. Give one example of circular motion and one example of motion in a straight line.5. A body moves in a straight line from O to P and then to Q. What is the value of (i)

distance travelled by the body, and (ii) displacement of the body.

7.2 GRAPHICAL REPRESENTATION OF MOTION

7.2.1 Position time graph

It is easy to analyze and understand motion of an object if it is represented graphically. Todraw graph of the motion of an object, its positions at different times are shown on y – axisand time on x – axis. For example, positions of an object at different times are given inTable 7.1.

In order to plot position – time graph for data given in Table 7.1, we represent time onhorizontal axis and position on vertical axis drawn on a graph paper. Next, we choose asuitable scale for this. For example, in Fig. 7.3, 1 cm on horizontal axis represent 2 s oftime interval and 1 cm on vertical axis represent 20 m, respectively. If we connect differentpoints representing corresponding position time data, we get a straight line as shown in

Table 7.1 Position of different objects at different times

Time (s) 0 1 2 3 4 5 6 7 8 9 10

Position (m) 0 10 20 30 40 50 60 70 80 90 100


1 2 3 4 5 6 7 8 9 10 11 12

20

40

60

80

100

Fi 7 3

Time (s)

Distan

ce

(m

)

0

Fig. 7.3. This line represents the position-time graph of the motion corresponding to datagiven in Table 7.1.

Fig. 7.3 Position-time graph for the motion of a particle on the basis of data given in table 7.1

We note from the data that displacement of the object in 1st second, 2nd second,………, 10th

second is the same i.e. 10 m. In 10 second, the displacement is 100 m. Therefore, velocity is100 m/10 s = 10 m/s for the whole course of motion.

Velocity during 1st second = 10 m/ 1s = 10 ms-1

Velocity during 2nd second = 10 m/1s = 10 ms-1 and so on.

Thus, velocity is constant i.e., equal to 10 m/s throughout the motion. The motion ofan object in which its velocity is constant, is called uniform motion. As you see in Fig.7.3, for uniform motion, position-time graph is a straight line.

ACTIVITY 7.2

Aim : To plot and interpret the graph of the motion (walking) of your friend.

What is required ?

A metre stick, stop watch, and a marker (chalk, etc.)

What to do?

(i) Go out to your college field with your friend.

(ii) Using a meter stick, mark positions, 0, 5m, 10m, 15m, 20m, 25m, 30m,35m, 40m, 45m, and 50m.

(iii)Ask your friend to walk down the line starting from position marked 0m.

(iv)As your friend starts walking, start the stop watch and record the reading of thestopwatch as he touches the marks 5m, ................, 50m.

What do you observe?

(i) Record your data in the following table:


Displacement (m) Time (s) Displacement (m) Time (s)

0 0 30m

5m 35m

10m 40m

15m 45m

20m 50m

25m

(ii) Plot a graph of distance (vertical axis) and time (horizontal axis).

What do you infer?

(i) Is the graph a straight line? If yes, what does it mean? If no, what does it mean?(ii) Did your friend travel this distance with uniform velocity?(iii)Calculate the average velocity of your friend for a dsiplacement of 20m, 40m and

50m. Are they same? Explain your result.

Like position-time graph, one can also plot displacement-time graph. Displacementis represented on the vertical axis and time interval on the horizontal axis. Sincedisplacement in each second is 10 m for data in Table 7.1, the same graph (Fig. 7.3) alsorepresents the displacement-time graph if the vertical axis is labelled as displacement.

How will the position-time graph look like for a stationary object or object at rest.Suppose an object is at rest at position x = 40 m. Then, its position-time graph will be astraight line parallel to the time axis as shown in Fig. 7.4 because at all times, it is at 40 m.

Fig. 7.4 Position time graph of a particle at rest7.2.2 Velocity – time graph

Take time on the horizontal axis and velocity on the vertical axis on a graph paper. Let 1cm on horizontal axis represent 2 s and 1 cm on vertical axis represent 10 ms-1. Plottingthe data in Table 7.2 gives us the graph as shown in Fig. 7.5.

1 2 3 4 5 6 7 8 9 10 11 12

20

40

60

80

100

Time (s)

Distance

(m

)

0


Fig. 7.5 Velocity-time graph for the motion of a particle on the basis of data given in table 7.2

Thus, we see that the velocity-time graph of motion represented in Table 7.1 and Table7.2 is a straight line parallel to time axis. This is so because the velocity is constantthroughout the motion. The motion is uniform.

Consider the area under the graph in Fig. 7.5.

Area = (10 ms-1) x 10 s = 100 m. This is equal to the displacement of the object in 10s.

Area under velocity-time graph = Displacement of the object during that time interval.

Though, we obtained this result for a simple case of uniform motion, it is a generalresult.

Let x be displacement of an object in time t, moving with uniform velocity v, then

x = v t (Uniform motion ) …(10.3)

In real life, objects usually do not move with constant velocity. We see that usually anobject starts from rest, picks up motion, moves some distance, slows down and finallycomes to rest. This means that the velocity during different time intervals of motion isdifferent. In other words, velocity is not constant. Such a motion is called non-uniformmotion. This change in velocity with time is a physical quantity called accelerationwhich we shall define next.

7.2.3 Acceleration

The acceleration of an object is defined as the change in velocity divided by the timeinterval during which this change occurs.

ntervali Time

yin velocit ChangeonAccelerati = …(10.4)

Its unit is m/s2. It is a vector quantity.

Suppose the velocity of a car changes from + 10 m/s to + 30 m/s in a time interval of2.0 s. Note that both velocities are towards the right, as indicated by + signs. Therefore,

Acceleration = s0.2

s/m10s/m30 − = + 10 ms-2

Table 7.2 Velocity-time data of an object

Time (s) 0 1 2 3 4 5 6 7 8 9 10

Position (m) 0 10 10 10 10 10 10 10 10 10 10

2 4 6 8 10

10

20

30

40

50

Time (s)

Velocity

(m

s)

–1


t (s)

V(m

s)

–1

0

The acceleration in the present case is +10 ms-2. This means that the car accelerates inthe + x direction and its velocity increases at a rate of 10 ms-1 every second.

If the acceleration of an object during its motion is constant, we say that the object ismoving with uniform acceleration. The velocity-time graph of such a motion is a straightline inclined to the time axis as shown in Fig. 7.6.

Fig. 7.6 Velocity-time graph of a particle moving with uniform acceleration

For a given time interval, if the final velocity is more than the initial velocity, thenaccording to Fig. 7.6, the acceleration will be positive. However, if the final velocity isless than the initial velocity, the acceleration will be negative.

What is the acceleration corresponding to motion represented in Fig. 7.6? It is zerosince there is no change in velocity with time. Thus, for uniform motion, the accelerationis zero and for non-uniform motion, the acceleration is non-zero.

Note: Please note that speed and velocity that we defined in the earlier section are, infact, average speed and average velocity for the time-interval under consideration. Unlessotherwise specified, terms ‘speed’ and ‘velocity’ wherever used refer to the ‘average speed’and ‘average velocity’.


1. Look at fig. 7.7.(i) What kind of motion does the graph represent?(ii) What does the slope of the graph represent?

2. Look at fig. 7.8.(i) What kind of motion does the graph represent?(ii) What does the area under the graph represent?

t (s)

d(m

)

0

Fig. 7.7

Fig. 7.8


3. Look at fig. 7.9.(i) What kind of motion does the graph represent?(ii) What does the slope of the line represent?(iii) What does the area under the curve represent?

4. A car starts from rest accelerates uniformly and attains a maximum speed of 20 ms-1 in5 seconds. In the next 10 s it slows down uniformly and comes to rest at the end of 10th

s. Draw a velocity time graph for the motion. Calculate from the graph (i) acceleration,(ii) retardation, and (iii) distance travelled.

5. A body moving with a constant speed of 10 ms-1 suddenly reverses its direction ofmotion at the 5th second and come to rest in the next 5 seconds. Draw a position - timegraph of the motion.

7.3 EQUATIONS OF MOTION

Consider an object moving with uniform acceleration, a. Let u be its initial velocity (attime t = 0), v, its velocity after time t and s, its displacement during this time interval. Letus see how these quantities are related to each other.

7.3.1 Relation between, v, u, a and t

According to the definition of acceleration, we have

ntervali Time

yin velocit ChangeonAccelerati =

ort

uva

−=

or, v = u + a t …(10.5)

With the help of this equation, we can find velocity of a uniformly accelerated objectafter a given time interval. Or, given any three of these quantities, fourth can be foundusing this equation.

Example 7.2: A car has an initial velocity of 25 ms-1. The brakes are applied and the carstops in 2.0 s. What is the acceleration of the car?

Solution: Using (10.5), v = 0, u = 25 ms-1, t = 2.0 s

O = 25 ms-1 + a (2.0s) hence, a = - 12.5 ms-2

It is negative. Negative acceleration is also called deceleration.

7.3.2 Relation between s, u, a and t

From equation (10.3), we have

Displacement = (average velocity) × ( time interval )

t(s)

V(m

s)

–1

O 10

Fig. 7.9


or, tuv

s

+=

2

But, v = u + at

Therefore, ( ) 2

2

1

2

1tatutatuus +=++=

2

2

1tauts += … (10.6)

If an object starts from rest, u = 0 and equation (10.6) reduces to

2

2

1tas = … (10.7)

Thus, we see that the displacement of an object undergoing a constant acceleration isproportional to t2, while the displacement of an object with a constant velocity (zeroacceleration) is proportional to t (Equation 10.3).

A body in free fall, falls with a uniform acceleration, called acceleration due to gravity(denoted by g) and having an average value 9.8 ms-2 near the surface of earth. For thismotion the equations of motion become

v = u + gt

s = ut + ½ gt2

Use these concepts to do the following activity:

ACTIVITY 7.3

Aim: To measure your reaction time.

What is required?

To do this activity, you need the help of your friend, a metre scale, and a stopwatch.

What to do?

(i) Take a metre scale and ask your friend to hold it vertically between his indexfinger and thumb.

(ii) Note the position of the metre scale with respect to his index finger.

(iii)Ask your friend to release the ruler and you must catch it (without loweringyour hand after catching it).

(iv)Note the position of the metre scale, when you catch it and find the distancethrough which the ruler falls. Let it be d.

(v) Repeat this activity 5 times and note the value of d each time.


What do you infer?

(i) The ruler is a freely falling object with u = 0, a = g(acceleration due to gravity = 9.8 m/s2)

Using equation of motion, 2

2

1tauts +=

we have 2

2

1rtgd =

or g

dtr

2=

(ii) Using the different experimentally values of d obtained, you can calculate tr

find the mass of all these values. What you get is your reaction time.(iii)Similarly, you can measure the reaction time of your friend. It is usually about 0.2 s.

Example 7.3: An object with an initial velocity of 4.0 m/s is accelerated at 6.0 m/s2 for2.0 s.(a) How far does the object travel during this period?(b) How far would the object travel if it were initially at rest?

Solution:a) Given u = 4.0 ms-1, t = 2.0 s, a = 6.0 ms-2

s = u t + ½ a t 2 = (4.0 ms-1) ( 2.0 s) + (1/2) (6.0 ms-2) (2.0 s)2

= 8.0 m + 12.0 m = 20 m.b) For u = 0,

s = 0 + ½ (6.0 ms-2) (2.0 s) 2

= 12 m

7.3.3 Relation between u, v, and sWe know that,

tvu

s

+=

2

and,t

uva

−=

On multiplying these two equations, we have

tuvt

uvas

+

−=

2 =

2

22 uv −

or, v2 = u2 + 2 a s …(10.8)

Equations (10.5), (10.6) and (10.8) are the three equations of motion.

Example 7.4: A bus starts from rest and moves with a uniform acceleration of 3ms-2. Whatwill be its velocity after moving a distance of 37.5 m?


Solution :

Given u = 0, a = 3 ms-2, s = 37.5 mv = u2 + 2 a s

= 0 + 2 (3 ms-2) (37.5 m)= 225 m2/s2 = (15 ms-1) 2

v = 15 ms-1

Example 7.5: A body is dropped from the top of a 3 story (h=15m) building. After howmuch time will it strike the ground? (g=10ms-2)

1Solution: s = ut + _____ gt2

2u = 0, g = 10 ms–2, s = 15m

∴ × 15 = 12

10 2t

⇒ t s= = =15

53 1732.


1. A ball is thrown straight up with an initial velocity of + 19.6 ms-1. It was caught at thesame distance above ground from which it was thrown:(i) How high does the ball rise.(ii) How long does the ball remain in air? (g=9.8 m/s2)

2. A ball is thrown vertically upwards.(i) What are its velocity and acceleration when it reaches the highest point?(ii) What is its acceleration just before it hits the ground?

3. A body accelerates from rest and attains a velocity of 10 ms-1 in 5s. What is itsacceleration?

4. A body starts its motion with a speed of 10 ms-1 and accelerates for 10 s with 10 ms-2.What will be the distance covered by the body in 10s?

5. A body starts from rest and covers a distance of 50m in 10 s. What is the average speedof the body?

LET US REVISE

! If a body stays at the same position with time, it is at rest.

! If the body changes its position with time, it is in motion.

! Motion is said to be rectilinear if the body moves in the same straight line all-the time,e.g, a car moving in a straight line on a level road.

! The motion is said to be circular if the body moves on a circular path: e.g, the motionof the tip of the hand of a watch.

! The total path length covered by a moving body is the distance travelled by it.

!


! The difference between the final and initial position of a body is called its displacement.

! Physical quanities are of two types (i) scalar: which have magnitude only, no direction(ii) vector: which have magnitude as well as direction.

! Distance, speed, mass, time, temperature etc. are scalar quantities, whereasdisplacement, velocity, acceleration, momentum, force etc. are vector quantities.

! Distance travelled in unit time is called speed, whereas, displacement per unit time iscalled velocity.

! Position-time graph of a body moving in a straight line with constant speed is a straightline sloping with time axis. The slope of the line gives the velocity of the motion.

! Velocity-time graph of a body in a straight line with constant speed is a straight lineparallel to time axis. Area under the graph gives distance travelled.

! Velocity-time graph of a body in a straight line with constant acceleration is a straightline sloping with the time axis. The slope of the line gives acceleration.

! For uniformly accelerated motion :v = u+at

1s = ut + _____ at2

2where u = initial velocity, v = final velocity, and s = distance travelled in t seconds.

TERMINAL EXERCISES

1. Explain whether or not the following particles have an acceleration:(i) a particle moving in a straight line with constant speed, and(ii) a particle moving on a curve with constant speed

2. Consider the following combination of signs and values, for velocity and accelerationof an object with respect to a one-dimensional motion along x-axis:

Velocity Acceleration Velocity Acceleration

a. Positive Positive e. Negative Negativeb. Positve Negative f. Negative Zero.c. Positive Zero g. Zero Positived. Negative Positive h. Zero Negative

Describe what an object is doing in each case, and give a real-life example for a car on aneast-west one-dimensional axis, with east considered as the positive direction.3. A car travelling initally at + 7.0 m/s accelerates at the rate of + 0.80 m/s2 for an interval

of 2.0s. What is its velocity at the end of the acceleration?4. A car travelling in a straight line has a velocity of + 5.0 m/s at some instant. After 4.0s,

its velocity is + 8.0 m/s. What is its average acceleration in this time interval?5. The velocity - time graph for an object moving along a straight line as shown in figure.

7.10.


Find the average acceleration of this object during the time intervals 0 to 5.0 s, 5.0s to15.0s, and 0 to 20.0s.

6. The velocity of an automobile changes over a period of 8 s as shown in the table givenbelow:

Time(s) Velocity (m/s) Time (s) Velocity (m/s)0.0 0.0 5.0 20.01.0 4.0 6.0 20.02.0 8.0 7.0 20.03.0 12.0 8.0 20.04.0 16.0

(i) Plot the velocity - time graph of motion.(ii) Determine the distance the car travels during the first 2s.(iii) What distance does the car travel during the first 4s?(iv) What distance does the car travel during the entire 8s?(v) Find the slope of the line between t = 0s and t = 4.0s. What does this slope

represent?(vi) Find the slope of the line between t = 5.0s and t = 7.0s. What does the slope

indicate?7. The position-time data of a car is given in the table given below:

Time(s) Position(m) Time(s) Position(m)

0 0 25 1505 100 30 112.510 200 35 7515 200 40 37.520 200 45 0

(i) Plot the position-time graph of the car.(ii) Calculate the average velocity of the car during first 10 seonds.(iii) Calculate the average velocity between t = 10s to t = 20s.

–8

–6

–4

–2

2

4

6

8

t (s)

5 10 15 20

V(m

s)

–1

Fig. 7.10


(iv) Calculate the average velocity between t = 20s and t = 25 s. What can you sayabout the direction of the motion of car?

8. Distance is always (a) less than; (b) greater than; (c) less than or equal to; (d) greaterthan or equal to, the magnitude of displacement.

9. The graph of x vs. t plot for an object with a uniform velocity in the x-direction is (a)a curved line; (b) a straight line; (c) a circle; (d) a point.

10. An object initially at rest moves for t seconds with a constant acceleration a. Theaverage speed of the object during this time interval is (a) at/2; (b) 2 at; (c) 1/2 at2 (d)1/2at.

11. A car starts from rest with a uniform acceleration of 4 m/s2. The distances travelled atthe ends of each of the first 4 seconds are, respectively, (a) 4, 8, 16, 32m, (b) 2, 8, 18,32m, (c) 2, 4, 8, 16m, (d) 4, 16, 32, 64m.


7.1

1. Zero2. Both A and B should be along the same direction. B should be equal in magnitude and

opposite in direction to A3. No. Though the magnitudes of their velocites are the same (25 ms-1), their direction

are different. Hence, VA ≠ V

B.

4. Motion of a second’s hand of a clock is a circular motion. A ball rolling on a horizontalsurface executes motion along a straight line.

5. (i) 25m, (ii) –5m

7.2

1. (i) uniform motion (ii) velocity of the object2. (i) uniform motion (ii) displacement of the object3. (i) uniformly accelerated motion

(ii) accleration(iii) displacement

4. refer section 7.2.25. refer section 7.2.1

7.3

1. (i) 19.6m (ii) 4s2. (i) v = 0, a = g (ii) g3. 2ms-2

4. 600m5. 5ms-1

Force & Motion INTRODUCTION:

In the previous lesson, you have studied about the motion of bodies in a straight line. You have learnt that there are three equations of motion, with the help of which you can solve problems involving initial velocity, duration of motion and the acceleration with which a body travels. But what is the cause of change in motion or cause of acceleration which is responsible for producing an increase in the velocity of a moving body? Newton formulated three laws regarding the motion of bodies. These laws are called Newton’s laws of motion. In this lesson you will learn about these laws. These laws will tell you why a motion occurs. These will help you to find out more about moving bodies.

If you push a body on a floor or ground, the body stops after moving through some distance. Why does it happen so? Why does a stone thrown upwards always come down to the earth? Why the edge of a knife is made sharp? Why do some bodies float on water whereas some other bodies sink in water? The answer to such questions will be discussed in the present lesson.

OBJECTIVES


• explain the cause of motion;

• define the terms inertia, force, mass and momentum;

• state the three laws of motion and explain their significance;

• establish a relationship between force, mass and acceleration;

• explain friction and the factors on which it depends;

• illustrate advantages and disadvantages of friction in day to day life;

• explain how friction is increased or decreased in different situations;

• state and explain the Newton’s law of gravitation;

• distinguish between mass and weight, and express the relationship between them;

• distinguish between thrust and pressure with suitable examples;

• state the principle of Archimedes and apply it to solve problems.

Force and Motion

If you put a ball on the ground, it will stay there. It does not move by itself. It will move only when you kick it. If you kick it hard, it moves faster. To move a heavy stone across a room lot of pushing has to be done. But to move a sheet of paper off your table requires a very little push. Can you think of a situation when a cart is moving without bullocks? No. It means something has to be done to move a body from rest or to make it move slow or fast. You can also stop a moving ball by catching it or putting an obstacle in its path. It means something is done to stop a moving body. Consider another example in which the volleyball players are hitting the ball from both sides. You will observe that in each hit the direction of the ball is changed. In this case also something is done to change the direction of motion of the body.

What is this something that changes the state of rest, or of uniform motion of a body? This something is called force. Thus, we can say that the force is something which when applied on a body changes or tends to change the state of rest or uniform motion of the body. Kick, push, pull and hit are some of the different ways of applying force on a body. Each one is called an action. You must note that force is a vector quantity, because it is always applied along a particular direction and has magnitude.

Newton's Laws of Motion

8.2.1 Newton’s first law of motion

Therefore, we conclude that everybody continues in its state of rest or of uniform motion in a straight line unless and until it is compelled by some unbalanced force to change that state. This is the statement of Newton’s First Law of Motion. So, Newton’s first law of motion may be used to define force. It also defines another concept called inertia. The property of a body by virtue of which it is unable to change its state of rest or of uniform motion in a straight line is called inertia. You can do another activity to understand the concept of inertia.

ACTIVITY 8.1

Aim: To study Newton’s first law of motion and inertia. What is required ? Two books and a smooth sheet of paper.

What to do? (i) Place the sheet of paper on the table with some part of it coming out of the edge of the table. Now stack two books on the paper. (ii) Remove the paper with a jerk and see the effect on books.

What do you observe? When the paper is removed with a jerk from below the books, the books do not change their position (Fig. 8.3).

Fig 8.3 Paper being removed with a jerk from below the books

What do you infer? We find that the books remain in their position unless something external is done. Even removal of paper from below them with jerk does not change their position.

ACTIVITY 8.2

Aim: To study the inertia of rest

What is required? A coin, talcum powder, a table with sunmica top or glass top.

What to do? (i) Strike the coin on a smooth floor and note the distance travelled by it. (ii) Now, sprinkle talcom powder on the floor and again strike the coin from the same place with the same force and note the distance travelled by it again. The distance travelled by the coin in straight line is different in the two situations.

What do you infer? We find that coin travels through much longer distance along a straight line on the floor when powder is sprinkled. The floor exerts a resistive force on the motion of the coin. But this resistive force is much less when powder is sprinkled on the floor, so coin travels much farther. If we imagine a completely smooth floor which offers no resistive force, the coin will continue to move on it with constant velocity unless some net external force is applied to stop it.

Inertia is a property common to all bodies in nature. You must have experienced that it is difficult to move a heavy body than a lighter one e.g., pushing a loaded box is more difficult than to push an empty box because heavy box has more inertia. So inertia of a body is characterized by the quantity called mass of the body. Thus, it can be said that the mass of a body is a measure of its inertia.

Some illustrations of first law of motion

(a) Why do you tend to fall while getting off a moving bus or why are you thrown forward when the moving bus stops suddenly and you are not cautious. What actually happens is that when a moving bus suddenly stops, your feet in contact with the bus are suddenly brought to rest while the rest of your body, which has acquired the same velocity as the bus, due to inertia of motion tends to move forward even after the bus has stopped.

(b) You would have noticed that when a moving trolley is stopped suddenly, sometimes the loaded goods fall from it. This is due to the inertia of motion of the goods. When the trolley stops suddenly, it comes at rest immediately. But because of inertia of motion, the goods placed in it try to remain in motion. Hence they fall from it. Now think, why does the ink comes out of a fountain pen when it is given a jerk?

Newton's second law of motion

You have seen that force changes or tends to change the state of motion of a body. When you throw a piece of stone in the air, you apply a force. Greater the force with which you throw the stone, the farther it goes, i.e., the greater the force, the greater is the change in motion of a particular body. But how does the motion of a body change when you apply some external force? To establish a relationship between the force and the acceleration produced in a body, Newton formulated his second law of motion. If you kick a football, it moves, but if you kick it very hard, it moves faster than before. Kicking harder means applying more force due to which football gains more acceleration and hence moves faster. It is seen that acceleration of a body is directly proportional to the force applied on the body.

Fig. 8.4 Passenger falling forward as the bus stops suddenly

The mass of the football is greater than that of the plastic ball. For the same force the acceleration produced in the plastic ball is greater than the acceleration produced in the football. So it can be said that the acceleration produced in a body depends on its mass and is inversely proportional to the mass.

Hence, we have, a = F/m ............... 8.1

Where, ‘a’ denotes the acceleration produced in a body of mass ‘m’ when a force ‘F’ is applied on it. Now, you know acceleration is a vector quantity and force is also a vector quantity, so the equation (8.1) may be expressed in the vector form as

a = F/m ...................... 8.2

Fig 8.5 Motion of (a) foot ball (b) plastic ball when the same force is applied on them

This shows that acceleration produced in a body is in the same direction as the applied force. Equation (8.2) represents Newton’s second law of motion which may be stated as the acceleration produced in a body is directly proportional to the unbalanced force acting on it and is inversely proportional to its mass. The direction of the acceleration is the same as that of the force.

Unit of force:

You can use equation (8.1) to define the unit of force. Equation (8.1) may be written as,

F = ma ............................ 8.3

In SI system of units, if m = 1 kg and a = 1 ms-2

then, F = (1 kg *1 m)/1s2 = 1kgm/s2

1kgm/s2 is called as 1 Newton whose symbol is N. Hence, the SI unit of force is Newton.1 Newton force is that force which on acting on a body of mass 1 kg produces in it an acceleration of 1 ms-2 i.e., 1N = 1 kg ms-2

You must note that equation (8.3) can be used to find out the acceleration or force applied or mass of a body provided any two of the three quantities namely force, mass and acceleration are known. If you put F = 0 in equation (8.3), you will get, ma = 0 But, mass m of a body can never be zero. Therefore, or a = v – u = 0 or v = u i.e., a moving body continues to move with the same velocity if no force acts on it. This is nothing but the first law of motion. So first law can be derived from the second law. Let us solve some problems using Newton’s Law of motion.

Example 8.1: What force accelerates a 50 kg mass at 4m/s-2?

Solution: Newton’s second law gives F = ma Here m = 50kg and a = 4ms-2 Therefore, F = 50 kg x 4ms-2 = 200 kg ms-2 = 200N (since 1N = 1kg ms-2 )

Example 8.2: If a force of 50 N acts on a body of mass 10 kg. then what is the acceleration produced in the body? Solution: Newton’s second law gives

F = ma or a = F/m Here, F = 50 N = 50 kg ms-2 and m = 10 kg

a= 50kgms-2 * 1/10 kg = 5ms-2

Momentum

You know that a moving body always has a mass and a velocity. These two quantities help us to define a new quantity called momentum. Thus, the momentum of a moving body is defined as the product of its mass and velocity, and its direction is same as that of its velocity.

So, we can say that all moving bodies have momentum. i.e. Momentum = Mass x Velocity p = m x v = mv You know, velocity is a vector quantity, so momentum is also a vector quantity directed along the velocity. In vector notation, p = mv Momentum plays an important role in the motion of bodies. We know that the acceleration of a body is defined as rate of change of its velocity.

v = Final velocity, u = Initial velocity and t = Time Newton’s second law of motion gives. F = ma Substituting (8.4) in the above equation we have,

F = ( Final momentum – Initial momentum ) / Time (according to the definition of momentum)

or F = ( Change in momentum ) / Time = Rate of change of momentum Hence the rate of change of momentum of a body is equal to the force acting on the body and is in the same direction.

This is another way of stating Newton’s second law of motion.

Unit of momentum: By definition, momentum is the product of mass and velocity. In SI units, the unit of mass is kg and that of velocity is m/s. Therefore, the unit of momentum is kg m/s or kgms-1 or Ns

8.2.5 Some illustrations of second law of motion

According to the Newton’s second law of motion the force is defined as

F = Rate of change of momentum

Force is large when time is small and when time is large, force becomes smaller, for the same change of momentum. The following examples are based on this concept.

(a) If a bundle tied with a string is lifted quickly by holding the string, the string snaps. Why? This is because a large force must be exerted on the string to quickly increase the momentum of the bundle.

(b) When a person falls on a cemented floor, why does he get hurt? The person has some initial momentum ‘mu’ which becomes zero when he comes to halt. Since the mass comes to rest within a very short time, very large force comes into action in order to produce a definite change in momentum (from ‘mu’ to zero), thereby hurting the person. On the other hand, if he falls on dry clay or husk or on a foam mattresses, he does not get hurt due to prolongation of time in making momentum zero and hence reduction of force.

(c) Why does a cricket player while catching a ball moves his hands backward? By doing so he increases the time duration in which the momentum of the ball becomes zero. As time increases, smaller force comes into action to produce the desired change in momentum, so his hands do not get hurt.

8.2.6 Newton’s third law of motion

You must have noticed that when you jump out of a boat suddenly, the boat moves in the backward direction. Why does this happen?

While jumping, your foot exerts a backward force on the boat (Fig. 8.6). This force is called the action. At the same time, a force is exerted by the boat on your foot, which makes you move forward. This force is known as reaction. Action and reaction forces are, equal in magnitude but opposite in direction.

Fig 8.6 A boy jumping out of a boat

Also you must have noticed that when an air-filled balloon is released, the balloon moves opposite to the direction of the air coming out of it. In this case the air coming out of the balloon exerts a force of reaction on the balloon and this force pushes the balloon backwards. If the air rushes out vertically downwards (action) the balloon moves vertically upwards (reaction). You can try it yourself at your home.

Therefore, forces always exist in pairs and they act on two different bodies. Newton’s third law of motion very clearly states that to every action there is always an equal and opposite reaction, and the action-reaction forces act on different bodies.

There are three significant features of this law:

(i) We cannot say which force out of the two forces is the force of action and which one is the force of reaction. They are interchangeable. (ii) Action and reaction always act on two different bodies. (iii) The force of reaction appears so long as the force of action acts. Therefore, these two forces are simultaneous.

8.2.7 An illustration of third law of motion

Working of jet plane and rockets: A jet plane takes in air, the fuel burns and then releases the burnt gases from the tail. As the burnt gases come out, the plane moves in the forward direction. If the force with which the gases escape is the action, then the force enabling the plane to move forward is the reaction.

Friction in Motion You might have noticed that a moving car begins to slow down the instant its engine is switched off. Why does it happen? In fact the car is slowed down by a force called friction, which exists between the surfaces of all materials which rub against each other.

8.3.1 Factors affecting friction

Friction is caused due to the irregularities i.e., elevations and depressions in the surfaces of sliding objects. These irregularities act as obstructions to motion. The direction of the frictional force is always in a direction opposite to the motion. Thus, if an object is to move at a constant velocity, a force equal to the opposing force of friction must be applied. In that condition the two forces exactly cancel one another and the net force on the body is zero; hence the acceleration produced in the body is zero. But zero acceleration does not mean zero velocity. Zero acceleration means that the body maintains its velocity, it neither speeds up nor slows down. The resistive force before the body starts moving on a surface is called static friction.

So it may be concluded that force needed to overcome friction, is necessary to maintain the uniform motion of a body. Air resistance is one type of frictional force. It is a common experience that it is difficult to walk on sand, but it is easier to walk on a metalled road. Greater the roughness, greater is the friction. More power is needed to develop same speed on the same road for a heavy truck than for a lighter truck. It is so because the heavy truck has greater normal reaction (reaction of road on the truck against the action or weight of the loaded truck) and hence greater frictional force.

8.3.2 Sliding and Rolling Friction

Once a body starts moving on a surface the friction between them is called sliding or kinetic friction. This friction is less than the static friction discussed above. You might have used a slide in your school or a park in your childhood to play with. Here force of friction is much less. You may be using a bicycle or a scooter to go from one place to another. The wheels are round and these roll over the road. Friction between wheels and road is rolling friction. This type of friction is least of all other types. Ball bearings do have rolling friction.

8.3.3 Advantages and disadvantages of friction

(a) Advantages of friction

• It helps vehicles to move. If there were no friction between vehicle tyres and the ground, the wheels of the vehicle might spin but the vehicle would stay where they were. Thus, vehicle tyres are designed to give as much friction as possible in all conditions.

• It helps us to walk. When you are walking, a force of friction is developed between the soles of your feet or shoes and the ground. This force causes us to move. Can you now tell, why we find it difficult to walk on slippery ground or even on the sand?

• We can easily walk, write on a page or black board due to friction.

(b) Disadvantages of friction

• Friction produces heating of the rubbing surfaces.

• Friction reduces efficiency of the machines as considerable amount of energy is wasted in overcoming friction.

• Friction causes wear and tear of surfaces and machine parts.

8.3.4 Control of friction

• Oil reduces friction by helping the surfaces slide over each other, move smoothly.

• Wheels of vehicles are usually mounted on ball or roller bearing to reduce friction.

• Some time friction is increased by making surface rough as in case of tyres of vehicles, stairs and ramps.

From what has been stated above one may conclude that the friction plays an important role in our daily life. This is the reason why friction is often termed as a necessary evil.

Force of Gravitation

It is every day experience that bodies like a ball thrown vertically upward comes back to the earth. Why does it happen so? We are even today fascinated how planets move around the sun and how various stars are there in their orbits or positions? All this has been possible due to the force of attraction between any two masses. Newton called it force of gravitation and he formulated a law connecting the force and masses of the two bodies involved. The interesting aspect of this gravitational force is that it is always attractive whatever may be the size of bodies.

8.4.1 Newton’s law of gravitation

On the basis of some observations, Newton found that the force of gravitation is directly proportional to the product of masses of the two bodies and inversely proportional to the square of the distance between the bodies. Mathematically,

F = G (m1*m2) / r2

Where G is called the universal gravitational constant. In S.I. units where m is measured in kilogram, F in newton and r in metre, G has a value 6.67 x 10-11 N m2/kg2. At once we see that for appreciable value of force, masses should be very large. The gravitational force due to earth is also known as gravity.

8.4.2 Acceleration due to gravity

Stand at the roof top of a three or more storeyed building with stones of different masses in your two hands and drop these together (Be careful don’t hurt anyone). Ask another person (an observer) to observe falling of the stones. You will find that both the stones fall simultaneously.

The earth’s gravity accelerates the bricks down. Since both reach ground together, this acceleration, called acceleration due to gravity (g), is same for both pieces and is same for any mass. That is ‘g’ is independent of the mass of the freely falling body. Its value changes from place to place on the Earth and it is 9.81 ms -2 at the equator. Its value is maximum at the poles.

8.4.3 Mass and weight

We know that acceleration due to gravity varies with geographical latitude and the gravitational force is an inverse square of force i.e. Fµ 1/r2 . However, the ratio of the gravitational force to the free fall acceleration for a given body at any point on the Earth is a constant.

The ratio F/g is a characteristic of a body and is known as the mass of the body according to Newton’s second law of motion. Thus, mass of a body is defined as the

ratio of the force of gravity acting on the body to the free fall acceleration. m = F/g. Mass is a scalar quantity and is measured in kilogram (kg). Mass is also defined as the matter contained in the body. At a given place the value of acceleration due to gravity is same for all masses-big or small. Hence force of gravity is proportional to the mass of the body.

The weight of a body at a given place is the force with which the Earth attracts the body towards it. The unit of weight is Newton. More massive a body is, more weighty it will be. Thus, the weight of the body W is

W = mass x acceleration or W = mg

Mass : Mass is the amount of matter contained in a body. It is a scalar quantity. Mass of a body is a constant quantity. Mass is measured with a beam balance. Unit of mass is kg. It is gravitational force with which Earth attracts a body towards it. It is the ratio of gravitational force on the body to the acceleration due to gravity.

Weight: Weight is force, hence a vector quantity. Weight of the body changes from place to place. Weight is measured with a spring balance. Unit of weight is N.

Example 8.3: A body weighs 49 N at a place where g=9.8 ms-2. What will be its weight at the pole where g=9.82 ms-2.

Solution: w=mg

weight at pole w = mg = 5 kg x 9.8 ms-2 = 49.10 N

Example 8.4: A 1kg body falls from a height of 60 m from rest on a planet where acceleration due to gravity is 120 ms-2. Calculate the velocity of the body when it touches the planets surface.

Solution : Initial velocity of the, body, u = 0 acceleration, a = g = 120 ms-2 height through which body falls = h = 60m. Using the equation of motion v2 = u2 + 2gh we have on substitution of values v2 = 02 +2 x 120 ms-2 x 60 m

= 1,4,400 m2 s-2 v = 120 ms-2

Example 8.5: Two bodies A and B weighing, 2N and 6N are dropped from the roof of a 10 m high building together. Which A or B will reach the Earth first?

Solution : Since the two bodies have same initial velocity (i.e. zero) and have same acceleration (g) i.e. acceleration due to gravity acts on both A and B, hence, both will reach Earth together irrespective of their masses.

8.4.4 Motion under gravity and free fall

The force with which Earth attracts a body, stationary with respect to Earth, is called the weight of the body. When a body falls freely from some height (say, top of a building) it does so due to a force called weight. When no other force like air resistance except gravity, acts on a falling body, it is called a free fall and body acquires acceleration under its weight.

There are many situations, like a person in a lift or satellite which has some acceleration with respect to the earth other than ‘g’, the weight of the person is not same. He exerts less or more force on his support than his actual weight. If the system, somehow, has acceleration a =g, the person feels no weight. This is known as the weightlessness. It has several implications for an astronaut.

Thrust and Pressure Rockets and jet planes eject burnt gases with force. The gas in turn react on the rocket or jet plane with force called thrust. Similarly water coming out of a plastic or rubber pipe exert thrust on it. Thrust and pressure are properties of fluids (gases and liquids). The total force exerted by a fluid (liquid or gas) on any surface in contact with it, is called thrust. Thrust is measured in newton (N). The thrust exerted by fluid at rest per unit area of the surface in contact with the fluid (liquid/gas) is called pressure. The air exerts pressure, you fill air in cycle or scooter/motor cycle tyres upto a certain pressure, the astronaughts and soldiers in places like Siachin wear pressure suits to avoid bleeding. Blood pressure becomes higher than atmospheric pressure at high attitudes. Under sea water, pressure put by it on the body is quite high. The normal atmospheric pressure is one atmospheric pressure and according to the definition

p = F/A, its value in Nm-2 is 1 atmosphere = 1.014 x 105 Nm-2 Normal atmospheric pressure is also measured in terms of centimetre or milimetre of mercury, 1 atm = 76 cm = 760 mm of mercury. The SI unit of pressure is Nm-2. Pressure is measured in pascal (Pa)

1 Pa = 1 Nm-2 The air or gas pressure is also measured in bar or torr. 1 atmos = 1.014 bar and 1 torr = 1 m m of mercury – 133.3 N m-2 1 atmos = 760 torr

A practical application of pressure is the shape of a paper pin, needle and nail. The tip of these is made very narrow so that these pierce the object with greater pressure.

Buoyancy : How a ship floats on sea? If there is no water below or insufficient water, the ship will sink to the sea bottom. The weight of the ship acts downward and water pushes it upward. Similarly a diver in a swimming tank is also pushed up when he jumps from the board into the water. The force exerted by water or any liquid or gas on a body immersed in it, in the upward direction, is called the up-thrust or buoyant force or simply buoyancy.

In the early days of space flights, buoyancy of Earth’s atmosphere posed a big problem in the re-entry of space vehicles in Earth’s atmosphere and their safe landing on Earth. The principle of buoyancy comes from Archimedes principle.

8.5.1 Archimedes principle

When you put a piece of stone on the surface of water it sinks but boat can float on it. Why does this happen? Greek scientist Archimedes gave the principle which could explain such things. According to Archimedes principle when a body is immersed, wholly or partially in a liquid (or fluid), it undergoes an apparent loss in its weight, which is equal to the weight of the liquid displaced by the body.

8.5.2 Applications of Archimedes principle

(i) Flotation of bodies : Suppose the weight of a body is W and weight of liquid or fluid displaced is w then body will float immersed or partially immersed when W = w or W < w. (ii) To determine the specific gravity of the body (iii) To find the volume of a body.

Example 8.6: The mass of a body in air is 1 kg. What will be its weight in the liquid of specific gravity 1.2 if it displaces 100 ml of this liquid? (Take g = 10ms-2). Solution : Weight of the body in air = 1 kg x 10ms-2 – 10N. Density of liquid = 1.2 x 103 kgm-3 Volume of the liquid displaced – 100 ml = 100 x 10-6 m3 Buoyant force = 100 x 10-6 m3 = 1.2 N Loss in weight of the body =1.2 N Weight of body in the liquid = 10N – 1.2N = 8.8 N.

In Text Questions

1. Is there any force applied when (i) you push the wall of a house? (ii) the speed of the cycle is increased? (iii) the player changes the direction of football by using his head?

2. What is the property by virtue of which a body tends to remain stationary?

3. When a bus suddenly starts, the passengers feel a backward jerk. Why?

4. If forces of same magnitude are applied on two bodies of masses 2 kg and 4 kg, which of the bodies will have more acceleration?

5. If a body of mass 5 kg moves with a velocity of 10 ms-1, then what is the momentum of the body?

6. If a force of 10N produces an acceleration of 2 ms-2 in a body, how much force would be required to produce an acceleration of 4 ms-1 in the same body?

7. What is the acceleration of an aeroplane moving in a circle around the Earth, if the passenger in it feels weightlessness?

8. Does any force act on a body in a free fall.

9. Density of kerosene is 0.8 g cm-3 and density of water is 1 g cm-3. Which one will exert greater buoyant force on a body?

10. A body is immersed in a liquid. If the liquid displaced by the body weighs 20g then what is the buoyant force acting on the body?

11. If the weight of a body is 10 g and the buoyant force is 7g, will the body sink or float?

12. Why is straw used to drink a soft drink?

13. Sailors generally say that a person is easily drowned in a river than in sea, why?

Terminal Questions

1. Define force. Is it a vector quantity? What is its unit?

2. State Newton’s first law of motion.

3. Explain why is it dangerous to jump (or alight) from a fast moving bus or train?

4. Why do the dust particles from the hanging blankets fall off by beating with a stick?

5. Which law helps you to find the magnitude of the force acting on a body of mass ‘m’moving with an acceleration ‘a’? State the law.

6. Define momentum. Is it a vector quantity? What is the unit of momentum?

7. How is the rate of change of momentum related to force?

8. Which will have greater momentum—a truck moving with a speed of 60 km/ h or a train moving with the same speed? Justify your answer.

9. Find the acceleration produced in a body of 2 kg mass when a force of 10N acts on it.

10. What force accelerates a 50 kg mass at 6 m/s?

11. A force of 60N accelerates a mass of 15 kg from rest. Find the velocity at the end of 6 seconds.

12. Explain the effect of friction on motion.

13. Give an example to show that friction is useful as well as harmful to us.

14. What is an inverse square law of gravitation?

What you have learnt

- If a body stays at the same position with time, it is at rest.

- If the body changes its position with time, it is in motion.

- Motion is said to be rectilinear if the body moves in the same straight line all-the time, e.g, a car moving in a straight line on a level road.

- The motion is said to be circular if the body moves on a circular path: e.g, the motion of the tip of the hand of a watch.

- The total path length covered by a moving body is the distance travelled by it.

- The difference between the final and initial position of a body is called its displacement.

- Physical quantities are of two types (i) scalar: which have magnitude only, no direction (ii) vector: which have magnitude as well as direction.

- Distance, speed, mass, time, temperature etc. are scalar quantities, whereas displacement, velocity, acceleration, momentum, force etc. are vector quantities.

- Distance travelled in unit time is called speed, whereas, displacement per unit time is called velocity.

- Position-time graph of a body moving in a straight line with constant speed is a straight line sloping with time axis. The slope of the line gives the velocity of the motion.

- Velocity-time graph of a body in a straight line with constant speed is a straight line parallel to time axis. Area under the graph gives distance travelled.

- Velocity-time graph of a body in a straight line with constant acceleration is a straight line sloping with the time axis. The slope of the line gives acceleration.

- For uniformly accelerated motion : v = u+at

s = ut + 1/2 at2

where u = initial velocity, v = final velocity, and s = distance travelled in t seconds.

10

Thermal EnergyYou are aware that energy is required for all types of activities. In the previous lesson youhave learnt about mechanical form of energy. Heat is also a form of energy, called thermalenergy. Fire has heat in it . When fuels like coal, petrol, wood, kerosene-oil are burnt, heatis produced. You would have noticed that in winter season, when it is cold, generallypeople rub their palms to warm up. Here, doing mechanical work against friction producesheat. You must have learnt that in ancient times man used to produce fire by rubbing twopieces of stone together. Even now a days we produce fire by the same method when werub the tip of a matchstick on the special surface of the matchbox.

Why do we need heat? We require heat to cook, to iron clothes, to have hot water forbathing in winter season, to melt solids, to vaporize the liquids, etc. Why do the wet clothesget dried when hanged in sunlight? Have you seen an iron smith heating an iron rod red hotand then beating it to give the required shape of a knife or a scissor? You must have got achance to see a gold smith working with flames of a lamp in designing an ornament. Whatis the use of flame? In thermal power plants coal is burnt to generate electricity. In steelindustry and glass industry, iron and glass are melted to give them definite shapes. Steamengine can pull a train due to the power of steam. In all these activities heat is used. Let uslearn all about heat and its effects in this lesson.

OBJECTIVESAfter completing this lesson, you will be able to• differentiate between heat and temperature;• explain that heat is transferred from one body to another when there is a temperature

difference between the two bodies;• describe construction, calibration and use of thermometers;• explain the effect of heat on matter resulting in thermal expansion of solids, liquids

and gases;• explain the constancy of temperature of a substance during change of phase even though

heated continuously;• state the factors upon which the total transferable heat of a body depends;• calculate heat flow from a hotter body to a colder body in contact;• predict the variation in melting point and boiling point of materials due to the presence

of impurities and with variation in pressure;• explain why the food gets cooked easily and quickly in a pressure cooker.

: 172 : Thermal Energy

10.1 WHAT IS HEAT?Heat is a form of energy. We call it thermal energy. It is measured in joule. Sunrays have heatin them. This heat is called radiant heat. It travels with the speed of light i.e. 3 x 10 8 m s -1.

10.1.1 How is heat produced?Rub your palms together. What happens? They become warmer, indicating generation ofheat. Here friction is generating heat. When you burn coal, wood or kerosene oil, fire isproduced. Fire has heat energy in it. Here, the chemical energy gets converted into heat bythe process of burning.

Fig. 10.1 Rubbing the palms Fig. 10.2 Fire has heat energy together makes them warmer

10.1.2 Heat is energy of molecular motionEvery material is made up of molecules, which arein a state of continuous random motion. This isdue to the heat in them. When we heat up thismaterial, this molecular motion increases. Thissuggests that heat is kinetic energy of molecularmotion.

Kinetic energy of a body in motion can beutilized in doing work against frictional forces. This results in the heating up of the body.It is due to transfer of kinetic energy from the moving body to the molecules. Let us performan activity to demonstrate conversion of mechanical energy into heat energy.

ACTIVITY 10.1

Aim: Demonstration of conversionof mechanical energy into heatWhat to do?i) Keep bicycle on its stand and

rotate the paddle with hand sothat the rear wheel rotates veryfast.

ii) With the help of a pad of clothon your finger tip, touch the rimof the wheel to stop the wheel. Fig. 10.4 Conversion of mechanical

energy into heat energy

Fig. 10.3 Molecular motion increaseswith absorption of heat

Brake rubber

Thermal Energy : 173 :


At the finger tip you feel that cloth has become hot.

What do you conclude?The kinetic energy of motion of the wheel has been transferred to the cloth due to frictionand it appears in the form of heat.

10.1.3 Heat can lead to work

You might have seen water boiling in a kettle. Dueto steam formed in the kettle, its lid moves up anddown. This shows that heat can do work.

You must have seen a steam engine pulling along array of coaches. Thus, heat can be utilized todo work. Thus, we can conclude that heat is a formof energy since it can do work. Also, heat andwork are inter convertible.

The device that converts thermal energy intomechanical work is called heat engine.

10.1.4 Temperature and need for its measurement

How will you measure the hotness of a given body? You may suggest that this can be donesimply by touching the body. It means feeling of hotness by our hand can be used toestimate how hot a body is. But sometimes it may be difficult (if the body is very hot andmay cause burns) and sometimes the conclusion may be confusing. Can you have a wrongsensation of hotness by touch?

ACTIVITY 10.2Aim: Our sense of touch may be misleading

What to do?

i) Take three bowls A, B and C. Fill ice cold water in bowl A, ordinary tap water inbowl B and hot water in bowl C (Fig. 10.6).

ii) Now dip your left hand in bowl C containing hot water and right hand in bowl Acontaining ice cold water and let them remain there for two minutes.

iii) Now take your hands out of both bowls and put both of them in bowl B containingtap water.

Fig. 10.6 Sense of touch may be misleading

Fig. 10.5 Heat can do work

R R L L

(a) (b) (c)


What do you feel?

You will be surprised to note that your left hand will give you the sensation that this water iscold, while the right hand will give you the sensation that it is warm.

Thus, confusing sensations can be felt by skin. The difficulty in using the sensation asa measure of hotness arises because of the fact that the terms hot and cold are relativeterms and cannot be used in the absolute measurement of hotness.

Therefore, there is a need of some standard for the measurement of the hotness of abody.

The degree of hotness of a body is called its temperature. It is measured by devicescalled thermometer. It is represented as a number on a thermometric scale.

10.1.5 Difference between heat and temperature

Heat is energy in transit, which is transferred from one body to another due to temperaturedifference between them. While heat is a form of energy, the temperature is the degree ofhotness of a body. Heat is measured in Joule while the temperature is measured in degreeFahrenheit (o F), degree Celsius (o C) or Kelvin (K).

10.1.6 Various types of scales for measurement of temperature

The thermometers in common use have two different types of scales of measurementsnamely Fahrenheit and Celsius scales of temperature. For scientific work, Kelvin scale oftemperature is more often used. However, the construction and working of thesethermometers is same.

It is obvious that a hotter body would show higher temperature and a colder body alower temperature on the same scale. The thermometers cannot have confusing or wrongsensations.

10.1.7 Construction and use of a thermometer

Mercury thermometers are the most common thermometers in use.Mercury is filled in a thin walled glass bulb joined at the end of acapillary tube by the process of repeated heating and cooling. Themercury is seen in the form of a thin dark thread in the capillary. Thespace above the mercury level in capillary is evacuated. The other endis now sealed. Mercury has the property of uniform thermal expansionover a wide range of temperatures. This means, the length of themercury thread in the thermometer increases by same amount for eachdegree rise in its temperature. The tip of the mercury thread can beeasily seen in the transparent glass tube as shown in Fig 10.7.

Calibration of mercury thermometer

To calibrate a scale on a thermometer, two fixed points are marked,the lower fixed point or ice point and upper fixed point or steam point.

To mark the ice point, the bulb of thermometer is placed in avessel containing mixture of water and crushed ice. When the level ofthe mercury becomes stable, a mark is put at the position of the tip of

Fig. 10.7 Mercurythermometer

Constriction

Mercury in bulb

Capillarytube


mercury thread in the glass tube. This is called ice-point. Next, the same bulb is placed insteam just above boiling water in a vessel. The position of the tip of mercury thread changesdue to thermal expansion of mercury in the bulb. A mark is again made on the glass tube atthis new position of the tip of the mercury thread. This is called steam point.

Now to mark a Celsius scale on this thermometer, zero is written at the ice point markand 100 is written at the steam point mark. The length between these two marks is thendivided into 100 equal parts. This now becomes a Celsius thermometer.

To mark a Fahrenheit scale, 32 is written on the ice-point mark and 212 is written onthe steam point mark. The length between these two marks is then divided into 180 equalparts. This now becomes a Fahrenheit thermometer.

Fig. 10.8 Method of calibration of a thermometer

In a clinical thermometer, the marks are shown only in the range 95 0F to 110 0F.[These are the two limits of human body temperature beyond which human beings cannotsurvive].

Kelvin scale can be marked on a Celsius scaleby writing 273 at ice point and 373 at steam point.Thus, each mark is calibrated with a value higherby 273 than on Celsius scale. The Kelvin scalebegins with the lowest possible temperature as itszero, which is –273.15 0C. This temperature is alsocalled absolute zero.

To measure the temperature of a hot body,the bulb of the thermometer, is put in contact withthat body. Mercury in the bulls expands, resultingin the increase of the length of the mercury threadin the glass capillary. The position of the tip of the mercury thread on the scale(calibrated on the capillary) is read. This gives the value of temperature. When you measuretemperature of a cooler body, mercury contracts, length of mercury thread decreases and itgives the value of temperature. As mercury does not stick to glass, the receding tip ofmercury thread does not leave any mercury in empty part of capillary, which could causeerror in the reading.

Fig. 10.9 Calibration of thermometers indifferent scales

Ice point

Steam point

Steam point

Ice point

212212

32 32 35

37

44

C F

110

98.6

95


ACTIVITY 10.3

Aim: To measure the temperature of a patient

What to do?

i) Take a clinical thermometer (also calledDoctor’s thermometer) (Fig. 10.7).

ii) Wash the thermometer in running cold water under a tap,rinse carefully and give a few jerks to bring tip of themercury thread below 95 0F.

iii) Now put the bulb end of the thermometer in the mouthunder the tongue of the patient for about 2 minutes.

iv) Now take it out gently and read it. This gives the bodytemperature of the patient.

v) Is it more than 98.6 0F? If yes! The patient has fever. It may be somewhere inbetween 97 0F and 98.6 0F, if the patient does not have fever.

vi) Wash it again in running tap water; hold it from the other end and give it 3-4 jerksso that the thermometer reading reduces to 95 0F.

vii) Now put the bulb of the thermometer under the armpit of the patient inside theshirt and keep it slightly pressed. Hold it for about 2 minutes.

viii) Take it out gently and note the thermometer reading.ix) Is it about 1.0 0F lower than before?

What do you conclude?

The mouth temperature called the body temperature is about 10 higher than armpittemperature.

To know the body temperature of an infant who cannot keep the bulb of the thermometerin his mouth, the temperature of the armpit is measured and then 10 is added to thisreading to find the body temperature and decide if he has fever.

10.1.9 Relation between Fahrenheit and Celsius scales of temperature

Let us solve the following examples:

Example 10.1: A thermometer reads the temperature of some hot liquid as 100 0F. Whatwould be the reading of the Celsius thermometer used to measure this temperature?

Solution : You have known that Fahrenheit scale starts from 32 0F instead of 0 0C. Both ofthese are the ice points. Also steam points on these scales are marked as 212 0F and 100 0C,respectively.

Thus, 180 divisions of F scale are equivalent to 100 divisions of C scale.

Hence, 1 division of F scale = 100/180 divisions of C scale

Now if F is the reading on the F scale, then

Number of divisions above ice point are = F – 32

Therefore, value of (F – 32) divisions of F scale = (100/180) x (F – 32) divisions of C scale

Fig. 10.10 To measure thetemperature of a pateint


i.e. reading of Celsius thermometer will be = (100/180) (F – 32) = C

or100

C

180

32F =−

5

C

9

32F =−

This becomes the required formula to convert any reading of F scale to C scale or viceversa.

In the present case F = 100

Celsius degree 78.379

340)32100(

9

5C ==−=

≈ 37.8 0C

Example 10.2: Which temperature has same numerical value on Fahrenheit scale andCelcius scale of temperature ?

Solution: Here, we are given F = C

Therefore, in the conversion formula 5

C

9

32F =−,

put F = C, we get

5

C

9

32C =−

5C – 160 = 9C → C = – 400

Thus –40 0C = –40 0F

Example 10.3: What would be the value of 80 0C on Kelvin scale?

Solution: Since Kelvin scale readings are higher by 273 than on Celsius scale, the valueon Kelvin scale is 80+273 = 353 K

Kelvin scale is used in system international (SI) to report the temperature. However,in laboratory we use only Celsius scale for measuring temperature.

On Kelvin scale, the temperature is mentioned in Kelvin onlyand not degree Kelvin.


State whether the following statements are True or False.

1. Heat can be measured in Kelvin. (T/F)2. –30 0F is a lower temperature than – 30 0C. (T/F)3. The numerical value of temperature of any hot body measured on Kelvin scale is

always higher than measured on Fahrenheit Scale. (T/F)4. Thermal energy can be measured either in calories or Joules. (T/F)


5. Pure alcohol can also be used as thermometric liquid. (T/F)6. When we touch a cold body, heat flows from our hand to the cold body. (T/F)

10.2 EFFECTS OF HEAT

When objects are heated, they may show a change in their shape, size, colour or sometimesin their state. However, the magnitude of change depends upon the quantity of heat absorbedby the object.

10.2.1 Solids expand on heating

Have you ever faced a problem of openingthe jammed metallic cap of an inkpot?Sometimes, it is too much tightly closed.Place the inkpot in a wide vessel containinghot water for few minutes. Now take it outand try to open the cap. It opens easily. Why?Metallic cap undergoes thermal expansionin its size (more than the mouth of inkpot which is made of glass) due to absorption ofheat from the hot water and therefore, gets loosened.

The phenomenon of expansion of solids is used for various purposes.

(i) Fitting of tyres on wheels: Do you know, how is the iron ring mounted on the woodenwheel of a horse-cart? The radius of the iron ring is slightly less than that of the woodenwheel. It, therefore, cannot be easily slipped on to the rim of wooden wheel. The ironring is, therefore, first heated to a higher temperature so that it expands in size and thehot ring is then easily slipped over to the rim of the wooden wheel. Cold water is nowpoured on the iron ring so that it contracts in size and holds the wooden wheel tightly(Fig. 10.12a)

(a) Fitting of tyres on wheels (b) Gaps in railway tracks at joints

(c) Thermostat in electrical appliance

Fig. 10.12 Some applications of thermal expansion

Fig. 10.11 Method to open tightly closedmetallic cap of an inkpot

Iron ring expandson heating

Aluminium

Brass

Aluminiumexpands more


(ii) Gaps in the railway track at joints: You must have noticed gaps at the jointsin a railways track. Why is it left like that? If this gap is not left then duringsummer the iron rail will expand due to hot weather and will get bent at the joints(Fig. 10.12b).

(iii) Thermostat in electrical appliances: Thermostat is a temperature control device. Itis a bi-metallic strip made up of two different metals having different expansivity. Asthe temperature rises, due to unequal thermal expansion, the strip bends. Due to thisthe contact breaks and the circuits gets disconnected. Similarly, it can be used to makecontact as temperature rises and thus, to switch on a circuit, as in case of a fire-alarm(Fig. 10.12c). Thus, bimetallic strip is a technical application of differential expansionof metals.

10.2.2 How to measure the expansivity of the material of a body?

All substances do not expand by the same amount when heated through the same differenceof temperature. Also it is seen that the same substance expands by a different amount whenheated to a different temperature. It is found that larger the rise in temperature, larger is theexpansion. It is understood that the ratio of change in length (∆L) to the original length (L)is directly proportional to the rise in temperature (∆t) of solid bodies ; i.e.

tL

L ∆∝∆

orL

L∆ = t∆α

α is a constant and depends on the nature of the material of the body. It is called linearcoefficient of thermal expansion of the material. It is measured in per degree celsius. It isdefined as fractional increase in length for each degree rise in temperature.

Example 10.4: The length of a steel rod at room temperature of 25 0C is 20.00 cm. Whatwould be its length when its temperature is raised to 325 0C? [Given linear coefficient ofthermal expansion of steel as 0.000012 0C-1 ].

Solution : Since ,

L

L ∝∆= t∆α and t = 300 °C

or ∆L = L α t = 200 x 0.000012 x 300 = 0.072 cm

Therefore, the increased length will be 20.00 + 0.072 = 20.07 cm.

Please note that the result is rounded off to 2nd decimal place because 20.00 cm, theterm with smallest decimal place in addition has 2 decimal places.


ACTIVITY 10.4

Aim: To study the expansion of water

What to do?

i) Take a small glass bottle (say a used medicine/ injectionbottle). Fill it with water up to the rim.

ii) Take the thin plastic tube of a used, empty ball-pen refill.Warm it, bend it and pass through a cork into the mouth ofthe bottle.

iii) Now heat the bottle gradually. Do you find droplets of watercoming out of the bent tube?


Liquids expand on heating

Mercury is a liquid. The property of thermal expansion of mercury has been used inthe construction of a thermometer. Different liquids expand by different extent for thesame rise in temperature.

Gases also expand on heating. It is important to know that unlike solids and liquids allgases expend by same amount for the same rise in temperature. Thus heating causes expansionof solids, liquids and gases. However, in case of liquids and gases we measure their volumeexpansivity. It is found that fractional increase in volume of liquids or gases is directlyproportional to rise in their temperatures, i.e.

tV

V ∝∆

or tV

V γ=∆

Where, γ is a constant called volume coefficient of thermal expansion, which is differentfor different liquids. It is defined as the fractional increase in volume for each degree risein its temperature. It is also measured in per degree Celsius. For gases this constant has theunique value 1/273 per Kelvin.

It is interesting to note that unlike other liquids, water expands when it freezes into ice.Also when water is heated from 0 0C to 4 0C, its volume decreases. But further heatingbeyond 4 0C results in volume expansion.

You must have noticed that if water bottles or cold drink bottles are left in the freezerof a refrigerator for some days, they crack. Similarly, there is bursting of water pipes underextreme cold conditions at hill stations. This is due to the fact that water expands on freezinginto ice. Table 10.1 shows the values of linear and volume coefficients of thermal expansionof some materials. It is seen that volume coefficient of thermal expansion is equal to threetimes the linear coefficient of thermal expansion.

Fig. 10.13 Expansion ofliquids


Table 10.1: Coefficients of linear expansion and volumeexpansion for some substances

Material Coefficient of linear Coefficient ofexpansion (oC-1) volume expansion (oC-1)

Quartz 0.4 x 10 -6 1.2 x 10 –6

Steel 8 x 10 -6 24 x 10 -6

Iron 11 x 10 –6 33 x 10 -6

Silver 18 x 10 –6 54 x 10 -6

Brass 18 x 10 –6 54 x 10 -6

Aluminium 25 x 10 –6 75 x 10 -6

Lead 2.9 x 10 –6 8.7 x 10 -6

10.2.3 Heating causes change of state of matter

When a solid material is heated, its temperature rises. When the temperature reaches acertain value, the solid starts melting. The temperature remains constant till whole of thesolid material gets melted. This temperature is called the melting point (M.P.) of the material.It is a characteristic temperature for the material. It does not depend upon the shape or sizeof the solid. Different materials have different melting points.

ACTIVITY 10.5

Aim: Determination of melting point of ice

What to do?

i) Take some crushed ice in a cooking utensil.Place a thermometer in it and note down itstemperature (it should be 0 0C) (Fig. 10.14).

ii) Now heat it on a gas stove slowly. Do yousee conversion of ice into water? Keep aneye on the level of mercury thread of thethermometer. Does it change?

iii) Keep on heating till whole of the ice getsmelted. What is the temperature? Is itconstant at 0 0C. Heat further. Do you find that the temperature of water is nowincreasing?


You will find that the ice melts at 0 0C and the temperature of ice-water mixture remainsconstant at 0 0C till whole of ice gets melted.

Repeat this activity for other solids to find their melting points. You can perform asimilar activity with boiling water to find its boiling point. You have to take care that

Fig. 10.14 Determination ofmelting point of ice


thermometer measures the temperature of steam a little above water surface. If it dips inboiling water the water must be quite pure.

Whenever there is a change of state between solid and liquid or liquid and gaseousstates, the temperature does not change even though the heat is either continuously absorbed(as in the process of melting or boiling) or continuously given out (as in the process offreezing and liquefaction) by the material under observation.

Table 10.2: Melting points and boiling points of some materials

Material Melting Latent heat of Boiling Latent heat ofpoint (oC) fusion (kj/kg) point (oC) evaporation

(kJ/kg)

Helium -271 - -268 25.1

Hydrogen -259 58.6 -252 452

Air -212 23.0 -191 213

Mercury -39 11.7 357 272

Pure water 0 335 100 2260

Aluminium 658 322 1800 –

Gold 1063 67 2500 –

10.2.4 Effects of impurities on melting point and boiling point

Pure substances have definite melting points and boiling points characteristics of thematerial. But on addition of impurities their values change. Let us study this with the helpof some activities.

ACTIVITY 10.6

Aim: To find out effect of impurities on melting point of ice

What to do?

i) Take two containers A and B. In container A, put some pure water and crushed pureice. In container B, ice is mixed with about 1/3rd its weight of powdered salt. Observethat in B some ice melts and a saturated solution of salt is formed.

ii) Measure the temperature of liquid in both the containers. Obviously, temperature of icein any container is same as that of its liquid. In which container is temperature lower?

iii) The temperature is lower in B.


Presence of impurities lowers the freezing point/melting point.


Activity 10.7

Aim: To find out effect of impurities on boiling point of water

What to do?

i) In the above activity 10.6, heat both the containers until the water starts boiling.ii) Note the boiling point of water in the both containers, keeping the bulbs of the two

thermometers inside the levels of respective boiling liquids.


The boiling point of salted water is higher than that of pure water.


Presence of impurities increases the boiling point.

10.2.5 Effect of pressure on melting point and boiling point

The melting and boiling points of a material also change with the change in atmosphericpressure. Let us study the effect of pressure on melting point and boiling point with thehelp of some activities.

ACTIVITY 10.8Aim: To study the effect of pressure on the melting point of a substance

What to do?i) Take an ice block, a wooden block and a wire.ii) Press the wire to first cut the ice block and then the wooden block. You cannot cut a

wooden block by pressing a wire on it though wood is softer. Why does the wirepass through the ice block easily?


The pressure applied through the wire melts the ice in immediate vicinity allowing thewire to pass through it. Thus, the melting point of ice is lowered with increase in pressure.

It should be noted that in case of all solids other than ice, the volume of liquid obtainedon melting is generally larger than solid volume. Water is an exceptional case. In suchsolids, which increase in volume on melting, the melting point increases with increase ofpressure.

All liquids expand on evaporation. Hence, increase in pressure will obstruct the changeof phase on boiling. This results in an increase in the boiling point of liquids with increasein pressure.

10.2.6 Cooking is easier in pressure cookerIn a pressure cooker, (which is air tight from all sides), when water together with vegetablesis heated, its temperature rises. Initially, when valve of the cooker is open, water boils toform steam at 1000 C. This steam so formed occupies larger volume than what it had inliquid state. Now the valve is closed. The steam, having no exit to come out, exerts pressure


on the surface of water in the cooker, whichstops boiling. More heat is now supplied. Thisincreases the temperature of water withoutallowing the remaining water to boil any more.Thus, inside a pressure cooker, there is steamand water at higher temperature and at highpressure. The higher temperature and pressurequickly softens the vegetable and causes thequicker cooking of food.

There is always a certain weight put on thenozzle of the lid of the pressure cooker. If theforce due to the pressure of the steam exceeds this weight, the weight gets lifted and someof the steam leaks out and reduces pressure. Do you now understand why it is calledpressure cooker?

The importance of pressure cooker for persons living at hill stations is very great. Theatmospheric pressure in hilly areas is lower due to the high altitude, and thus, water startsboiling at a lower temperature. In such a situation if the ordinary utensil is used for cookingfood (especially food like rice and pulses), it will take a long time, resulting in wastage ofprecious fuel.

10.2.7 Latent heat

We have already discussed in the previous parts of this section that the temperature does notchange during the change state even though heat is continuously supplied to the material.What happens to this heat supplied? It is used up wholly in changing the state of the substance.Therefore, it does not appear in the form of rise in temperature of the body. This is, thereforecalled latent heat (or hidden heat). Its value is constant and is different for different materials.

Latent heat of a material is defined as the amount of heat required to completelychange the state of unit mass of that substance either from solid to liquid or liquid togaseous state. It is generally denoted by capital letter L and is measured in Joules perkilogram (J/kg). When material changes from solid to liquid, it is called as latent heat offusion and when the state changes from liquid to vapours it is latent heat of evaporation.

Do you understand why does water filled in a clay pitcher become cold even whenplaced inside a room? In this case, water drops leaking through the fine pores of claypitcher absorb heat for evaporation from the water inside. Therefore, the inside water getscooled.

Example 10.5: How much thermal energy is required for complete melting of 10 kg of iceat 0 0C to form water at 0 0C?

Solution: Thermal energy for melting m kg ice at its melting point

= m L

= 10 x 335 kJ

= 3350 kJ

Fig. 10.15 Cooking is easier in a pressure cooker


Heat required for a mass m kg of a substance for change in state at its melting point orboiling point is = mL joule.

10.2.8 Sublimation

Some solid substances when heated directly change to gaseous state without becomingliquid. This process is called sublimation.

ACTIVITY 10.9

Aim: To study the sublimation of camphor

What to do?Take some camphor tablets in a spoon and heat thespoon slowly over a gas stove.

What to observe?Do you see fumes coming out and camphor graduallyvanishing without melting?

What do you conclude?This shows that camphor sublimates on heating.

Naphthalene balls (used for preserving woolen clothes) and iodine are also sublimesubstances.


Fill in the blanks with the correct choice.

1. A bimetallic strip is used as a thermostat in the electrical device named ___________(geyser, camera, T.V.)

2. If the mass of a substance is doubled, its melting point will _____________ (be lowered,be raised, remains same)

3. When solid ice is heated, the volume of the water formed on melting is _________ theinitial volume of the solid ice. (more than, less than, same as)

4. Latent heat of evaporation is measured in _______________ (J, J/k, J/kg)

5. The water containing little salt dissolved in it boils at a temperature _______ 100 0C.(higher than, lower than, equal to)

10.3 THERMAL EQUILIBRIUM

When two bodies at different temperatures are brought in contact heat energy will alwaysflow from the body at higher temperature to the body at lower temperature, till both thebodies acquire the same temperature. The two bodies are then said to be in thermalequilibrium.

Fig. 10.16 Camphor sublimateson heating


ACTIVITY 10.10Aim: To study the state of thermal equilibrium

What to do?

i) Take a steel tumbler. Fill it 2/3rd with tap water. Put a thermometer in it and measureits temperature.

ii) Now take a large heavy metallic spoon which can be inserted in the tumbler. Heat iton a flame and put it in the tumbler and keep an eye on the temperature scale of thethermometer.


Does the temperature of the water rise? Does the temperature stops rising after sometime? Touch the spoon with the thermometer bulb and note the temperature of the spoon.Is the temperature of spoon same as that of water?


The heat energy keeps on flowing from the hot body to the cold body till both acquiresame temperature. This is called state of thermal equilibrium.

10.3.1 Can we measure the amount of heat transferred?Heat gets transferred from a hotter body to a cooler body in contact. The larger the

quantity of heat transferred, larger would be the rise in the temperature of the colder bodybefore a state of thermal equilibrium is achieved.

Therefore, the heat energy transferred is proportional to the rise in temperature of thecold body. Similarly, heat energy lost by the hot body is proportional to the fall in temperatureof the hot body.

ACTIVITY 10.11

Aim: To study the factors on which the heat transferred from a hot body to a cold bodydependsWhat to do?i) Take two identical vessels A and B and put equal amount of tap water in both of

them.ii) Now take another larger vessel C containing some water and heat it on a gas stove till

it boils. Note its temperature.iii)Now pour a small quantity of water from vessel C into vessel A and larger quantity of

water into vessel B. Note the new temperatures of water in vessels A and B.


The temperature of water in vessel B is more than that of water in vessel A.

What do you conclude?The vessel B, in which larger quality of boiled water was added, has been given largerquantity of heat. Thus, the quantity of heat transferred not only depends on the temperatureof the hot body but also depends upon its mass.


The quantity of heat (H) transferred from a hot body is proportional to (i) mass (m)and also to (ii) fall in temperature (t).

H ∝ m x tor H = s x m x t

Where, s is a constant of proportionality and is called specific heat of the material ofthe body. It is a characteristic constant of the material of the body and does not dependupon the shape or size or mass of the body. Since s = H/m x t, specific heat of a materialcan be defined as the amount of heat required to raise the temperature of unit mass of thatsubstance through unit degree. In S.I., it is measured in J/kg 0C or Jkg-1 0C -1.Using the concept of conservation of energy

Heat given by hot body = Heat received by colder body

Example 10.6: How much thermal energy is required to raise the temperature of 10 kg ofwater form 25 0C to 100 oC? [Given specific heat of water s = 4200 J kg-1 C-1].

Solution: Heat required = m x s x t = 10 x 4200 x (100 –25) J = 315 0 kJ

Example 10.7: A hot iron ball of mass 1.0kg and specific heat 3000 J kg-1 0C-1 at temperature60 0C is placed in water of mass 3.0kg at a temperature 25 0C. Calculate the final temperaturewhen thermal equilibrium is achieved. Neglect the heat sharing by the vessel containingwater.

Solution: Let the final temperature of the mixture be θ 0CThen, heat given by the iron ball = ms t = 1 x 3000 (60- θ)JHeat taken by water = 3 x 4200 (θ – 25) JSince heat given = heat taken

1 x 3000 x (60- θ) = 3 x 4200 (θ – 25)or 180000 – 3000 θ = 12600 θ – 315000or 15600 θ = 495000

This gives θ = 31.7 0C


Which of the following is the correct alternative?

1. Two iron balls of radii r and 2r are heated to same temperature. They are dropped intotwo different ice boxes, A and B, respectively. The mass of ice melted(a) will be same in the two boxes (b) in A will be twice than in B(c) in B will be twice that melted in A (d) in B will be 8 times that

melted in A

2. An iron ball A of mass 2 kg at temperature 20 0C is kept in contact with another ironball B of mass 1.0 kg at 20 0C. The heat energy will(a) flow from A to B only (b) flow from B to A only(c) not flow form A to B or B to A (d) flow from B to A as well as

A to B


3. When solid ice at 0 0C is heated, its temperature(a) rises immediately. (b) falls(c) does not change until whole (d) first rises then falls back to 0oC

of it melts

4. Which of the following bodies when gently dropped in a vessel containing water at200C will cause highest rise in the temperature of water?a) An iron ball of mass 1.0 kg at temperature 50 0C.b) A brass ball of mass 2.0 kg at temperature 40 0C with specific heat half that of

iron.c) A block of ice of mass 0.1 kg at temp –10 0C.

5. When steam at 100 0C is heated its temperaturea) does not changeb) increasesc) decreases

LET US REVISE

• Heat is a form of energy while the temperature is the degree of hotness of the body.• Heat energy is measured in joule while the temperature is measured either in degree

Fahrenheit (0F) or degree Celsius (0C) or in Kelvin (K).• Mercury is used as a thermometric substance, because it is opaque and does not stick

to the walls of the glass capillary. Also it has uniform coefficient of thermal expansionover a wide range of temperature.

• A Fahrenheit scale of temperature is related to Celsius scale of temperature by the

relation 5

C

9

32F =−

• The Kelvin scale is related to Celsius scale by the relation K = 273 + °C• All substances expand on heating i.e. a rise in temperature.• Linear coefficient of thermal expansion of a solid material is defined as the increase in

length per unit length per degree Celsius rise in temperature. It is measured in 0C-1.• Volume coefficient of thermal expansion of a solid material or a liquid or gaseous

material is defined as the change in volume per unit volume per unit rise in temperature.It is also measured in 0C-1 .

• Volume coefficient of thermal expansion of a solid material is equal to three times itslinear coefficient of thermal expansion.

• Different substances expand to different extents when heated for same rise intemperature.

• Bi-metallic strip is a technical application of differential expansion of solid metals. Itcan be used as an on/off switch in electrical circuits in response to a rise in temperature.

• Melting point and boiling point of a material are characteristic temperatures for thatmaterial. They do not depend upon their shape or size.

• Melting point of a substance decreases while its boiling point increases with mixingof impurities.


• Melting point and boiling point change with rise in pressure. The solids (like ice) whichcontract in volume on melting show a fall in their melting point with rise in pressure. Theboiling point of all liquids increases with rise in pressure.

• The temperature of substances remains constant when heat energy is supplied at theirMelting point and boiling point. This is used in changing their phases and is calledlatent heat. It is measured in joule per kg.

• Heat always flows from a body at higher temperature to another body in contact atlower temperature. It keeps on flowing till both the bodies acquire a common finaltemperature and a state of thermal equilibrium is achieved.

• Heat transferred is equal to mass × specific heat × change in temperature.

• In all heat transfer cases; heat given by hot body is equal to heat taken by cold body.

TERMINAL EXERCISES

Descriptive type questions.1. What is the difference between the temperature of a hot body and its thermal energy?

2. What happens to the temperature of a body when it changes its state from liquid to solid?

3. On what factors does the thermal expansion in a wire depend?

4. What is the difference in the units of specific heat and latent heat of substances.

5. Name any two uses of a bimetallic strip.

6. If you have a mercury thermometer without any calibration, how will you make a (i)Celsius scale (ii) Fahrenheit scale for it?

7. Why is the mercury used as a thermometric substance?

8. Why does a bimetallic strip bend on heating?

9. Heats of fusion and vaporization of substances are often referred to as latent heats.Why?

10. When some water in a tea kettle is heated on a gas stove, it always takes a much lessertime for the water to start boiling than for all the water to vaporize? Why is it so?

11. Why is the steam-burn far more serious than the one obtained from a spilling hotwater.

12. A solid substance expands on melting. What will happen to its freezing point when thepressure is reduced, just like at a hill station?

13. At what temperature the numerical value of Fahrenheit scale will be just double of thaton Celsius scale? (Ans. 160 0C or 320 0F)

14. A 50 cm silver bar becomes shorter by 1.0 mm when it is cooled. How much was itcooled. Given coefficient of linear expansion for silver = 18 x 10-6 C –1.

15. The iron rim of a wagon wheel has an internal diameter of 1.000 m when the temperatureis 150 0C. What would be its diameter when it cools off to 25 0C? (Coefficient of linearexpansion for iron = 12 x 10-6 0C -1)

16. How much heat energy is required to change 200 g of ice at –20 0C to water at 70 0C?[Given latent heat of fusion of ice = 335 kJ/kg and specific heat of ice =2100 j/kg 0C]


17. A 2.0 kg block of iron at 100 0C is dropped into a 0.75 kg of water contained in a 0.325kg copper Calorimeter. If the initial temperature of water and Calorimeter was 12 0C,what will be the final temperature.Given specific heat of iron = 105 cal kg -1 °C-1; specific heat of water = 1000 calkg–10C-1; specific heat of copper = 93 cal kg–10C-1 (use 1.0 cal = 4.25J)

18. A heavy box of mass 200 kg is pulled along the floor for 15 m. If the coefficient ofsliding friction is 0.4, how much heat energy is developed?

19. A 50 g bullet of lead at 27 0C fired form a rifle moves with a velocity of 200 m s-1.What temperature would it attain when it stops after the impact? [Given specific heatof lead = 130 J/kg 0C]. Assume that entire heat generated by impact goes to the bulletand not to target.


10.1

1. F 2. T 3. F 4. T 5. F 6. T

10.2

1. hot water geyser 2. remains same 3. less than

10.3

1. (d) 2. (c) 3. (c) 4. (a) 5. (b)

GLOSSARY

Heat: A form of energy which gives us sensation of warmth.Latent heat of fusion of a solid: The amount of heat required (in joules) to convert 1

kg mass of the solid into its corresponding liquid state at its melting point.Latent heat of vaporization of a liquid: The amount of heat required to convert 1 kg

of the liquid into its corresponding gaseous state at a constant temperature.Principle of Calorimetry: In case no heat is lost to the surroundings and no change of

state is taking place, the heat lost by hot body is equal to the heat gained by the cold body,when these are brought into contact.

Specific heat of a substance: Defined as the amount of heat required (in joule) toraise the temperature of 1 kg of a substance by 1 0C (or 1 K).

Sublimation: The process in which a solid changes into its gaseous state directlywithout passing through liquid state.

Temperature: A numerical measure of hotness of a body which determines the directionof flow of heat. Heat always flows from a body at higher temperature to a body at lowertemperature.

Thermal equilibrium: Implies that the two bodies are at the same temperature andhence no net heat transfer is taking place between them.

Thermal expansion: Implies the increase in the size of an object on heating.Principleof Calorimetry: In case no heat is lost to the surroundings and no change of state is takingplace, the heat lost by hot body is equal to the heat gained by the cold body, when these arebrought into contact.

Thermometer: A device used for measuring temperature.Thermostat: A temperature control device usually made of a bimetallic strip.

11

Light EnergyCan you read a book in the dark? If you try to do so, then you will realize, how much weare dependent on light. Light is very important part of our daily life. We require light for anumber of activities. Even the plants on which we depend, need light for their foodproduction. Without light we feel helpless. Truly speaking, life is not sustainable withoutlight.

It is an experience from our early childhood that objects become visible in presence oflight. You see the objects when the light after reflection from them falls on your eyes andthus makes their image at the retina of your eye. In fact, light is a form of energy and henceit is invisible, although the presence of light gives us the ability to see the things around us.

You may have seen in torches that there is a curved sheet of metal around the bulb.Can you think why is it so? We are very fond of looking at the image of our face in alooking glass. Do you know how the image is formed? You would also have noticed thatwhen a rod is placed in a tumbler of water, it appears bent. What has caused the rod to bend? We see that the stars twinkle on a clear night, that on a clear day the sky appears blue, atthe time of sunset or sunrise the sky near horizon appears orange red. Have you ever triedto find out the reason for such natural events? In the present lesson you will find theanswers to all such questions. You will also study about some man-made otpical instrumentslike microscope and telescope in this lesson.OBJECTIVESAfter completing this lesson, you will be able to:• recognise the importance of light in day to day life;• define the reflection of light and state the laws of reflection;• describe the image formation by plane and curved mirrors with suitable ray diagrams;• use mirror formula and define magnification;• define refraction of light and state the laws of refraction;• give some examples from nature showing refraction of light;• explain the refraction of light through prism and rectangular glass slab;• describe the types of lenses and explain the image formation by convex and concave

lenses with the help of ray diagrams;• use lens formula and define magnification;• explain the power of lens and define dioptre;• describe briefly the construction and working of the instruments, like simple micro-

scope, compound microscope and astronomical telescope.

: 192 : Light Energy

11.1 REFLECTION OF LIGHT

Can you think how does an object become visible to you. When we see an object, we do sobecause light from the object enters our eyes. Some objects such as sun, stars, candle,lamp etc. may emit their own light, called luminous objects. Some other objects may bounceback a part of the light falling on them from other luminous objects. This bouncing back ofthe light after falling on any surface is called reflection of light. The light bounced backfrom the surface is called reflected light.

Thus, when a beam of light travelling through a mediumcomes in contact with an object, a part of it gets bouncedback (however, a part of it is absorbed and some part of it isable to penetrate through the object). This phenomenon iscalled reflection of light.

Some objects having smooth and shiny surfaces reflectlight better than others. A smooth shining surface, whichreflects most of the light incident on it is mirror. The reflectionof light from a plane mirror is shown in Fig 11.1

While studying the reflection of light, you will come across different terms related toit. They are given below :

• Ray can be defined as the direction of propagation of light.• Beam of light consists of a number of rays.• Incident ray is the ray of light that falls on the reflecting surface.• Normal is the name given to a line drawn at 900 to the surface at the point where the

incident ray strikes the surface.• Angle of incidence is the angle between the normal and the incident ray.

11.1.1 Laws of reflection of light

Suppose, a ray of light (IO) falls on a reflecting surface AB at O, after reflection it goesalong OR, as shown in Fig 11.1. The reflection of light from the surface takes place accordingto the following two laws:

(i) First law of reflection: The incident ray, the reflected ray and the normal at the pointof incidence, all lie in the same plane.

(ii) Second law of reflection: The angle of incidence is equal to the angle of reflectioni.e., ∠i = ∠r

11.1.2 Types of reflection

Depending on the nature of the surface the reflection oflight can be of two types:

(i) Regular reflection: When the reflecting surface isvery smooth and the rays of light falling on it arereflected straight off it, then it is called regularreflection, as shown in Fig. 11.2.

Reflection

ray

Incidentray

Norm

al

IR

N

O

Angle

of

reflection

Angle

of

incid

ence

r l

Fig. 11.1 Reflection of lightfrom a plane mirror

Incident rays Reflected rays

Smooth plane surface

Fig. 11.2 Regular reflection

Light Energy : 193 :

(ii) Diffused reflection : When the reflection of lighttakes place from rough surfaces, the light is reflectedoff in all directions, as shown in Fig. 11.3. It dependson the angle of the incidence on the part of the surfaceit hits. This is called diffused reflection.

Do you know ?

The rough surface diffuse or scatter the light falling on it and prevent the formationof image. Light is reflected from the paper of this book also but the surface ofpaper is much rougher than mirrors. That is why no image is formed by the paper.

You might have seen people putting frosted window glass pane? Have youever thought why are frosted glass used? The frosted glass has a rough surfacewhich does not allow the light to form clear images. Instead, the rough surface ofglass diffuses the light and no clear image can be seen through it .

How do we see non-luminous objects?

Sunlight or light from a lamp incident over non-luminous objects undergo regular as wellas diffuse reflection. When these reflected rays strike the retina of our eyes, an image ofthat object is formed in the eye, and thus we are able to see the objects.

11.1.3 Formation of images due to reflection

You know that a mirror is a good reflector of light rays. Daily at least once a day, you mustbe using a mirror to see your face. What do you actually see in the mirror? You see yourimage.

The images are of two types – real and virutal.

(i) Real image: The images which are obtained by the actual intersection of reflectedrays, are called real images. The real images can be cast on a screen. In case of sphericalmirrors real images are formed on the same side of the mirror as the object.

(ii) Virtual image: The image obtained when the rays appear to meet each other butactually do not intersect each other, are called virtual images. They cannot be cast ona screen. Virtual images are formed behind the mirror.

To understand the formation of image by a plane mirror, let us do an activity.

ACTIVITY 11.1

Aim : Image formation by a plane mirrorWhat is required?A plane mirror, a few pins and a sheet of paper.What to do?(i) Spread the sheet of paper over a soft, smooth wooden plank or a piece of card

board.

NormalIncident

rays

Normal

Reflectedrays

Rough surface

Fig. 11.3 Diffused reflection


(ii) Put the mirror M1 M

2 in a vertical position over the sheet as shown in figure

11.4

(iii) Put two pins, one at ‘A’ little far from the mirror and the other one very near tothe mirror at ‘B’ so that, the line AB makes an angle with the line M

1 M

2

showing the position of the mirror.

(iv) Look at the images A and B of the two pins through the mirror, put two otherpins at C and D so that all four pins A, B, C and D are in the same straight line.

(v) Now, look at the images of all these pinsclosing one of your eyes and moving your facein side ways. If the image of the two earlierpins and the two pins you have put just now,appear to be moving together you can say yourobservation is free-from parallax error.

(vi) Join the positions of pins by straight lines.

(vii) Keeping the first pin as it is, take out otherthree pins and repeat the experiment describedabove by putting the pins in new positions.This way take a few more readings.

What do you observe?Besides the formation of image of the pins by the mirror, you are able to trace thedirections of various incident and reflected rays.

To understand the formation of image, you may consider the light rays emergingout of the object A. We have drawn only three rays namely (a), (b) and (c). Theserays after striking the mirror M

1 M

2 get reflected in the directions (d), (e) and (f),

respectively, (as shown in the figure 11.4) obeying the laws of reflection.

It is clear that these reflected rays never meet with each other in reality. However,they appear to be coming (emerging) out from the point A´, inside the mirror i.e., ifthe reflected rays (d), (e) and (f) are extended in the backward direction, they willall meet with each other at A´. Thus, at A´ we get the image of object A.

From the above activity we find that the image formed by a plane mirror has thefollowing characteristics:This image is virtual (i.e., not real), erect and same in size as the object.• The object distance and the image distance from the mirror are found to be equal

i.e., OA = OA´.Hence, the image of a point in a plane mirror lies behind the mirror along the normal

from the object, and is as far behind the mirror as the object is in front. It is an erect andvirtual image of equal size.

11.1.4 Few important facts about reflectionPut your left hand near a plane mirror. What do you see? You will find that on reflection,the image of the hand appears as right hand as shown in Fig. 11.5 (a). Similarly, the number2 will appear in an inverted fashion on reflection as shown in Fig. 11.5 (b).

A

A

O M2

a b c

a

b

c

B

B C O

Fig. 11.4 Image formation by aplane mirror


Hence, due to reflection in a plane mirror left handedness is changed into righthandedness and vice-versa. This is known as lateral inversion.

For example a left handed screw will appear to be right handed screw on reflection asshown in Fig. 11.5(c).

Fig. 11.5 Lateral inversion in image formed by a plane mirror

Do you know ?

(i) If you are approaching towards a plane mirror, even your image will also appearto be approaching towards you.

(ii) A woman can see her full image in a plane mirror whose height is half of herheight. See the ray diagram in Fig. 11.6 and try to understand why this happens.

Fig. 11.6 Size of plane mirror to see full image


1. Name four luminous objects.2. Name the phenomenon of bouncing back of light from a rigid surface.3. What is the relationship between the angle of incidence and the angle of reflection?4. Although the light is reflected from the book you read, why is your image not visible

in it?5. Give two differences between diffused and regular reflection.

11.2 REFLECTION AT CURVED MIRRORS

A curved mirror is a section of a hollow sphere whose inner or outer surface has beenpolished. Thus, there are mainly two types of spherical mirrors-convex mirror and concavemirror.

mirror mirror

(a) (b)

mirror

(c)

B

AGH

F

C

h

E

f


ir

Incident ray

Angle ofincidence

Angle of

reflection

Reflected ray

Curvedmirror

Fig. 11.7 Reflection of light bycurved mirrors

R

Radius of curvature

Principleaxis

x y

R

Radius of curvature

Principleaxis

x y

Hollow glass sphere Hollow glass sphere

Concave mirror Convex mirror

(i) Convex mirror: It is a mirror in which the reflection takes place from the outer or thebulging side (i.e. the polishing is on the inner side) as shown in Fig 11.7 (a).

(ii) Concave mirror: It is a mirror in which the reflectiontakes place from the hollow side (i.e., the polishingis on the outer-side) as shown in Fig. 11.7 (b).

For understanding the reflection at spherical mirrors,certain important terms are very useful. They are as shownin Fig 11.8 and defined below.(i) Pole (P): It is the mid-point of the spherical mirror.

(ii) Centre of curvature (C): It is the centre of thehollow sphere of which the spherical mirror is a part.

(iii) Radius of curvature (R): It is the distance between the pole and the centre of curvatureof a spherical mirror.

(iv) Principal axis: It is the imaginary line joining the pole with the centre of curvature.

(v) Principal focus (F): The rays of light parallel and close to the principal axis of themirror after reflection, either pass through a point (in concave mirror) or appear to becoming from a point (in convex mirror) on the principal axis; this point is calledprincipal focus of the mirror.

(vi) Focal Length (f): It is the distance between the pole and the principal focus of themirror.

Fig. 11.8 Some terms in image formation by spherical mirrors

Relationship between focal length and radius of curavture

Focal length (F) of a spherical mirror is equal to half of the radius of curvature (R) of thatmirror. In mathematical terms it can be written as,

fR=2

11.2.1 Rules of image formation by spherical mirrors

The ray diagram for image formation by mirrors can be drawn by taking any two of thefollowing rays :

(i) Central ray: The ray of light striking the pole of the mirror is reflected back at thesame angle on the other side of the principal axis (Ray no. 1 in Fig. 11.9).


(a) (b)

P

1

2

3

3

2

1

1

2

1 2

PC

F

F

CA

D

P

(a) When the object issituated at a

(b) (c)AF

B

ACB F

BB

’

A'

(e)CB'

A

B

D

P

D

P

(b) Object beyond c (c) Object at c

(d) Object between then c and f (e) Object at f (f) Object between f and p

BF

A

C DPB

CF

A'

B'

Real, inverted, highly diminishedimage at focus

Real, inverted highly diminishedbetween C and F

Real, inverted highly image of the samesize as object at C

Real, inverted, enlarged imagebeyond C

Real, inverted, highly enlargedimage at infinity

Virtual, erect, enlarged imagebehind the mirror

(ii) Parallel ray: For a concave mirror the ray parallel to the principle axis is reflected insuch a way that after reflection it passes through the principal focus. But for a convexmirror the parallel ray is so reflected that it appears to come from principal focus (Rayno.2 in Fig 11.9).

(iii) Ray through centre of curvature: A ray passing through the centre of curvature hitsthe mirror along the direction of the normal to the mirror at that point and retraces itspath after reflection (Ray no.3 in Fig 11.9).

Fig. 11.9 Image formation by spherical mirrors (a) Concave mirror (b) Convex mirror

Now, let us see how images are formed by concave and convex mirrors when theobject is placed in different positions.

(a) Formation of image by concave mirror

Using the above said rules of image formation, the ray diagram for the image formed fordifferent positions of object are given below:

Fig. 11.10 Formation of image by a concave mirror


In all these diagrams we have considered two rays starting from a point at the top ofthe object. The image is formed where these rays intersect after reflection.

(b) Formation of image by convex mirror

In case of convex mirror, the formation of the image is shown in Fig 11.11. The incident ray AQparallel to principal axis is reflected such that it appears to come from focus F. The incident rayAN towards the centre of curvature being normal tothe mirror is reflected back along the same path.These two reflected rays appear to be coming fromthe common point A´, which is the image of point A.

The image formed by convex mirror isbetween pole P and focus F, virtual, diminished,and erect.

In convex mirror, whatever may be the position of the object infront of the mirror, theimage formed is always virtual, erect, diminished, (i.e, smaller than the pize of the object)and is situated between the pole and the focus.

11.2.2 Uses of mirrors

The different types of mirrors have different uses in our daily life. Let us study them oneby one.

(i) Plane mirror is used• in looking glasses,• in construction of kaleidoscope, telescope, sextent, and periscope etc.,• for seeing round corners,• as deflector of light.(ii) Concave mirror is used• as shaving and makeup mirrors,• as a reflector in search light, head light of motor cars and projectors etc,• for converging solar radiation in solar cookers,• as mirror for the dentists,• in flood lights to obtain a divergent beam of light to illuminate buildings,• in reflecting telescopes large concave mirrors are used.(iii) Convex mirror is used• as a rear view mirror in motor cars, buses and scooters, etc,• as safety viewers at dangerous corners and on upper deck buses

11.2.3 Sign convention and mirror formulaTo measure distances with respect to a curved mirror, following convention is followed:(i) All distances are measured from the pole of the mirror.(ii) The distances measured in the same direction as incident light, are taken as positive.(iii) The distances measured against the direction of incident light, are taken as negative.(iv) The distances above the principal axis are taken positive, whereas, below it are taken

negative.Using the sign convention, the relationship between object distance (u), image distance

(v) and the focal length for a curved mirror is given by,

A

B P

N

B'

F C

A'

Fig. 11.11 Formation of image by convex mirror


1 1 1

f u v= +

You can use this formula to find out any of the quantities, provided the other two aregiven.

11.2.4 Magnification in spherical mirrors

Often we find that a spherical mirror can produce an enlarged or magnified image of anyobject. The ratio of the size of the image to the size of the object is called linearmagnification.

i.e., linear magnification (M) = = −size of image (I)

size of object (O)

v

u

Where, v = image distance from mirror, and u = object distance from mirrorPositive value of M tells that image formed is erect while negative value of M indicatesthat an inverted image is formed.


1. What is the focal length of a plane mirror?

2. Write the position and nature of image formed by a concave mirror when the object isplaced between the focus and centre of curvature.

3. List any two differences between real and virtual images.

4. What type of mirror is used to view the rear objects by an autodriver?

5. If an object of 5cm size is placed infront of a concave mirror, the size of the imageformed by it is 7.5 cm, what is the linearmagnification of the mirror?

11.3 REFRACTION OF LIGHT

When a light ray passes from a less dense mediumto a more dense medium (e.g., from air to glass), itbends towards the normal (Fig. 11.12) and when itpasses from a denser medium to a less dense medium(e.g., from glass to air) it bends away from the normal(Fig. 11.12). This phenomenon of deviation of lightrays from their original path, when they pass from one medium to another, is called refractionof light.

ACTIVITY 11.2Aim : To study the refraction through a glass slab

What is required ?A glass slab, drawing sheet, pencil, drawing board, alpins, protector, and a scale.

What to do?(i) Place glass slab on a drawing sheet fixed on a wooden drawing board, sketch a

2

31

2

13

r

i

i Air

r

(a) (b)

Fig. 11.12 Refraction of light


pencil boundary. Draw a line OC meeting theboundary line obliquely.

(ii) Fix the pins A and B on that line. Now look forthese pins through the other side of the glass slab.

(iii) Take a pin and fix it on the sheet such that A, Band E are in a straight line.

(iv)Now fix another pin F such that it is in a straightline with pins A, B and E. Remove the slab and thepins.

(v) Draw a line joining the points F to E to meet theboundary at D.

(vi)Join point C to D by a dotted line.

What do we observe?As shown in Fig. 11.13, the line ABC gives the direction of incident ray on theglass slab while the line DEF gives the direction of emergent ray. The line CDgives the direction of refracted ray. Draw normals N

1CN

2 at C and N

3DN

4 at D to

the boundaries. Now check the indication of these rays. Do you find that the refractedray D has slightly bent towards the normal to the boundary at C?


The ray of light when goes from a rarer (air) to a denser (glass) medium, it bendstowards the normal. Also, the ray of light when goes from denser (glass) to rarer(air) mediums it bends away from the normal.

11.3.1 Refractive index of the medium

When the light travels from one medium to another medium, the speed of light changes.Aray of light from a rarer medium to a denser medium slows down and bends towards thenormal. On the other hand the ray of light going from a denser medium to a rarer medium,is speeded up and bends away from the normal. It shows that the speed of light in differentsubstances varies. Therefore, different substances have different abilities to bend or refractlight. We call this bending ability of a material as the index of refraction or refractive indexof that material.

The refractive index (µ) of a material is defined as the ratio of the speed of light invacuume to that in the material medium.

Therefore, refractive index of a medium (µ) =speed of light in vacuum

speed of light in medium

The refractive index of a rarer medium is less as compare to that of a denser medium.

11.3.2. Laws of refractionThe extent to which a ray bends, depends not only on the refractive index of medium, butalso on the angle of incidence. The laws of refraction are :(i) First law of refraction: The incident ray, the refracted ray and the normal at the point

of incidence all lie in the same plane.(see fig. 11.13)

Fig. 11.13 Refraction througha glass slab

A

B

E

F

D

C

O


(ii) Second law of refraction: The ratio of the sine of the angle of incidence to the sine ofthe angle of refraction is constant and it is equal to the refractive index of the medium.This law is also called as the Snell’s law of refraction.

Refractive index (µ) = =sine of angle of incidence

sine of angle of reflection

sin

sin

i

r

11.3.3 Application of refraction of light

(i) If you look at a coin placed at the bottomof a container full of water, you will noticethat it appears to be raised as shown in Fig.11.14. You know that an object is visibleonly when the rays of light from the objectreach your eyes. In the first case, whenthere is no water in the container, the coinwill not be visible to you from the side ofthe container as shown in Fig. 11.14(a),because the rays of light traveling in astraight line do not reach your eyes. But on pouring the water into the container, therays of light from the coin change their direction as they travel from water (densermedium) into air (rarer medium) and thus, reach your eyes. Thus, the coin becomesvisible to your eyes. The rays now appear to be coming from C1 instead of C. In thisway, the coin appears to be raised.The ratio of the actual depth of the coin to the apperent depth of the coin is equal to the

refractive index of the liquid of the container.

Refractive index (µ) =actual depth

apparent depth

(ii) Another example of refraction observed in our daily life is the twinkling of stars.Visibility of the sun before actual sunrise or after actual sunset can also be explainedon the basis of refraction of light.

(ii) You would have observed that a pencilhalf kept in water in a glass appears tobe bent. When the part of a pencil is keptinside the water in a glass, it appears tobe broken or bent with respect to the partoutside the water as shown in Fig 11.14(b). This is also due to the bending oflight rays when they pass from water toair.Try to explain these events and discuss your answer with your teacher or fellow students.


1. What happens when a ray of light passes from one medium to another of differentdensity?

Water

Actualdepth

Apparentdepth

Air

1 Rupee

1 Rupee

Fig. 11.14 (a) Apparent depth of a coin inwater

Position of the pencilas it appears when seenfom above

Actual positionof the pencil

Ray of light suffersbending here

Air

Water

Fig. 11.14 (b) The pencil inside water appearsbent

C

C´


2. Why do the stars twinkle at night?3. What happens to a ray of light, if it enters a glass block along its normal?

11.4 REFRACTION THROUGH CURVED SURFACE

In the present discussion under this section, we will confine ourselves to the refraction oflight through lenses only. Do you know what is a lens? A lens is a portion of a transparentrefracting medium bounded by two spherical surfaces. Because the lenses are made fromspheres, they are called as spherical lenses. They are mainly of two types :

• Convex lens• Concave lens

(i) Convex lens: A convex lens is thick inmiddle and thin at the rim. It makes theparallel rays of light to converge andcome to a point. Hence, it is also calleda converging lens. The convegingproperty of a convex lens is shown inFig. 11.15(a).

(ii) Concave lens : A concave lens is thinin the middle and thick at rim. It makesthe parallel rays of light to spread froma point. Hence it is also called adiverging lens. The diverging property of concave lens is shown in Fig. 11.15(b).

The point at which the incident rays parallel to principal axis will converge upon afterrefraction in a convex lens is called its principal focus. Where as in a concave lens thepoint from where incident rays parallel to the principal axis of the lens appear to be coming,is called as its principal focus (F).

11.4.1 Rules of image formation by lenses

In order to draw the image formed by any lens only two rays are required. These two raysare:(i) A ray parallel to the principal axis of the lens after refraction, converges upon (appears

to diverge off) the principal focus of a convex (concave) lens.(ii) A ray towards the optical center falls on the lens symmetrically and after refraction

passes through it undeviated.Let us now see the image formation in cases of convex and concave lens in different

situations of the objects.(a) Image formation by convex lensAccording to the above said rules of image formation, the position and nature of the imageformed for different positions of object is shown by the following ray diagrams: (see Fig.11.16).(i) If the object is placed between the optical centre O and first focus F

1, the image is

formed on the same side of lens and it is virtual, upright and magnified.(ii) If the object is at first focus F

1 ,the image is at infinity and it is real, inverted and very

much magnified.

Fig. 11.15 Types of lenses(a) Convex lens (b) Concave lens


Fig. 11.16 (a) Object placed between Fig. 11.16 (b) Object at the first focusoptical centre and first focus

(iii) If the object is between F1 and 2F

1, the image is beyond 2F

2 on the other side of the lens

and it is real, inverted and larger in size.(iv) If the object is at 2F

1, the image is at 2F

2 on the other side of the lens and it is real,

inverted and is of same size as object.

Fig. 11.16 (c) Object is between F1 and 2F

1Fig. 11.16 (d) Object is at 2F

1

(v) If the Object is beyond 2F1, the image is inbetween F

2 and 2F

2 on the other side of the

lens and is real, inverted and diminished.(vi) If the object is at infinity, the image is at F

2 on the other side of the lens and is real,

inverted and very much diminished.

Fig. 11.16 (e) Object is beyond 2F1

Fig. 11.16 (f) Object is at infinity

(b) Image formation by concave lens

The image formed by a concave lens is always smallerthan the object, erect and virtual and is formed betweenfocus and optical centre on the same side as the objectwhatever be the position of object as shown in Fig.11.17.

11.4.2 Sign convention and lens formula

In case of the spherical lenses,

(i) all distances in a lens are to be measured fromoptical centre of the lens,

(ii) distances measured in the direction of incident ray are taken to be positive,(iii) distances opposite to the direction of incident ray are taken to be negative.

Rays coming fromthe object at Infinity

I

F

(Image formed at F)

F(Object at F)

(Refracted parallel rays meet atinfinity. So , image is formed at infinity.

O F

2F 2FF F

I

O

Image is formedat 2F

2F F O F

2F

O

O

2F F O F

2F

Image betweenF and 2F

(Object beyond 2F)F FO

O

(Object isbetweenF and O)

Eye of the observer

P

Q

P'

Q'

O

Fig. 11.17 Image by concave lens


(iv) the height of the object or image measured abovethe principal axis are taken positive whereas belowit, are taken negative.Using the above mentioned sign convention, in Fig.

11.18 let us assume, distance of object PQ from theoptical center O = OQ = (-u), distance of iamge P´Q´from the optical center O = OQ´ = (+V), and focal lengthof lens = OF´

2 = (+f).

The relationship between u,v and f for a lens is as shown below:

1 1 1

v u f− =

This is called lens formula.

Focal length for convex lens is positive, whereas, for concave lens it is taken negative.

11.4.3 MagnificationYou would have noticed that in case of some lenses, the size of the image of an object isenlarged whereas in some other cases it is diminished. If we take the ratio of the size of theimage to the size of the object for a particular lens it remains constant for that lens. Thisratio of the size of the image to that of the object is called as the magnification of the lens.

i.e., magnification (m) = size of image

size of object= I

O

also, m = I

O

v

u=

A positive value of m tells that the image is erect and negative value of m tells that theimage is inverted.


1. If an object is placed at the focus of a convex lens, what will be the position and natureof the image?

2. Draw the ray diagram to show the image formed by a concave lens.

11.5 DISPERSION OF WHITE LIGHT

We are sure, you must have observed seven brillient colours of light in your surrounding.The separation of white light into its constituent seven colours is called dispersion of light.

11.5.1 Dispersion of light through glass prism

When the white light passes through a glass prism, it gets splitted into seven differentcolour rays.

In fact, the white light is supposed to be made up of seven colours. Different colouredlight have different wavelengths. The refractive media like glass have different values ofrefractive indices for different colours. You should know that as we go from violet to redwavelength of light increases . The violet part of incident white light get refracted of thesurface PQ at angle <r

0 which is different than angles of refraction for other colour-rays.

Q

P

F1

f

u v

F2

Q'

C2

OC1

+

+–

– P'

Fig 11.18 Sign convention in lenses


As a result of which different coloured light rays are seperated from each other. Thus, onemergence through the face PS, they get further separated resulting in the dispersion andforming a spectrum.

ACTIVITY 11.3

Aim : To produce a spectrum using a prism and sunlight.

What is required ?

A shoe box, knife, a transparent white paper.

What to do ?(i) Take an empty shoe box. Make a rectangular opening on its cover with a knife

and close it with a transparent white paper to see the spectrum.(ii) Make a thin slit with knife on the

opposite side cardboard of the shoe box.(iii) Place the prism on a block inside the

box as shown in the figure 11.19.(iv) Turn the slit-side face of the box

towards sunlight.(v) Do you see coloured strips on the

transparent paper?What do we conclude ?We can see that a brilliant pattern of the colours is formed in the sequence of

Violet, Indigo, Blue, Green, Yellow, Orange and Red which can be written asVIBGYOR.

If you repeat the same activity with a glass slab, you will find that a glass prismshows dispersion of white light but a glass slab does not? Can you think of Why?The emergent beam refracted through the other face of glass slab is a parallel beamand therefore, does not get separated. To produce dispersion, the emergent beamshould be divergent.


1. What is the sequence of colour in a spectrum of white light formed by prism?2. Which colour has minimum wavelength? Violet, yellow or green.3. For which colour the value of refrative index is more – orange or blue?4. The emergent beam in a prism producing spectrum is convergent or divergent?5. Why does a glass slab not produce a spectrum of white light?

11.6 OPTICAL INSTRUMENTS

You are advised to wash your hands before taking any food. Do you know why? Because,there may be harmful germs on your hands, which are not visible to you with your nakedeyes. Then how do we see such minute germs, bacteria or other things. For this purpose weuse microscope. Do you know or have you ever seen it? A microscope is an opticalinstrument used to see very-very small objects by forming their magnified image at theleast distance of distinct vision from the eye.

least blending

most bendingGlass prism

White light

(Sun light)Red

OrangeYellow

GreenBlue

IndigoViolet

Spectrum

Fig. 11.19 Formation ofspectrum by a prism


There are a number of instruments and devices that make use of the light. For example,lens camera, pin hole camera, microscope, telescope and projector etc. are the opticalinstruments. Here, we will study about the microscope and telescope only.

Least distance of distinct vision

The minimum distance of an object from a normal eye up to which it is clearly visible, iscalled least distance of distinct vision.

11.6.1 Microscope

There are two types of microscopes-simple microscope and compound microscope. Let usstudy about them one by one.

(a) Simple Microscope

A simple microscope consists of just a convex lensof small focal length. We know that a convex lensproduces an erect and magnified image when theobject is placed at a distance less than its focal length.This property is made use of in a simple microscope.In other words, a simple microscope is nothing but amagnifying glass.

The magnification in the case of convex lens Fig.11.20 is given by

mv

f= −1

Where, v is the image distance and is the focal length of the lens.Now, taking the image distance to be equal to 25 cm, i.e. the least distance of distinct

vision, the magnification m turns to be.

mf f

= − − = +125

125

Thus, we see that the magnification increases with the decrease in the focal length ofthe lens, even if the focal length of a convex lens is very small, say 1 cm. But the magnifyingpower of a simple microscope cannot exceed beyond a certain limit. To get highermagnification a compound microscope, therefore, has to be used.Example 11.1: Find the magnifying power of a simple microscope having the focal lengthequal to 2.5 cm.

Solution : We know that for a simple microscope,

m = 1 + 25

fGiven, f = 2.5 cm Therefore, substituting value of f, we get

m = 1 + 25

2 51 10 11

.= + =

A

B' F1 B O F2

Fig. 11.20 Image formation by simplemicroscope


(b) Compound microscopeIn order to see very minute objects, we use compound microscope.In a compound microscope, unlike the simple microscope, the magnification takes placein two stages.

(i) Construction

It consists of two convex lenses. The lens towards the object is known as objective, whereas,the other lens towards eye of the viewers is called eyepiece. Both the eyepiece and objectiveare of short focal lengths. But the focal length of the objective is shorter as compared tothat of the eyepiece as shown in Fig 11.21.

Fig. 11.21 Image formation by a compound microscope

(ii) Working

Consider figure 11.21. Let the object is placed at a distance slightly greater than the focallength of the objective. A real inverted and magnified image is formed by the lens on itsother side. The eyepiece is so adjusted that this image is within its focal length. The imageacts like an object for the eyepiece which produces a virtual, erect and enlarged finalimage. It is inverted image of the object.

(iii) Magnifying power of the microscope

Let u1 = distance of the object AB; v

1 = distance of the image from the objective lens L

1 :

u2 = distance of A

1 B

1 from the eyepiece L

2; v

2 or D = the distance of its image A

2B

2 from

L2.

Now, with the eye placed very close to the eyepiece, the magnifying power (m) of thecompound microscope is given as:

m = mo x m

e

Where, me is the magnification of eye piece and m

o is the magnification of the objective.

Since the eyepiece acts like a simple microscope, so its magnifying power is,

mfe

e

= +125

where, fe = focal length of the eye piece.

m mfe

e

= +125

Hence, mv

u fe

= +1

1

125

Thus, it is clear that the magnifying power of the compound microscope is greaterthan that of a simple microscope. From this equation it is clear that the magnifying powerof the compound microscope can be increased if,

B

fo

fo'A fe'

A'' A'

L'

fe

Eye pieceObjective

f2

B'

B

O'


• u is small, that is the object AB is placed very near to the objective. It is possible onlywhen the focal length of the objective is very small, since the object is to be placedbeyond the focus to give real, inverted and magnified image.

• v1 is greater, that is, the distance between the image and the objective is large. Again

the object has to be placed near the focus of the objective. So the length of themicroscope should be large.

• fe, the focal length of the eyepiece is very small.

11.6.2 Telescope

A telescope is an optical instrument used to view the distant objects. There are mainly twotypes of telescopes:

• Refracting telescope• Reflecting telescope

(i) The refracting telescope

The refracting telescope is normally used to observe the astronomical or heavenly bodies,therefore, it is known as astronomical telescope also. It consists of two convex lensesarranged in a tube. The lens towards the object is called objective and it is of larger focallength. The other lens towards the eye is called the eye lens or eye piece and it has a shortfocal length. Fig. 11.22.

Fig. 11.22 Astronomical telescope

The objective forms an inverted upside down and real image of a distant object. The eyelens acts like a magnifying glass taking the image formed by the objective as its object.

ii) The reflecting telescope

The objective of this type of telescope is a sphericalconcave mirror of large focal length. Fig. 11.23shows a reflecting telescope. The parallel rays froma distant object fall on the concave mirror. Beforebeing focused at the focus, the rays are interceptedby a small convex mirror, M

1M

2 inclined at 45o

with the axis of the objective mirror. Thus, theimage is shifted towards the eyepiece. Theeyepiece magnifies the image as usual. The mirror M

1M

2 is so small that it does not obstruct

much of the incident light. Hence, the brightness of the image is not affected. The mirrorM

1M

2 can be replaced by a totally reflecting right angled isoscles prism.

Objective

Eye piece

Real Inverted andDiminished

Q

Eye

P

From

o

Eye piece

Objective

Complexmirror

Fig. 11.23 Reflecting telescope


Most of the telescopes used for serious astronomical observations are reflecting telescopes.


1. Write down the uses of a microscope and a telescope.2. What type of lens is used in a simple microscope?3. What is the difference between reflecting and refracting telescopes?4. What do you mean by an objective and an eyelens?

LET US REVISE

• Light is a form of energy which itself is not visible but makes other things visible.• When the light ray (called incident ray) strikes a polished surface, it is bounced back

in the same medium (forming reflecting ray), and the phenomenon is said to be reflectionof light.

• The reflection of light always takes place according to the two laws of reflection.• The reflected light forms images. Images are of two kinds real image and virtual image.• The image formed by a plane mirror lies along the normal from the object, is as far

behind as the object is in front and it is erect, virtual and of equal size.• Plane mirror gives an image in which left handedness turns into right handedness and

vice-versa, i.e. it causes lateral inversion.• The image formed by a concave or convex mirror depends on the position of the

object in front of the mirror.• When a ray of light passes from one medium to another medium of different density,

it bends and this phenomenon is called refraction of light.• Refraction of light is caused by the change in the speed of light as it passes from one

medium to another of different density.• The ability of any medium to bend light ray is called the refractive index of the medium.

It is defined as the ratio of the speed of light in vacuum to that in the medium.• A lens is bounded by two surfaces which may be spherical. There are two categories

of spherical lenses convex lens and concave lens.• Convex lens makes a parallel beam of light rays to converge and come to a point.

Where as concave lens makes the light rays to diverge.• Magnification is the ratio of the size of the image to the size of the object.• A microscope is an instrument used to see very small objects by forming their enlarged

images.• A telescope is an instrument used for observing distant objects like the stars.

TERMINAL EXERCISES

1. What is reflection of light? Explain it with the help of a ray diagram.2. State and explain the laws of reflection.3. What is an optical image and how is it formed?4. Name two types of images and distinguish between them.5. Explain the formation of images with the help of ray diagrams for the following cases:

(i) a plane mirror(ii) a convex mirror(iii) a concave mirror, for an object lying between focus and center of curvature.

6. Define the focus of convex and concave mirrors. Give relationship between focallength and radius of curvature.

7. What is refraction of light? State laws of refraction of light.8. Define refractive index of a medium.9. Explain why do the start twinkle?10. Why is a convex lens is also called converging lens?11. With the help of ray diagram show the image formed by a convex lens when the object

is placed between F and C.12. An object is placed at a distance of 30 cm from a convex lens of focal length 20 cm.

Find the nature and position of the image formed.13. What is a microscope? Explain briefly the principle of simple microscope with a

suitable diagram.14. What is a telescope? Explain briefly the principle of refracting telescope.15. What is the difference between objective lens and eye lens in a telescope?


11.1

1. The sun, candle flame, fire, lighted electric bulb2. Reflection of light3. Angle of incidence = angle of reflection4. Because of diffuse reflection5. Regular reflection:

i) It takes place at smooth and shiny surfaces.ii) Reflected rays are in a particular direction.Diffused reflection :i) It takes place at rough surfaces.ii) Reflected rays are in different directions.

11.2

1. infinity2. image is beyond C, real, inverted and magnified3. Real image

i) They are formed by actual intersection of reflected raysii) They can be casted on the screen

Virtual image

i) They are formed by the reflected rays which appears to be coming from a pointthey do not intersect actually.

ii) Cannot be casted on screen4. Convex mirror

5. Magnification (m) = = =I

O

7 5

515

..

11.31. It deviates from its original path2. Because of multiple refraction of the light coming from the stars3. It is refracted without deviation

11.41.The image will be at infinity, real, invested and magnified.2. Ray diagram

11.51. Violet, indigo, blue, green, yellow, orange, red2. Violet3. Blue4. Divergent5. Because the emergent beam is parallel

11.61. Microscope is used to see very small objects by making an enlarged image.

Telescope is used to see for distant objects by making their image nearer to the eye.2. Convex lens3. Reflecting telescope consists of a concave mirror as objective whereas refracting

telescope consists of a convex lens on objective.4. The lens towards the object is objective lens and the lens towards the eye is eye-lens.

12

Electrical EnergyThe tiny electrons, about which you have studied in lesson 3. on structure of atoms, exhibitvery interesting behaviour when at rest and very useful effects when in motion. Electricalenergy is the energy, basically associated with the electrons and other such particles calledcharged particles.

The wonderful genie of electrical energy brings all comforts to our command just withthe press of a button. It is impossible to think of a world devoid of electrical energy. Westart feeling very uncomfortable even if electricity, in our houses, is not available even fora short duration. Would you not like to know the nature of electrical energy, and the way itworks. This is exactly what you are going to study in this lesson.OBJECTIVESAfter completing this lesson, you will be able to:• cite examples of production of static electricity from everyday life;• describe experiments to show the existence of two types of charges and state Coulomb’s

law for the force between them;• define the terms electrostatic potential energy, potential difference, electric current

and electric resistance;• state Ohm’s law and describe its experimental verification;• apply Ohm’s law for finding equivalent resistance of series and parallel combinations

of resistances;• describe experiments to illustrate thermal and magnetic effects of electric current;• define the commercial units of electric power and electric energy;• solve numerical problems based on Coulomb’s law, Ohm’s law, combination of resistors

and consumption of electric power and electric energy in our houses.

12.1 ELECTROSTATICS

Ordinarily, if you bring a plastic comb near a piece of paper, you would not find anyattraction between them. But, if you comb your dry hair with a comb and bring it close toa small piece of paper, you will find that the piece of paper is attracted towards the comb.We say that the comb gets charged or electrified in the process of combing. Thales ofMiletus (600 BC), a Greak philosopher, knew that amber when rubbed with fur acquiresthe property of attracting small bits of wood. However, the systematic study of electricitystarted with Dr. Gilbert, the personal physician of queen Elizabeth-I, who published hiswork in 1600 AD about charges and magnets. It was Dr. Gilbert, who using the word

: 214 : Electrical Energy

Insulating stand

Silken thread

Glass rod A

Glass rod BStirrup

“electron” for amber coined the word electricity. Dr. Gilbert, through his experience alsoindicated that the process of charging is not limited to amber only. Many other materials,like glass, ebonite and sealing wax can also be charged similarly.

The electricity (or charge) developed on a body, when it is rubbed in intimate contactwith another body is called frictional electricity. It was realised that metals cannot becharged that way, whereas, non-metallic solids can be charged.

12.1.1 Nature of charges

A French chemist Charles Dufay, while performing experiments on charged bodies, foundthat charge acquired by a glass rod on getting it rubbed with silk is different from thecharge acquired by an ebonite rod rubbed with wool. Let us perform the activity performedby Dufay to understand the difference.

ACTIVITY 12.1Aim : To identify two different types of chargesWhat do you need?Two glass-rods, two ebonite rods, a piece of silk, a piece of woolen cloth, aninsulating stand with which a stirrup is hanging vertically with the help of a silkenthread.What should you do?(i) Rub a glass-rod with a piece of silk and place it on the stirrup so that it stays

horizontally. Let it come to rest.(ii) Rub the second glass-rod with silk and bring it close to one end of the first

glass-rod. Observe carefully the position of first glass rod.(iii)Rub an ebonite-rod with a piece of wool and bring it close to the end of glass

rod on stirrup as in step (ii). What difference do you note in the position of theglass-rod ?

(iv)Repeat the experiment by placing an ebonite-rod on the stirrup instead of glass-rod.

Fig. 12.1 Glass-rod rubbed with silk has a charge different than the charge acquired by eboniterod rubbed with wool


We observe that (i) two charged glass-rods repel each other, (ii) two charged ebonite-rods also repel each other, but, (iii) a charged glass-rod attracts a charged ebonite rod.


We conclude that:

(a) Glass rod A hanging ona stirrup

(b) Glass rod A moves awayfrom Glass rod B

(c) Glass rod A movestowards the ebonite rod

Ebonite rod

Electrical Energy : 215 :

(i) Charge developed on glass-rod on rubbing it with silk has a different naturethat the charge developed on ebonite rod rubbed with wool.(ii) Like charges repel each other while unlike charges attract each other.

Dufay called the charge acquired by glass-rod on rubbing it with silk as vitreouselectricity and the charge acquired by ebonite-rod on rubbing it with wood as resinuouselectricity. Later, Benjamin Franklin termed the former as positive charge and the latteras negative charge.

Dr. Gilbert also constructed a device for detecting charge. Such a device is calledelectroscope. Let us also construct a simple electroscope and do another activity using it.

ACTIVITY 12.2

Aim : To verify that in the process of charging by friction, equal and oppositecharges are developed on the bodies rubbed together

What should you need?

A pith-ball with acqua-dag coating, a small silk thread, an insulating stand, anebonite-rod, 4" long woollen cap which fits on the ebonite rod.

Fig. 12.2 Using pith ball electroscope to show that equal and opposite charges are produced byfriction.

What should you do?

(i) Pass the silk thread through the pith ball, put a knot at the lower end and attachthe other end with the insulating stand as shown in Fig. 12.2 (a)

(ii) Insert the ebonite rod in woollen cap, rub them with each other for some timeand then touch the ebonite rod with path ball. The pith ball will, thus, getnegatively charged, which is indicated by its repulsion with ebonite rod.

(iii) Now put woollen cap on the ebonite rod, and bring the rod close to the pithball. Is there any attraction or repulsion shown by the pith ball?

(iv) Check again with and without woollen cap on ebonite rod one by one.What do you observe?We observe that (i) negatively charged ebonite rod repels negatively charged pithball, (ii) woollen cap attracts the negatively charged pith ball, (iii) when eboniterod with woollen cap is brought near the pith ball no attraction or repulsion takesplace.What do you conclude?We conclude that ebonite rod has negative charge and woollen cap has equal amountof positive charge.

Pith ball

Insulating stand

Silkenthread

Woolen cap

(A) (B) (C) (D)


Remember : Charging by friction always produces equal and opposite charges on the twobodies which are rubbed in intimate contact.

How to explain this?

A material as such may be neutral but it is made of atoms. An atom possesses a positivelycharged nucleus surrounded by negatively charged electrons. When we rub two materialsin intimate contact with each other, some of the weakly bound electrons from one body aretransferred to the other body. The body which gains electrons becomes negatively chargedand the body which loses electrons becomes positively charged.

The charge of an electron (e) = 1.6 x 10-19C

If a body gains n electrons it will acquire a negative charge q = n e .........(12.1)

12.1.2 Force between electrical charges : Coulomb’s law

In the previous section we have seen experimentally that we can give different amounts ofcharge to bodies by friction. Also, that some of its charge can be transferred from a chargedbody to an uncharged body by contact. We have also learnt that like charges repel eachother while unlike charges attract. The factors on which this force of attraction or repulsiondepends was studied first by the french physicist Charles Augustin de Coulomb. Coulombpresented the inferences of his experiments in the form of a law which is stated below.

Coulomb’s law

The magnitude of the force of attraction (or repulsion) between two point charges isdirectly proportional to the quantity of charge present on each of them and inverselyproportional to the square of the distance separating them.

If a charge q1 is placed at a distance from a similar charge q

2, then the two charges will

continue to repel each other with a force,

kq1q

2f = ____________ (12.2), r2

where k is a constant of proportionality depending on the nature of the medium inwhich the charges are placed.

In SI units, k = 9 x 109 Nm2 c-2, for vacuum (or for air)

Charge is a scalar quantity. Its SI unit iscoulomb. Equation 12.2 may be used to define1C. If q1=q2 q2=1C and r=1 m, f=9x109 N.Thus, 1C charge is the charge which whenplaced at a distance of 1 m from an equal likecharge in vacuum, experiences a repulsiveforce on 1N.

Coulomb is a very big unit of charge.Normally charge acquired by bodies are of the order of micro coulomb or at the most millicoulomb. You may recollect that

1 micro coulomb = 10–6Cand 1 milli coulomb = 10–3C

Fig 12.3 Coulombian force between two chargesseperated by a distance r


12.1.3 Electric potential

Consider a big charge ‘Q’ fixed at a point. Let us call it source charge. At a very largedistance from ‘Q’ a small charge ‘q’ will experience negligibly small force. As we bring‘q’ towards ‘Q’ the magnitude of force between Q and q increases. If Q and q both are ofthe same nature (i.e. both positive or both negative) there will be a force of repulsionbetween them. Hence in moving charge ‘q’towards ‘Q’ work will (1890) have to be doneon the charge ‘q’. This work will be stored upas potential energy in the charge.

It is because of this electrostatic potentialenergy, that a charge when left of itself in theregion surrounding a fixed charge, moves fromone point to another point. The electrostaticpotential energy possessed by a charge q when it is at a distance r from charge Q is givenby :

KQqU = ________ .................. (12.3)

r

In electricity, potential energy per unit charge is called electrostatic potential. Potentialis more significant than potential energy itself. Using equation 12.3 we can say that potentialat a point

U KQV = _____ = _______ ------------ (12.4)

q rElectrostatic potential is a scalar quantity and its SI unit is JC-1, the other name for

which is volts (V).

The potential at a point is 1 V if a + 1C charge placed at that point possesses apotential energy of 1J.

It may be noted that potential due to a positive source charge, at any point around it, ispositive and decreases with distance. Whereas the potential due to a negative source chargeis negative at any point around it and increases with increasing distance.

The importance of electrostatic potential lies in the fact that it is this quantity whichdetermines the direction of flow of charge. Positive charges always move from higherpotential to lower potential. On the other hand, negative charges move from lower potentialto higher potential.

Example 12.1 : How many electrons make one coulomb?

Solution : Let n electrons make 1 C

Since charge is built by the excess or deficiency of electrons only

Charge on 1 electron is 1.6 x 10-19C

Charge q = + n en = q

= 1

= 6.25 x 1018 electrons

e 1.6 x 10–19

Fig. 12.4 Potential energy of charge q place ata distance r from charge Q


Example 12.2 : Two point charges having magnitudes 1 microcoulmb and 2 microcoulombsrespectively are kept separated by a distance of 2 m. Calculate (i) the force of repulsionbetween them, (ii) the electrostatic potential energy of the charge system.

Solution :KQq = 9 x 109 x 1 x 10-6 x 2 x 10-6

(i) F = = 4.5 x 10–3 Nr2 (2)2

KQq = 9 x 109 x 10-6 x 2 x 10-6

(ii) U = = 9 x 10–3 Jr 2

Example 12.3 : Calculate the potential at a point, distant 30 cm from a 60 microcoulomb negative charge.

KQ = –9 x 109 x 60 x 10-6

Solution : V = = –1.8 x 106 voltsr 30 x 10-2


1. What type of charge does an ebonite rod acquire when it is rubbed with wool? What isthe nature of the charge acquired by wool?

2. When a glass rod is rubbed with a piece of silk it acquires +10 micro coulomb ofcharge. How many electrons have been transferred from glass to silk ?

3. If the charge on two particles be doubled and separation between them be halved, howmany times will become the Coulombian force between them?

4. A charged particle placed at a distance of 50 cm from a fixed charge has a potentialenergy of 10J. If the charge of the particle is 1 micro coulomb(i) what is the potential at the position of the particle(ii) what is the value of the fixed charge?

5. Define the unit of (i) charge (ii) potential

12.2 CURRENT ELECTRICITY

Can charge produced at one place be transferred to some other place without actuallymoving the charged body? Yes, you will say, by connecting the charged body to an unchargedbody through a metallic wire. But, can you do so by holding the naked wire in your hand?You will say, no, the wire should be insulated. What you know by sheer experience today,was shown by Stephen Gray in 1729 by extensive and expensive research of severalmonths.

12.2.1 Electric cells–Sources of potential difference

As you have learnt in the previous section, positive charge flows from higher to lowerpotential. So if you want to pass charge continuously, from one body to another bodythrough a wire, you have to maintain a potential difference between them. You are familiarwith a device which can be used to maintain potential difference between the two ends ofa wire-the dry cell. The dry cell is a type of electric cell.

An electric cell is a device which converts chemical energy into electrical energy.A group of cells is called battery. In a torch having many cells you are using a battery

of cells.


1 2 3 4 5

Connectingwire

Zincstrip

Lemon

LED

ACTIVITY 12.3Aim : To construct a battery of cells and use it to light up an LED

What you need?5 lemons, 5 thin strips of copper, 5 thin strips of zinc, an LED, copper connectingwires.

What to do?(i) Arrange the lemons in a line on a table.(ii) Insert one copper and one zinc strip in each of the lemons as shown in Fig. 12.5

Fig 12.5 A battery of 5 lemon cells used to light up an LED

(iii) Connect zinc strip of first lemon with the copper strip of the second lemon andthis way connecting all the cells, leaving one copper strip free at the end of firstlemon and one zinc strip at the end of last lemon.

(iv)Connect LED between these free strips.(v) Repeat the experiment using cotton threads instead of copper wires with LED.(vi)Repeat experiment using 3V bulb instead of LED.What do you observe ?You will observe that (i) the LED glows continuously when connected across thebattery using copper connecting wires, (ii) LED does not glow when we use cottonthread instead of copper wire, (iii) 3V bulb does not glow in this arrangement.What do you infer ?We infer that (i) the LED glows continuously, because, continuous charge flowsthrough it due to a constant potential difference applied across its ends with thehelp of battery of lemon cells, (ii) copper wire conducts charge but cotton threaddoes not, (iii) The arrangement does not supply enough charge to glow a 3V bulb.

Remember:

1. There are two types of substances: (i) those through which electric charge can floweasily are called conductors. All metals are good conductors of electricity, (ii) Thosethrough which charge does not pass are called insulators. Non-metals are generallyinsulators. It is the structure of the material which determines whether it will conductcharge or not. A conductor has a large number of free electrons, whereas insulatorshave none.

2. A directed flow of charge is called electric current. The electric current flowing througha conductor is defined as the charge flowing through any section of the conductor in 1second.

Copper strip


Qi.e. I = ------------- (12.5)t

Current is a scalar quantity and its SI unit is ampere (A). Current through a conductoris 1A if 1C charge flows through it in 1 second. The current flowing through a conductoris measured with the help of a device called ammeter.

3. As a convention, the direction of flow of positive charge is taken as the direction offlow of electric current. Thus in an electric circuit current is considered to be flowingfrom the positive terminal of the battery towards negative terminal.

4. In conductors it is the negatively charged free electrons which move to constitutecurrent. They flow in opposite direction to the direction of conventional current.

5. A dry cell bears a marking 1.5V. This figure indicates the maximum potential differencethat can be applied to this cell, when no current is being drawn from it. This is calledemf of the cell. The emf of a cell is its characteristic property. Actual potential differencewhich we can apply with the help of the cell is slightly less than its emf.

12.2.2 Electric circuits and Ohm’s law

When we connect some devices like electric bulb across a cell through connecting wires,current flows through the arrangement in a closed path. This type of arrangement of cells,conductors and bulbs is called electric circuit. In circuit diagrams various components arerepresented by definite symbols, some of which are given in Fig. 12.6.

Fig. 12.6 Some important symbols used in electric circuits

In this list of symbols, voltmeter is a device used to measure potential difference betweenany two points of a circuit, galvanometer - a device to detect current and rheostat to changecurrent in the circuit.

During contact programme request your tutor to show you various electrical devicesthey use in laboratories.

12.2.2a Ohm’s Law

In section 12.2.1 we have seen that current flows through a conductor when we apply apotential difference between its ends with the help of an electric cell. The question ariseshow does the value of current flowing through a wire change when the potential differenceapplied across it is changed. To answer this let us perform the following activity.

CellCell Battery

Variableresistance

Fixedresistance

Switch Closed key

Connectingwire

Wires connectedtogether

Wires crossing withoutbeing connected

GalvanometerVoltmeterAmmeter

Lamp


ACTIVITY 12.4

Aim : To find the relation between the current flowing through a wire and thepotential difference applied across it

What you need?A dry cell, a voltmeter (range 0–1.5V), an ammeter (range 0–1A), a standard fixedresistance coil (1 Ohm), rheostat (0–1 Ohm), connecting wires and a plug key.

What to do?

(i) Connect the fixed resistor (R),ammeter (A), dry cell (D), plug key(K) and rheostat (Rh) is series (endto end) and voltmeter (V) in parallelto R, as shown in Fig. 12.8.

(ii) When the key K is open check thatthe readings in ammeter andvoltmeter is zero.

(iii) Insert the plug in the key and move the sliding contact of the rheostat so thatthere is some small reading in ammeter and voltmeter. Record these readings.

(iv) Increase the value of current with the help of rheostat. Record ammeter andvoltmeter readings again.

(v) After changing the readings 4 to 5 times, record the corresponding values currentand voltage from ammeter and voltmeter.

(vi) Plot a graph between ammeter and voltmeter readings.

What do you observe?You will observe that : (i) on increasing ammeter reading voltmeter reading increasesin the same proportion. (ii) the voltage-current graph is a straight line as shown inFig. 12.8.

What do you conclude?We conclude that the current flowing through a wire is directly proportional to thepotential difference applied between its ends.i.e. V ∝ Ior V = RI ........................... (12.6)Here, R is a constant of proportionality and is called the resistance of the givenwire.This observation was first made by George Simon Ohm and is called Ohm’sLaw.

Remember :1. The law can be applied only to conducting wires and that too when its temperature and

other physical conditions remain unchanged. If the temperature of the conductorincreases its resistance also increases.

2. ‘R’ i.e. resistance of wire, is a constant for a given wire. It can be easily shown thatresistance of a wire depends on :

Fig 12.7 Circuit diagram to showrelationship between voltage and

current

Resistance (R)

Ammeter CellPlug key

V

A .

Rheostat (Rh)


Fig 12.8 Graph showingvariation in voltage with the

variation in current

Slope = Resistance (R)

Current (I)/amperes

Pot

enti

al d

iffe

renc

e (V

)/vo

lts

R1

R1

R2 R

3

V3V

2V

1

• its length – longer the wire, more the resistance• its thickness – thicker the wire, lesser the

resistance• the nature of material – copper wire has lesser

resistance than iron wire of same length andthickness.

3. Resistance is a scalar quantity and its SI unit is Ohm(Ω).

4. The resistance of a wire can never be negative.

Ohm is the resistance of a wire across, which when 1Vpotential difference is applied, 1A current flows throughit.

12.2.3 Combinations of resistors

In electrical circuits, we connect a number of devices having different resistance values.This we can do in two different ways.

(a) Series combinationIn this combination a number of resistances are connected end to end, so that, samecurrent flows through all of them (Fig. 12.9), when the combination is connected to acell.

Fig 12.9 Series combination of resistances

If we measure voltage across each of the resistances with the help of a voltmeter, wewill find that more the resistance more the potential difference across it. Thus, voltageacross r

1, i.e., V

1 = Ir

1, across r

2 is V

2 = I r

2 and so on.

Also, the total voltage across the combination is the sum of the voltage across individualresistors.i.e. V = V

1+V

2+V

3+ ---------------- (12.7)

= Ir1+Ir

2+ Ir

3 + ----------------

V___ = r

1+r

2+ ---------------- (12.7a)

IV

gives the resistance of the combination RIR = r

1 + r

2 + -------------- (12.8)

The resistance of a number of resistances in series is equal to the sum of the resistances ofthe component resistors.


Fig. 12.10. Parallel combinationof resistances

R1

R1

R2

R3

(b) Parallel combinationResistances are said to be connected in parallelwhen one end of all the resistors is connectedto the positive terminal of the battery and theother end to the negative terminal, as shown inFig. 12.10.

In parallel combination equal potentialdifference is applied across each resistor. Thecurrent drawn from the cell is inverselyproportional to the resistance, i.e.

I1 = V

, I

2 = V

, I

3 = V

r1

r2

r3

Also, total current drawn from the cell by the combination is equal to the sum ofcurrents drawn by the individual resistors. If the resistance of the combination be R,then

I = VR

and I = I1 + I

2 + I

3-------------------(12.9)

V

= V

+ V

+ V

R r1

r2

r3

⇒ 1

= 1

+ 1

+ 1

-----------------------(12.10)R r

1r

2r

3

The reciprocal of the resistance of a combination of a number of resistors connected inparallel is equal to the sum of the reciprocal of the individual resistances.

Remember :

1. Normally all the appliances in our household circuits are connected in parallel. But thechain of small bulbs that we use for decoration on Deewali has the bulbs connected inseries.

2. As we add resistances in series the circuit resistance increases but when we connectresistances in parallel the total resistance is smaller than the smallest of the resistancesinvolved.

Example 12.4 : Find the equivalent resistance of the following combination of resistors.

Fig. 12.11.


Solution :

(a) Here all resistors are connected in series

R=r1+r

2+r

3+r

4+r

5+r

6=1+2+3+3+2+1=12Ω

(b) Here we have two series combination of 3 resistors in parallel.R

1 = 1+2+3 = 6Ω, R

2 = 1+2+3 = 6Ω

R1×R

26 x 6 36

R = _________ = _______ = _____ = 3ΩR

1+R

26 + 6 12

(c) Here we have 3 parallel combinations of 2 resistances each connected in series.r

1 x r

21 x 1 1

R = _________ = _______ = ___ Ωr

1 + r

21 + 1 2

2 x 2R = _________ = 1Ω

2+23 x 3 9 3

R = _________ = ___ = ____ = 1.5 Ω3 + 3 6 2

R = R1 + R

2 + R

3 = ½ + 1 + 3/

2 = 3Ω


1. Define the units of (i) current (ii) resistance

2. A number of bulbs are connected in a circuit. Decide whether the bulbs are connectedin series or in parallel, when (i) the whole circuit goes off when one bulb is fused (ii)only the bulb that gets fused goes off.

3. When the potential difference across a wire is doubled, how will the following quantitiesbe affected (i) resistance of the wire (ii) current flowing through the wire.

4. How will the readings of ammeter and voltmeter change in the adjoining circuit (Fig.12.12), when an extra resistance R is connected (i) series with the battery (ii) parallelto the resistance R. Assume ammeter, voltmeter and cell to be ideal devices.

5. What is the reading of ammeter in the adjoining curcuit (Fig. 12.13).

Fig. 12.12 Fig. 12.13

12.3 Effects of electric current

When current is passed through a conductor some changes take place in and around itsmaterial. These changes, produced due to electric current, are called effects of electriccurrent.


There are two effects of electric current flowing through a conductor that we comeacross in our day to day life. They are :

(i) Thermal effect (ii) Magnetic effect

Let us study these effects of electric current one by one.

12.3.1 Thermal effect of electric current

When current is passed through a conductor it gets heated up. To study the heating effectof electric current let us perform the following activity.

ACTIVITY 12.5

Aim : To study thermal effect of electric current

What you need?Two pieces of the element of electric heater (one of which has 10 turns and theother 20 turns), two dry cells, connecting wires.

What to do?(i) Attach connecting wires to the free ends of 10-turn coil permanently.(ii) Touch the free ends of the connecting wires to the two terminals of dry cell,

thus passing current through it. Detach the contacts after 10 seconds. Nowtouch the coil and feel it.

(iii) Repeat the experiment by passing current for 20 seconds.(iv) Place two dry cells in contact, making series battery and repeat the second

step.(v) Repeat steps 2,3,4 with 20-turn heater coil.

What do you observe ?You will observe that(i) On passing current through a conductor it gets heated up.(ii) The coil is found to be heated when current is passed for a second.(iii) The coil is found to be hotter when greater voltage is applied across it.(iv) When same voltage is applied across bigger coil less heat is produced in it.What do you infer ?Thus, we conclude that(i) Current has a heating effect, i.e. when current is passed through a conducter it

gets heated up.(ii) More heat is produced in a conductor when

• more potential difference is applied across it• current is passed through it for more time (t)• more current is passed through the same conductor.

Fig. 12.14. Study of thermal effect


Thus it can be seen that heat produced Q α VIt or Q = kVIt

If V = 1 volt, I = 1 A, t = 1 second, Q = 1 J K = 1

Then Q = V I t ............................ (12.12)

According to Ohm’s Law : V = IR Q = I2 Rt = V2 t (12.13) R

12.3.2 Magnetic effect of electric currentAll of you, I am sure, might have played with magnetics. A magnet has such interestingproperties that you cannot resist possessing one. A pivoted magnetic needle always staysin north-south direction and is used as a magnetic compass. A magnet attracts small piecesof iron, nickel and cobalt. It also attracts unlike poles of another magnet and repels likepoles. But a stationary magnet does not attract or repel a stationary charge. Still electricityand magnetism are intimately related. In fact, magnetism is just an effect of electric current.This was for the first time discovered by H.C. Oersted, in 1820, accidently. Let us performan activity to understand oersted’s discovery.

ACTIVITY 12.6Aim : To study the magnetic effect of electric currentWhat do you need?A compass needle, a dry cell, connecting wires, a thick copperwire, two woodenstands

What to do?1. Place the magnetic needle on

the table. It will stay in north-south direction.

2. Stretch the thick copper wireover the magnetic needle,using wooden stands, so thatthe wire is parallel to the axisof the magnetic needle.

3. Attach connecting wires atthe two ends of the thickcopper wire.

4. Touch the free ends of the connecting wires to the two terminals of the battery.Observe the magnetic needle carefully.

5. Touch the reverse terminals of the battery with the free ends of the connectingwires, observe the magnetic needle again.

What do you observe ?(i) The magnetic needle gets deflected whenever an electric current is passed

through the thick copper wire.(ii) The deflection in magnetic needle gets reversed when the direction of flow of

current through the wire is reversed.

Fig. 12.15 Set-up to study the magnetic effectof electric current

Copper wire

Wooden stand

Magneticcompass


We conclude that a magnetic field is developedaround a conductor when electric current is passedthrough it. This observation is called magnetic effectof electric current. The magnetic field around aconductor carrying conductor is in the form of closedcircular loops, in a plane perpendicular to the conductor,and is given by right hand grip rule. According to therule, hold the conductor is your right hand with thumbpointing in the direction of electric current, then, thecurling figures point is the direction of the magneticfield.

Making use of these devices, scientists have deviseda number of electric gadgets that we use in our housesfor our comfort. You will learn about some of thesedevices in the next lesson.


1. Which will produce more heat in 1 second – a 1 ohm resistance on 10V or a 10 ohmresistance on the same voltage? Give reason for your answer.

2. How will the heat produced in a conductor change in each of the following cases?(i) The current flowing through the conductor is doubled.(ii) Voltage across the conductor is doubled.(iii) Time for which current is passed is doubled.

3. 1 A current flows though a conductor of resistance 10 ohmsfor 1/2 minute. How much heat is produced in the conductor?

4. When plug is inserted in key K, indicate the direction ofmagnetic field developed around wire AB in Fig. 12.17.

5. Name a household electric device based on (i) thermal effect(ii) magnetic effect of electric current.

12.4 ELECTRIC ENERGY AND ELECTRIC POWER

We have seen in the previous section that when V volts is applied across a conductor andI amperes flows through it, then the energy produced in the conductor in t seconds is givenby

Q = VItActually this equation holds true in whatever form the electrical energy may be

consumed. The electrical energy consumed in one second is called electric power and isgiven by

P = Q = VI (12.14)t

Equation 12.13 then becomes

Q = Pt = VIT = qv (12.15)

Fig. 12.16 Right hand grip rule

Magneticcompass

Direction ofmagnetic field

Direction ofelectric current

Fig. 12.17


12.4.1 Commercial units of electrical energy and electric power

In SI units, energy in each of its form, has the same unit, joule (J). But in our houses weconsume so much of electrical energy daily that joule proves to be a small unit for practicalpurposes. Therefore for commercial purposes we use a very big unit for measuring electricalenergy, called kilo watt hour (kWh)

IkWh = 1000 x 3600 = 3.6 X 106 J. (12.17)

For electric power also commercially we use a bigger unit, horse power (HP).

1 HP = 746 watt (12.18)

12.4.2 Electric power generation and consumption in India

One of the criteria, for judging the progress of a nation and the standard of living of itspeople, is the electrical energy generated and used by it. India has come a long way in itsefforts to generate and use electrical energy. Till 31st December 2001, we had developed atotal installed capacity of more than 103 billion watts of power utilities, with high targetsof capacity additions in our future plans. Though per capita electricity available to ourpeople is still very low as compared to the per capita electricity available in developed andoil rich countries, the situation is not very unsatisfactory when it comes to the considerationsof resources available.

There are three types of large scale electricity, generating plants :

(i) Hydroelectric power plants - where potential energy of water stores in a dam isused for generating electricity.

(ii) Thermal power plant - where a fossil fuel is burnt to produce steam which runsa turbine to convert mechanical energy into electrical energy.

(iii) Atomic power plant - where nuclear energy obtained from a fissionable materiallike uranium is used to run a turbine.

Some of the important power plants of India are listed below :

1. Hydel power plants(a) Bhakra -Nangal hydroelectric power plant, Punjab(b) Rihand hydel power house, Uttar Pradesh.(c) Periyar hydroelectric power station, Tamilnadu(d) Iddika hydroelectric power house, Kerala.(e) Umiam hydroelectric power station, Assam

2. Thermal power plants(a) Badarpur thermal Power Station, Delhi(b) Talcher thermal power house, Orissa.(c) Barauni thermal power station, Bihar.(d) Neyveli thermal power station, Tamil Nadu.(e) Namrup thermal power station, Assam.

3. Nuclear power plants(a) Tarapore atomic power station, Maharashtra.(b) Rajasthan atomic power station, Rana Pratap Sagar, Kota.


(c) Madras atomic power station, Kalpakkam, Tamil Nadu.(d) Narora atomic power station, Uttar pradesh.Of the total electrical power generation facilities available in India about 25%

are hydel, 7% thermal, 2.5% nuclear and the rest use other resources like windenergy, solar power, geothermal energy or oceanic energy. This shows that mainthrust by now has been on thermal power plants which use coal, natural gas anddiesel as fuel. Because, we have to import fossil fuels the production is not to thefull installed capacity. Thus a change in shift to other sources becomes imperative.

In India all the major plants produce A.C. (alternating current) at 50 hertz,11000 volts or more. This power can be further stepped up to higher voltagesusing transformers and hence can be transmitted to long distances without muchloss of power.

Example 12.5 Find the resistance of the filament of 100W, 250V electric bulb.

Solution : P = V2

P

= 250 x 250 = 625 Ω 100

Example 12.6 Calculate the energy consumed in a 2 kW electric heater in 2hours. Express the result in joules.

Solution : Q = Pt = 2 x 2 kWh = 4 kWh

= 4 x 3.6 x 106J = 14.4 x 106 J

Example 12.7 How much time will a 2 kW immersion rod take to raise thetemperature of 1 litre of water from 300 to 60°.

Solution : Q = Pt

Q = mcθmcθ = pt ....(12.19)

Mass of 1 litre of water (m)= 1 kg

Specific heat of water (c) = 4.18 x 103 J kg-1 C-1

Rise in temperature of water (θ) = 60 – 30 = 300C.

P = 2 kW = 2000W

Substituting in equation (12.19) we get

1 x 4.18 x 103 x 30 = 2000 x t

t = 125.4 x 103 = 62.7 s2 x 103

Example 12.8 : How many kilowatt hour of energy will be consumed by a 2HPmotor in 10 hours?Solution : P = 2 HP = 2 x 746 W = 1.492 kW

Q = Pt = 1.492 x 10 kWh = 14.92 kWh


Example 12.9 : A potential difference of 250V is applied across a resistance of 1000 ohm.Calculate the heat energy produced in the resistance in 10 s.Solution : Q = V2t = 250 x 250 x 10 = 625 J

R 1000


1. Which has a higher resistance a 40W, 220W bulb or a 1 kW electric heater?2. What is the maximum current that a 100W, 220V lamp can withstand?3. How many units of electricity will be consumed by a 60W lamp in 30 days, if the bulb

is lighted 4 hours daily.4. How many joules of electrical energy will a quarter horse power motor consume in

one hour.5. An electric heater is used on 220V supply and draws a current of 5 A. What is its

power?

LET US REVISE

• When two bodies are rubbed together in contact, they acquire a peculiar property ofattracting small bits of paper. We say the bodies are electrified or charged by friction.

• Charges are of two types. Charge acquired by a glass rod rubbed with silk is positiveand that acquired by an ebonite rod rubbed with fur is negative.

• Like charges repel each other and unlike charges attract each other.• The force between two charges is given by Coulomb’s law according to which

K q1 q

2F = ______________

r2

• Force per coulomb of charge at a point is called electric field, E = F/q• Work is done in moving a charge against electric field which is stored up as potential

energy of the charge. Hence, when charge is placed at a point in the field it possessespotential energy.

• Potential energy per coulomb of charge at a point is called potential. Positive chargealways moves from a higher potential to a lower potential and vice-versa.

• Electric cell is a device with the help of which we can apply a potential differencebetween the two ends of a wire due to which current will flow through the wire.

• Ohm’s law states that current flowing through a conductor is directly proportional tothe potential difference applied between its ends, provided temperature and otherphysical conditions of the conductor remain unchanged.

• Ratio of voltage applied across a conductor and the current flowing through it is calledresistance of the conductor. S.I. Unit of resistance is ohm.

• Resistances may be connected in two different independent ways (i) in series, (ii) in parallel.

• In series, total resistance of the combination is equal to the sum of the individualresistances.

• In parallel, reciprocal of the combined resistance is equal to the sum of the reciprocalsof the individual resistances.


• Current when passed through a conductor produces two effects.(i) Thermal effect,(ii) Magnetic effect.

• Commercial unit of electrical energy is kWh and that of electric power is HP.• India is gradually moving towards its target of providing enough electric power to its

people but still we have a long way to go.

TERMINAL EXERCISES

A. Multiple choice type questions.1. A charged conductor ‘A’ having charge is touched to an identical uncharged

conductor ‘B’ and removed. Charge left on A after separation will be :-(a) Q (B) Q/2 (C) Zero (D) 2 Q

2. J C-1 is the unit of(a) Current (b) Charge (c) Resistance (d) Potential

3. Which of the following materials is an electrical insulator?(a) Mica (b) Copper (c) Tungsten (d) Iron

4. The device which converts chemical energy into electrical energy is called(a) electric fan (b) electric generator (c) electric cell (d)electric heater.

5. The resistance of a conductor does not depend on its(a) temperature (b) length (c) thickness (d) shape

B. Fill in the blanks.1. When current is passed through a conductor its temperature __________.2. A current carrying conductor carries a __________ field arount it.3. The director of magnetic field around a current carrying conductor is determined

using __________.4. Unit of electric power is __________.5. of the two wires made of the same material and having same thickness the

longer one has __________ resistanceC. Descriptive type questions.

1. Name the instruments used to measure (a) current (b) potential difference.2. Name the quantity measured by the unit (a) NC-1 (b) C S-1

3. Give a one word name for the unit (a) J C-1 (b) C S-1

4. What is the potential difference between the terminals of a battery if 250 J ofwork is required to transfer 20C of charge from one terminal of the battery tothe other.

5. Give the symbols of (a) cell (b) battery (c) resistor (d) voltmeter.6. What is the conventional direction of flow of electric current? Do the charge

carriers in the conductor flow in the same direction ? Explain ?7. Out of ammeter and voltmeter which is connected us series and which is

connected in parallel ?8. You are given two resistances of 3 ohm and 6 ohm, respectively. Combining

these two resistances what other resistances can you obtain?


9. Two resistances when connected in series give 8 Ω and when connected inparallel give 1.5 Ω. What is the value of these resistances ?

10. Which effect of electric current can be utilized in detecting a current carryingwire concealed in a wall? Name the scientist who discovered this effect.

11. Two resistances are connected in seriesas shown in fig. 12.18.(i) What is the current through 5 ohmresistance?(ii) What is the value of R?(iii) What is the value of V?

12. In the circuit shown alongside (Fig. 12.19), find(i) Total resistance of the circuit.(ii) Ammeter (A) reading(iii) Current flowing through 2 ohm resister

13. For the circuit shown alongside (Fig.12.20), find the value of :(i) Current through 6 ohm resistor.(ii) Potential difference across 12 ohmresistor

14. You are given three resistors of 1 ohm, 2ohm and 3 ohm. Show by diagrams, howwill you connect these resistors to get (a) 6/11 Ω (b) 6 Ω (e) 1.5 Ω

15. A resistor of 8 Ω is connected in parallel with another resistor of X Ω. Theresultant resistance of the combination is 4.8 ohm. What is the value of resistorX

16. In the adjoining circuit (Fig. 12.21),find :(i) Total resistance of the circuit.(ii) Total current flowing throughthe circuit(iii) The potential difference across4 Ω resistor

17. What is the fuel used in :(a) an atomic power plant (b) a thermal power plant

18. What is the (a) frequency (b) voltage of electricity, supplied in our homes ?19. Name two of each of the following types of power plants in India. Also give

their location.(a) Hydel power plants (b) Thermal power plants (c) Nuclear power plants

Fig. 12.19

Fig. 12.18

Fig. 12.20

Fig. 12.21


20. What are the three types of electric power plants in India? How do they differfrom one another?


12.1

1. Negative charge. Wool acquires postive charge.q

1

0

x

1

0

-6

2. n = = 6.25 x 1013 electronse 1.6 x 10–19

q1q

2 2q

1 × 2q

2

3. F = k ________ ⇒ F´ = K _______________ = 8F r2 (r/2)2

U 104. (i) v = ____ = _______ = 107V

q 10–6

KQq Ur 10 × 0.5 5(ii) U = __________ = r = ______ = ____________________ = ____ × 10–3C

r Kq 9 × 109 × 10–6 95. (i) Unit of charge is Coulomb. IC charge is the charge which when placed at

a distance of 1 m from an equal like charge repels it with of force of 9 x109 N.

(ii) Unit of potential is 1 volt. Volts is the potential at a point in an electricfield such that if IC positive charge is brought from outside the field tothis point against the field 1 J work is done.

12.2

1. (i) Unit of current is ampere. 1A is the current in a wire in which 1C chargeflows in 1 second.

(ii) Unit of resistance is ohm .1 ohm is the resistance of a wire across whichwhen 1V potential difference is applied 1A current flows through it.

2. (i) If the whole circuit goes off when one bulb is fused the bulbs are connectedin series.

(ii) If only one bulb goes off and the rest of the circuit remains working thebulbs are connected in parallel.

3. (i) Resistance of the wire remains unaffected(ii) current flowing through the wire is doubled.

4. (i) When R is connected in series readings of voltmeter and ammeter willreduce to half.

(ii) When R is connected in parallel to R, reading of ammeter is doubled butreading of voltmeter remains unchanged.

5. 1A.

12.3

1. Q

= V

´ This implies that more the resistance less the power. Therefore, more t R heat will flow is 1s in 1 ohm resistor.

2. (i) Heat produced becomes four times (ii) heat produced becomes four times


(iii) heat produced is doubled.3. Q = I2Rt = 1 x 10 x 30 = 300 J.4. refer section 12.3.25. (i) Electric heater (ii) Electric fan

12.4 V2

1. R = _____ = 40W lamp has higher resistance. P

P 100 52. I = ____ = ______ = ____ A.

V 220 113. Q = Pt = 60 × 30 × 4 = 7200 Wh = 7.2 kWh.

7464. Q = Pt = ________ × 3600 J = 675400 J.

4

5. P = VI = 220 × 5 = 1100 watt.

13

Electricity in Our HomesToday, electricity has become so essential part of our life that we can not think of lifewithout it. Our households are full of electrical appliances such as electric bulb, electricbell, electric fan, electric iron, electric heater, refrigerator, washing machine, dish washer,radio, television, air conditioner and so on. In a way, we can say that man has gained apartial control over forces of nature with the help of electrical energy. Electricity runs ourindustries and pumps out water for irrigation of our fields. Use of electricity in the transportsystem is also increasing.

In the previous lesson you studied about the story of the wonderful genie of electricalenergy. In this lesson also, we will continue with the same story and highlight its uses inour daily life. We will try to understand how electrical energy is generated, distributed andused. The principle, construction and working of electric motor, generator and some of thedomestic electric gadgets will be explained. The important features of distribution systemsand domestic wiring systems will also be highlighted in this lesson.

OBJECTIVES

After completing this lesson, you will be able to:• explain the principle and working of an electric motor;• demonstrate the flow of electric current in a closed loop of conducting wire when

magnetic field associated with it is changed;• explain the principle and working of an a.c. generator;• draw circuit diagram to indicate how wiring is done to supply electric power to various

devices in our houses or in industry;• state the hazards involved in using electric energy and describe safety measures to

minimize them;• highlight the importance of fuse and earthing in electrical circuits; and• identify different household electrical appliances and explain their construction and working.

13.1 ELECTROMAGNETISM

In the previous lesson we have seen that a magnetic field is developed around a currentcarrying conductor due to which a magnetic needle placed alongside shows deflectionfrom its N-S orientation. Fig. 13.1 shows magnetic field due to a circular loop of wirecarrying current. It is evident here that the magnetic field due to the loop at its centre is

: 236 : Electricity in Our Homes

quite strong, because, every part of the current loop isproviding field in the same sense. The current-carryingloop, in fact, behaves as a magnet, one face of it beingNorth Pole and the other South Pole.

If we make a cylindrical coil of insulated wire havingmany turns (called solenoid) and pass current through it,the coil will behave as a temporary magnet – one endserving as a north pole and the other as a south pole.Hold your right hand above the coil with the curlingfingers pointing in the direction of the current, then thestretched thumb indicates that end of the coil, whichfunctions as the north pole. The stretched thumb also indicates the direction of the magneticfield inside the coil. This rule is called the right hand thumb rule. If you wind the coil ona cylindrical tube of cardboard and study the magnetic field by placing a magnetic needleinside it we can see that the magnetic field inside the coil increases when,

i) the current flowing through the coil is increased,

ii) number of turns in the coil is increased,

iii) length of the coil is decreased, and

iv) a soft iron bar is introduced in the card boardtube.

13.1.1 Electromagnets

A current-carrying solenoid with soft iron core is calledan electromagnet (Fig. 13.2).A comparison of an electromagnet and a permanentmagnet is given in the table below.

Table 13.1 : Distinction between a bar magnet and an electromagnet

Bar magnet Electromagnet

Bar magnet is a permanent magnet. An electromagnet is a temporary magnet. Itremains magnet only for the duration thecurrent flows through it.

The strength of a bar magnet cannot The strength of an electromagnet can bebe changed. changed by changing the amount of current

flowing through it.

Bar magnet is a weak magnet. Electromagnet produces comparativelystronger field.

The poles of a permanent magnet Just by reversing the direction of currentflow,

cannot be easily reversed. we can reverse the polarity of electromagnet.

Fig. 13.1 Magnetic field due to acurrent-carrying loop

Fig. 13.2 A solenoid carryingcurrent behaves like a magnet

Electricity in Our Homes : 237:

13.1.2 Force on a current carrying conductor placed in a magnetic field

Oersted’s experiment showed that a current carrying conductor deflects a magnetic needle.Newton’s third law of motion suggests that a current-carrying conductor placed in a magneticfield should experience a force.

ACTIVITY 13.1

Aim : To study the forece experienced by a current carrying conductor in a magneticfield.

What do you need ?

A U-shaped magnet, two batteries, two rheostats, one tapping key, one plug key, twoammeters, mercury in a shallow dish, a flexible joint J and connecting wires.

What to do ?

(i) Place the dish containing mercury between the pole pieces of the electromagnet.

(ii) From a rigid support T, suspend flexible joint J.Let the thick copper wire AB hang on J so that itslower end B just touches mercury.

(iii) Connect the positive terminal of the battery with Jand the negative terminal to a rheostat which isconnected to an ammeter and a tapping key. Wirefrom the other end of the tapping key is dipped inmercury.

(iv) Connect a battery, a rheostat, an ammeter and aplug key across the electromagnet as shown infigure 13.3.

(v) Insert plug in key K2 and press K

1.

(vi) Repeat the experiment by increasing current in theelectromagnet and the wire AB, one by one.

What do you observe ?

You will observe that (i) on pressing key, the wire ABswings out of the pole pieces of the electromagnet, (ii) the amplitude of swing increaseswhen the strength of the electromagnet is increased by increasing the current in it orwhen the current in the wire is increased.

What do you infer ?

(i) A current carrying conductor placed in a magnetic field experiences a force due towhich the conductor moves out of the mercury cup, as a result of which the circuitbreaks and wire falls back and swings in and out of the mercury cup.

(ii) The force increases with an increase in the current flowing through the conductor.

(iii) The magnitude of the force also increases with the increase in the strength of themagnetic field.

N

S

+

–

+–

A

K1

A

ST

B

K2

Fig. 13.3 Experimental set-up todemonstrate force on a current

carrying conductor


The direction of the force experienced by aconductor placed in a magnetic field is perpendicularto the direction of current as well as magnetic fieldand is given by Fleming’s Left Hand Rule. Accordingto this rule, if you hold the forefinger, the central fingerand the thumb of your left hand at right angles toeach other in such a way that the forefinger points inthe direction of magnetic field and the central fingeris in the direction of current, then, the thumb willindicate the direction of the force experienced by theconductor (Fig. 13.4).

For example, if we consider a horizontal magneticfield running west to east in which a conductor hangsvertically and the current in it flows in a verticallydownward direction, then the wire will experiencea force in southward direction as shown inFig. 13.5.

13.1.3 Electric motor

Faraday’s discovery of force experienced by acurrent carrying conductor placed in a magneticfield was a great discovery, because it helped in the development of electric motor. Anelectric motor is a device that can convert electrical energy into mechanical energy. Electricmotor, as you already know, is the main component of electric fans, mixer-juicer-grinders,electric churners, centrifuges, etc.

a) Construction of electric motorAn electric motor (Fig. 13.6) consists of the following parts:

i) Field magnet (N-S), which is a U-shaped permanent magnet with cylindricallycurved pole pieces.

ii) Rectangular coil (ABCD) with large number of turns of insulated copper wiremounted on a rotor shaft R and having asoft iron core (core not shown in figure).

iii) Split ring commutator (XY), which isa copper ring split in two part X and Yand mounted on the rotor shaft. One freeend of the coil is welded with X and theother with Y.

iv) Contact brushes (PQ) kept in contactwith the half-rings of the commutator toprovide for the convenience of supplyingcurrent to the coil through them.

Magnetic field

Current

Force

Fig. 13.4 Diagram to illustrateFlemings Left Hand Rule

Force

East

B

South

Fig. 13.5 Diagram showing application ofFlemings Left Hand Rule

+ –Battery

YQ

P X

D

A

BC

R

S

N

Fig. 13.6 Construction of an electricmotor


b) Principle and working of electric motor

When the contact brushes P and Q are connected to a battery, as shown, current flows throughthe coil. It can be seen by applying Fleming’s Left Hand Rule that on the strand DC the forcedue to magnetic field acts in vertically downward direction where as on BA it acts in verticallyupward direction. These two, unlike equal parallel forces acting on the coil, tend to rotate it.The coil makes a half rotation. Then the strands AB and CD interchange their positions andso do half-rings Q and P. Now the current in AB and CD again provides for a pair of equal,unlike parallel forces giving rotation to the coil in the same sense (anti-clockwise). Thus thecoil and the shaft attached with it rotate continuously till the current is passed.


1. State Fleming’s Left Hand Rule. What is the rule used for?2. Name the rule you will use to find the direction of magnetic field due to a current loop.

State the rule.3. Give two points of difference between an electromagnet and a permanent magnet.4. Give two factors on which the strength of an electromagnet depends.5. State the principle of working of an electric motor.

13.2 ELECTROMAGNETIC INDUCTION

It again occurred to Faraday that if electric current can give rise to magnetic field, a changingmagnetic field should also give rise to electric current. This phenomenon of producing electriccurrent in a coil, by changing magnetic field associated with it, is called electromagneticinduction. One of the experiments that Faraday performed to demonstrate the phenomenonof electromagnetic induction you can also try in the form of the following activity.

ACTIVITY 13.2

Aim : To induce current in a coil by changingthe magnetic field associated with it

What do you need?A strong magnet, a tightly wound cylindricalcoil of insulated copper wire mounted on aninsulating pipe, a galvanometer

What to do?Hold the coil horizontally. Connect free ends to a galvanometer. Hold the magnet at a distancefrom the coil. Move the magnet towards the coil. Move it away from the coil. Move themagnet towards and away from the coil at a faster rate.

What do you observe?i) The galvanometer shows a deflection whenever the magnet is moved relative to the

coil.ii) The galvanometer pointer swings in one direction when the magnet moves towards

the coil and in the opposite direction when the magnet moves away from the coil.

G

N S

Fig. 13.7 Electric current induced ina coil due to a moving magnet


iii) The galvanometer shows deflection only when the magnet is moving, the pointer ofthe galvanometer returns to zero position when the magnet is stopped at any position.

iv) The deflection in the galvanometer is more when the magnet moves faster.

What do you infer?An electric current is induced in a coil whenever the magnetic field associated with thecoil is changed. When magnetic field threading the coil increases, the current flows inone direction but when the field associated with the coil decreases, it flows in oppositedirection. More rapidly the field is changed greater is the magnitude of the inducedcurrent. This phenomenon of a changing magnetism into electricity is calledelectromagnetic induction and is the principle behind an electric generator.

13.2.1 Electric generator

Electric generator is a device that converts mechanical energy into electrical energy.

a) Principle of an electric generatorIf we rotate a rectangular coil of wire in a uniform magnetic field about an axis perpendicularto the field lines, the magnetic field associated with the coil will change continuously. Thiswill induce a continuously changing current in the coil.

Depending on the way the energy istapped out and the type of current we getin the output, generators are of two types:

• Direct current (d.c.) generators:These provide steady andunidirectional current output.

• Alternating current (a.c.)generators: These provide an outputcurrent, which varies continuously inmagnitude and periodically indirection.Most of the generators in use these

days are a.c. generators. Let us study theconstruction and working of this generator.

b) Construction of an alternatingcurrent generator

The construction and working of an a.c.generator is shown in Fig. 13.8.

An a.c. generator consists of the following parts:

i) Rectangular coil ABCD made of very large number of turns of insulated copper wire,wound over an insulated frame and mounted on a rotor shaft.

ii) Rotor shaft R attached to a rotating system (viz. a turbine, not shown in the figure)with the help of which it can be rotated rapidly between the pole pieces of the fieldmagnet.

S1

S2

B1

B2

Rotating

system

Rl

N S

b c

a d

R

Pivot

Fig. 13.8 An alternating current (a.c.) generator


S1

S2

B1

B2

b c

a d

S1

S2

B1

B2

b b

cc

a

a

d d

S1

S2

B1

B2

c b

d a

S1

S2

B1

B2

S1

S2

B1

B2

b c

a d

t=o

B=o

I=Io

t=T/4

B=max

I=o

T=T/2

B=max

I= – Io

T=3T/4

B=max

I=o

t=T

B=o

I=o

Io

Io

O

iii) Field magnet N-S is a strong horse shoe type permanent magnet.iv) A pair of slip rings S

1 and S

2, which are metallic rings, mounted on the rotor shaft

insulted from each other and insulated from the shaft. One free end of the coil wire(say, a) is connected to S

1 and the other (say, d) is connected to S

2.

v) Contact brushes B1 and B

2 are metallic (or carbon) brushes, kept in contact with S

1

and S2, through which current is taken out and passed in external circuits or appliances

connected across them. The appliance connected across the generator is shown by RL

in Fig. 13.8 and is called load.

c) Working of an alternating current generator

To understand the working of an a.c. generator let us consider that the coil, to begin with,is parallel to the magnetic field lines and starts rotating in anti-clockwise direction. Themagnetic field entering into the face ABCD of the coil increases from zero to some finitevalue and continues to increase till the coil becomes normal to the field. The rate, at whichthe magnetic field linked with the coil changes, is the maximum in the beginning and thenit decreases continuously. Thus the induced current in the coil is maximum at time, t = 0,and decreases with passing time. When the coil becomes normal to the field the rate atwhich flux changes becomes zero and hence current in the coil is zero.

When the coil further rotates the face of the coil through which magnetic field entersstarts changing and hence the direction of field-change and hence direction of current isreversed. The field entering into the face DCBA now continues increasing till the coilagain becomes parallel to the field at which position we find that the field linked with thecoil is zero but change in magnetic flux is maximum resulting in maximum current.

As the coil rotates further the flux linked with DCBA increases with a lesser rate ofchange in field. Thus the current in the coil decreases and attains a zero value when the coilis normal to the field. Then again the face of the coil turning towards the north pole isreversed. The current starts flowing in the initial direction and attains a maximum valuewhen the coil comes in the position we started with.

Fig. 13.9 shows the positions of the coil at crucial stages of rotation and the current inthe coil at these instants.

Fig. 13.9 Current in the coil of generator as it rotates


Thus, a continuously varying current which changes its directions after every T/2seconds is obtained. Such a current is called alternating current.

d) Direction of induced current: Fleming’s Right Hand Rule

The direction of induced current that is indicated in Fig. 13.9 is the actual direction weobtain for the given situation. The direction of current induced in a conductor moving in amagnetic field is given by Fleming’s Right Hand Rule. According to the rule, stretch thethumb, the forefinger and the central finger of your right hand at right angles to eachother and hold them in such a way that theforefinger points in the direction of magneticfield and thumb in the direction to motionof the conductor then the central finger willpoint in the direction of the induced current(Fig. 13.10).

Using this law now you can easily checkthat the direction of induced current inFig. 13.10 is towards north as shown.


1. What is meant by electromagnetic induction?

2. State the principle of working of an electric generator.

3. State Fleming’s Right Hand Rule and its use.

4. An LED is connected across a long solenoid (see the adjoining figure). When a strongpermanent magnet is moved towards the solenoid, the bulb glows. Explain.

5. Who discovered the phenomenon of electromagnetic induction?

13.3 DISTRIBUTION OF ELECTRIC POWER

Generators are to be constructed near the source of energy. You cannot construct a dam forevery house or a thermal power plant for every industry. Power is generated on large scaleat one place and then it is distributed to consumers situated at far off places from thegenerating stations. How is it done?

The conductor system through which electric power is conveyed from a generatingstation to consumer’s premises may be divided in two parts:

• Transmission System• Distribution System

13.3.1 Transmission system

Long distance transmission of DC power is technically impractical and financially non-viable. Because, we have to transmit power at the same voltage at which it is generated.But we have developed devices, which can increase or decrease the magnitude of alternatingvoltage. These devices are called transformers.

A transformer, which increases the level of voltage, is called step-up transformerand the one, which decreases the level of voltage, is called step-down transformer.

Magnetic field

Motion

Vertically

downward

Induced

current

North

West B

Fig. 13.10 Diagram to illustrateFleming’s Right Hand Rule


G

Power

station

(11 kv)

Step-up

Transformer

(139 KV)

Electric

pole

Electric

pole

Town

substation

Step down

Transformer

Distribution step

down

Transformer

220 v

Distribution line

In a transformer, if voltage is increased current decreases in the same proportion, and vice-versa. Thus, using a step-up transformer we can transmit power at high voltage and lowcurrent. Due to low current, the power losses and voltage drops on lead wires reduce tovery low values. Also the cost of the distribution system decreases substantially. A typicaltransmission system is shown in Fig. 13.11.

Fig. 13.11 A typical transmission system

In India, at the generating station the power is normally generated at 11000V, 50Hz.Using a step-up transformer this voltage is stepped up to 13.2 kV. Transmission is done atthis high voltage. At the town substation, this voltage is first reduced to 33 kV with thehelp of a step-down transformer and then 33 kV is stepped down to user level 220V, 50 Hzusing a step-down distribution transformer.

13.3.2 Distribution system

The distribution system is the arrangement, whichprovides power from town substation to the consumer.It involves feeders, distributors, sub-distributors andservice mains. Normally there are two types ofdistribution systems:

• Tree system

• Ring system

These days, mostly, it is the ring system that weuse. The arrangement of various components of ringmode distribution is shown in Fig. 13.12.

13.3.3 Domestic wiring circuit

Through the distribution system the electricity reaches to an electric pole in our street fromthe pole two insulated copper wires come to our house. One of these wires is at a highpotential of 220V. It is called live wire and is denoted by L. Normally, we use for live wire,a wire having red-coloured insulation. The other wire is ground potential of zero volt. It iscalled neutral wire and is denoted by N. For neutral wire we can use any colour except redor green. All the appliances in our house are connected in parallel to these two wires. Atypical household circuit is shown in Fig. 13.13.

S

S

S

F1

F2

Substation

Distributor

Service Mains

Feeder Feeder

S

S

S

Fig. 13.12 A ring mode distributionsystem


Fig. 13.13 A typical domestic electric wiring circuit from electric pole to a roomconsisting of bulb, a fan and a plug prompt.

Three specific features of the circuit are to be specially noted. These are as follows:

(i) All the appliances are connected in parallel.(ii) All fuses and switches are to be placed on live wire.(iii) A third wire of green colour called earth wire also runs along with live and neutral

wires. Bodies of all electrical appliances are earthed through this wire.


1. In domestic electrical wiring, on which wire do we normally place switch to operatelight?

2. Which effect of electric current is used in the working of an electric fuse?

3. Why are domestic appliances connected in parallel? Give one reason.

4. Name the electrical device used to increase or decrease the magnitude of voltage. Canit be used with direct current?

5. What are the advantages in transmission of electric power at high voltage?

6. Switching off a bulb in one room has no effect on other lamps in the same building.Why?

13.4 HAZARDS IN USING ELECTRICAL ENERGY

When used with care, electricity is the most convenient form of energy. But carelessness inthe use of electrical energy may prove to be fatal for the operator as well as for the installation.For example,

• If a person touches a live wire he or she may get a severe shock which may lead evento death.

• Short-circuiting due to damaged wiring or overloading of the circuit can cause electricalfire, which may damage the building.

Defects in household wiring like loose connections, defective switches, sockets, plugs,etc. may also cause sparking which may lead to an electrical fire. Some precautions andsafety measures are, therefore, suggested in the next subsection below.

Room -1

S1

S2

Fan

Main

fuse

Main

switch

Meter

Main

fuse

L

N

E

From electric pole

Sub

circuit

fuse

B

To other rooms


13.4.1 Precautions in the use of electrical energy

a) For household wiring always use good quality wires having proper thickness andinsulation. Plugs, sockets, switches and electrical appliances used should be ISI marked.All the wire connections should be tight and all the wire joints should be covered withan insulating tape. Defective switches, sockets, plugs, etc. should be immediatelyreplaced.

b) All the switches in your household electrical wiring circuit should be placed on thelive wire of the circuit so that when the switch is off, the appliance is disconnectedfrom the live wire and on touching the device you do not get shock.

c) Switch off the mains before you start working on a repair job on the electrical circuit.In case it is necessary to work on live circuit put on rubber gloves, rubber shoes anduse tools with insulted handles.

d) In case of an electrical accident switch off the main switch of electrical supply. Try toinsulate the person who has received a shock. In any case do not touch him directly.Never use water to extinguish fire arisen due to electrical sparking.

e) Ensure that the safety measures of earthing and fuse are properly done in your householdelectrical circuit. Ensure that the fuses are placed on live wires and are of proper currentrating.

13.4.2 Causes of electrical hazards

There are basically three causes behind most electrical hazards:

• Current leakage• Short-circuiting• Overloading

a) Current leakage

Some times, due to wear and tear or due to excessive heating, the insulation covering ofconnecting wires is removed and the wire becomes naked. This naked live wire may touchthe metal case of an electrical appliance raising its voltage to the level of the main voltage.The metal body of the appliance if connected to ground, current will flow through it to theground resulting in current leakage. Body of a person also acts like a conductor and aperson who touches the metal case of a current leaking appliance may get a shock.

b) Short circuiting and over loading

These are two faults in electrical circuits due to which heavy current may flow through thecircuit resulting in the overheating of conducting live wires and consequent fire in theinstallation.

• Short-circuiting takes place when a naked live wire touches a naked neutral wire.Normally sub-standard wires wear out soon and may cause short-circuiting.

• Overloading of electrical circuit results, when the number of appliances operated onthe circuit at the same time exceeds the limits the circuit wiring can withstand. Weknow that in household circuits all the appliances are connected in parallel. In parallelcircuits as we add more and more resistors (appliances) in parallel more current isdrawn from the supply.


A slight omission causing short circuiting or overloading may, therefore, cause fireand damage the whole installation. A safety device against this is a fuse.

13.4.3 Safety devices used in electrical circuits

a) Electric fuse

Electric fuse is a weak link in the electrical circuit. It is a short piece of a thin wire made ofa lead-tin alloy. The material that a fuse wire is made of has a low melting point and highresistance as compared to the material of live wire. Consequently, when current in thecircuit starts increasing the specified limit the fuse wire gets heated up and blows off. Thefuse wire thus saves the installation from getting damaged.

An electric fuse wire is always placed in series with the main supply on the live wire.Depending on the circuit specification we use fuse wires of different current rating normally 5Afor domestic lights and 15A for domestic power. 15A fuse wire is thicker then a 5A fuse wire.

b) Earthing the electrical appliance

To avoid shock due to current leakage in electrical appliances the metal body of the applianceis earthed, i.e. connected to earth. For this purpose a separate wire, called earth wire, runs allthrough the circuits along with live and neutral wires. Metal bodies of all the appliances arekept connected with the earth wire. The free end of the earth wire is attached to a copper plateburied deep in the ground. This leaves the body of electrical appliances at the same potential(zero) as the earth and hence when we touch the metal body we do not get shock. Earthing isthus a safety device incorporated in an electric circuit to protect the operator.

The above discussion shows how important the provisions of earthing and fuse are inour electrical circuits.


1. Why do electricians wear, rubber shoes or rubber hand gloves while working on electriccircuits?

2. State two hazards associated with the use of electricity.3. Why is a fuse wire is made of a tin-lead alloy?4. What is the usual rating of an electric fuse used in the following:

(i) lighting circuits(ii) power circuits

5. What will you do if you see a person in contact with a live wire?6. Along with live wire and neutral wire, a third wire is also used in domestic electrical

wiring. What is the name of this third wire? What is the purpose of this wire?

13.5 HOUSEHOLD ELECTRICAL APPLIANCES

There is a long list of electrical appliances that we use in our houses for convenience in ourwork and comfort. Some of these appliances are based on thermal effect an electric currentand the other are based on magnetic effect. We have already studied these effects in theprevious lesson. In this section we will study the construction and working of some of theimportant electrical appliances we use in our houses.


13.5.1 Electrical appliances based on thermal effect of electric currentWhen potential difference is applied across a conductor, the free electrons in it move upthe potential to minimize their potential energy. In doing so they collide with other particlesof the material of the conductor on their way. As a result the conductor gets heated up. Theheat so produced can be utilized in a number of electrical appliances, such as electric bulb,electric iron, electric oven, electric water heater, electric room heater, soldering iron, electrickettle, etc. A brief description of some of these devices is given below.

a) Electric lampsGenerally two types of electric lamps are used in our houses for producing light

• incandescent lamps• fluorescent lamps.

i) Incandescent lamp: An incandescent lamp, which is also called an electric bulb, isbased upon the principle that when a conductor having a high melting point is heatedelectrically to a high temperature it becomes incandescent and starts emitting light.If we closely examine an electric bulb we find that a small coil of tungsten wire (meltingpoint 3300 K) called filament is mounted on insulating supports inside the bulb. Thetwo ends of the filament are taken to the base of the bulb by two thick metal leads. Thisentire assembly is enclosed in a sealed glass bulb filled with noble gases at low pressure(Fig. 13.14).When the bulb is put in its holder and switched on, currentflows through the filament heating it to incandescence.The inert gas in the glass bulb prevents the filament fromgetting oxidised at that high temperature.

ii) Fluorescent lamp: A fluorescent lamp has a filamentsealed in one end of a glass tube and another electrodeat the other end. The tube contains little amount ofmercury that vaporizes when the filament gets hot. Anelectric current is then set up through the mercuryvapours from one end of the tube to the other. Themercury vapours give off both visible and ultra violetlight. The ultra violet light strikes the fluorescent coatingon the inside surface of the glass tube and causes it toglow brilliantly. The colour of the light given off by thelamp depends upon the material used for the fluorescentcoating. Fluorescent lamps produce a great deal of light with little heat. This is thereason why their efficiency is comparatively high.

b) Electric iron or electric press

An electric iron basically has four parts:• base plate,• heating element,• pressure plate, and• upper cover with handle of insulating material

such as bakalite.

220 v 60 w

Glass Bulb

Filament

Brass cap

Holder pins

Base Contacts

Fig. 13.14 Incandescent lamp

To on/off

switch

Mountain

panel

Lamp

Base

Tube filled with Argon gas,

and mercury vapour

Fig. 13.15 Fluorescent lamp


Rubber sealingCoil

Container

Lid

Handle

The heating element consists of fine nichrome strip wound ona mica sheet, which is further enclosed, between thin strips of mica.In electric irons, mica is used for electrical insulation. Mica has aspecial property that it acts as an insulator for electricity but as aconductor for heat.

The heating element is placed between the base plate and thepressure plate. The pressure plate protects the heating element anddoes not allow it to move from its position. When an electric currentpasses through the heating element, heat is generated in it that istransferred to the base plate by conduction. The heated base platecan now be used to iron clothes.

c) Electric heater and electric heat radiators

These devices have heating elements madeof nichrome wire in the form of spiralarranged in the grooves of ceramic plateor rod. The electric stove and room heatersare two such devices (Fig. 13.17).

In heat radiators, the heating elementis mounted in a frame having a highly polished concave reflectorat its back. When electric current is passed through the heatingelement, it gets heated and starts radiating heat. The heat radiationfalling on the concave polishedreflector are also reflected in theforward direction.

d) Electric kettle

It is an electrical device in which heating effect of electriccurrent is used (Fig. 13.18). You might have seen electrickettles of different shapes and sizes being used in houses,offices and restaurants. In an electric kettle, the heatingelement is fitted at the bottom of the vessel in such a way thatmaximum heat from the heating element is conducted to the vessel.

e) Electric immersion heater

An immersion heater consists of a heating element made of nichromewire housed in a metallic tube. The nichrome wire is separated fromthe metallic tube by filling the intervening space with magnesiumoxide powder that is an electrical insulator and a reasonably goodconductor of heat. The metallic tube is coiled in a few turns so thatits long length may be accommodated in a smaller space and assuch may remain in contact with the liquid to be heated. The designof an electrical immersion heater is shown in Fig. 13.19.

3-Pin plug top

Element

Code

Terminal

(i) Base Plate

(ii) Cast iron

pressure plate

(iii) Heating element

Fig. 13.16 Electriciron

Fig. 13.17 (a) Electricstove (b) Room heater

(a) Electric stove

(b) Room Heater

H.E.

Fig. 13.18 Electric kettle

Fig. 13.19 Electricimmersion heater


13.5.2 Electrical appliances based on magnetic effect ofelectric current

All the devices based on magnetic effect of electric currentsuch as electric bell, electric fan, mixer-grinder and ammeter,voltmeter etc. make use of an electromagnet and/or the motordescribed in section of this lesson. Below is given a briefdescription of two of these devices, viz. electric bell andelectric fan.

(a) Electric bell

If you closely examine an electric bell you will see that it hasa U-shaped electromagnet as shown in Fig. 13.20. The twoends of the windings of the electromagnet are connected tothe power supply (battery or the mains) through a make and break arrangement and a pushbutton called the bell push (a kind of switch).

When the bell push (B) is pressed the electric current flows through the coils of theelectromagnet and the soft iron core of the electromagnet becomes magnetized. Thismagnetized iron core attracts the armature, consequently the hammer attached to the armaturehits the gong and a loud sound is produced.

But as soon as the armature is attracted by the electromagnet the circuit is broken at thecontact screw. The electromagnet no more remains magnet. The armature due to springaction returns back to originalposition. The process is repeatedagain and again. The hammerperiodically keeps on hitting the gongtill the push button is not released.

(b) Electric fan

Electric fans are used to circulate airin a room. Mostly a fan uses apermanent capacitor type motor.Three blades are symmetricallyattached to the rotor shaft of themotor. As soon as the switch is madean electric current flowing throughthe motor rotates the blades which circulate air of the room. Fig. 13.21 shows the variousparts of a ceiling fan.


1. Which material is the filament of an incandescent lamp made of?2. Name a substance that is good conductor of heat but a bad conductor of electricity.3. What is the role of a polished concave surface behind the element of a room heater?4. Is fluorescent tube based on heating effect of electric current?5. Name the energy transformation that takes place in an electric fan.

Hammer

Gong

Soft-iron

core

Coil

BSpring

Armature

Screw

Contact

AC Mains

Fig. 13.20 Electric bell

Back CoverDecorating cup

Motor

Blade

Heat bolt

Bottom canopy

Upper cover

Condenser

Split pin

Bubin

Bolt

Shakle

Top Canopy

Hanging rod

Bolt to hold

Canopy

Condenser housing

Terminal connector

Back Cover

Fig. 13.21 Parts of a ceiling fan


LET US REVISE

• A current-carrying coil behaves as a magnet, one end of which behaves as the northpole and the other as the south pole. The polarity of the coil is determined by using theRight Hand Thumb Rule.

• The strength of an electromagnet depends on (i) strength of current flowing throughthe coil, (ii) number of turns per unit length of the coil, and (iii) the nature of the corematerial.

• A current-carrying conductor placed in magnetic field experiences a force. Themagnitude of the force depends on (i) strength of the magnetic field, (ii) current flowingthrough the conductor, (iii) length of the conductor, and (iv) orientation of the coilwith respect to the field.

• The direction of the force experienced by a current carrying conductor placed in amagnetic field is given by Fleming’s Left Hand Rule.

• An electric motor is a device, which converts electrical energy into magnetic field.

• When magnetic field associated with a closed coil is changed a current is induced inthe coil, which lasts, so long as the change in magnetic field is continued. Thisobservation first made by Faraday is called electromagnetic induction.

• Electric generator is a device, which converts mechanical energy into electrical energy.The device is based on the phenomenon of electromagnetic induction.

• Long distance transmission of a.c. power has become possible by the use of devicescalled transformers. There are two types of transformer (i) step-up transformers, whichincrease the level of voltage (ii) step-down transformers, which decrease the level ofvoltage.

• In domestic wiring circuits, fuse and earthing are very important safety provisions.Careless use of electric power may be dangerous. Therefore, we must take properprecaution in the use of electricity.

• Electric bulb, electric heater, electric iron, electric kettle, etc. are examples of appliancesbased on thermal effect of electric current.

• Electric bell, electric crane, electric fan, electric mixer-juicer-grinder are appliancesbased on magnetic effect of electric current.

TERMINAL EXERCISES

A. Objective type questions.

1. Name the device which converts

i) electrical energy into mechanical energy

ii) mechanical energy into electrical energy

iii) high voltage into low voltage


iv) low voltage into high voltage

v) electrical energy into light energy2. Fill in the blanks.

i) The electricity in our home has voltage __________________ volts, frequency____________Hz and it provides ____________ current.

ii) The power plant in which ____________ or ____________ is used as basicfuel is called thermal power plant, whereas Uranium–235 is used in____________ power plant.

iii) In a hydroelectric power plant ____________ of water stored in a dam isconverted into ____________ of rotation of a turbine, which in turn changesinto ____________ energy.

iv) In the statement for Fleming’s Left Hand Rule central finger points in the directionof ____________, forefinger points in the direction of ____________, and thumbpoints in the direction of____________

v) _______________, ____________ and/or ____________are at the root of allelectrical hazards.

B. Descriptive type questions.

1. A circuit has a fuse of 5A. What is the maximum number of 100 W (220V) bulbs thatcan be safely used in the circuit?

2. Explain the principle and working of an a.c. generator.

3. Explain the construction and working of the following:

i) Electric motor

ii) Electric iron

iii) Electric bulb

iv) Electric fan

v) Electric bell

4. Distinguish between the following:

i) Permanent magnet and electromagnet

ii) Electric generator and electric motor

5. Explain the role of earthing and fuse in electric circuits.

6. With the help of suitable diagrams show how electrical energy is

i) Transmitted from generating station to town substation.

ii) Distributed from town substation to the consumer site.


7. Draw a domestic electric wiring diagram from electric pole to one room of the househaving a bulb, a socket and a fan. Provide separate fuse for each room and separateswitch for each device.

8. Describe an experiment to demonstrate:i) Force experienced by a current-carrying conductor placed in a magnetic fieldii) Phenomenon of electromagnetic industries

9. Enlist the precautions one must observe in the use of electrical energy.


13.1

1. Fleming’s Left Hand Rule is used to find the direction of force experienced by acurrent carrying conductor placed in a magnetic field. According to this rule,stretch the forefinger, the central finger and thumb of your left hand at right anglesto each other and hold them in such a way that the forefinger points in the directionof the magnetic field, central finger points in the direction of electric current, thenthumb will point in the direction of motion of (or force) in the conductor.

2. The direction of magnetic field due to a current-carrying loop can be determinedusing Right Hand Thumb Rule. According to the rule, hold the right hand abovethe coil with the curling fingers pointing in the direction of current, then thestretched thumb will indicate the direction of magnetic field.

3. i) An electromagnet is much stronger than permanent magnet.

ii) The strength and polarity of an electromagnet can be changed but that of apermanent magnet is fixed.

4. The strength of electromagnet depends on (i) the current flowing through theconductor, and (ii) number of turns in the coil.

5. When a current-carrying coil is placed in a uniform magnetic field it experiencesa pair of equal, opposite and parallel forces due to which the coil rotates.

13.2

1. The phenomenon of setting up electric current in a coil by changing magneticfield associated with it is called electromagnetic induction.

2. When a coil of wire is rotated in a uniform magnetic field, the magnetic fieldassociated with the coil is changed due to which current is set up in the coil.

3. We use Fleming’s Right Hand Rule to determine the direction of the currentinduced in a conductor when the conductor moves in a magnetic field crossingthe field lines. The law states, stretch the forefinger, the central finger and thethumb of your right hand at right angles to each other, and hold it in such a waythat forefinger points in the direction of field, thumb points in the direction ofmotion of the conductor, then, the central finger will point in the direction ofinduced current.


4. When the magnet moves towards the coil, the magnetic field threading the coilincreases inducing a current in it. Due to the induced current the LED glows.

5. Michael Faraday

13.3

1. Live wire

2. Thermal effect

3. So that different appliances may draw different currents needed by them.

4. Transformer; No it cannot be used with d.c.

5. Transmission at high voltage provides for low current due to which the powerlosses and voltage drop in lead wires substantially decreases and transmissioncan be done at low cost.

6. Because all the lamps are connected in parallel.

13.4

1. The insulating rubber shoes or gloves do not let the current flow through thebody of the electrician to earth and he is saved from getting any shock.

2. i) Electricity may give shock to the careless operator.

ii) Short circuiting or over loading may cause fire.

3. Tin-lead alloy has low melting point and high resistance, hence blows off muchbefore the rest of the circuit wiring is heated appreciably.

4. i) Light circuit 5 A

ii) Power circuit 15 A

5. We will switch off the main switch and try to insulate the body of the person (sothat he does not remain in contact with earth) without directly touching him/her.

6. The third wire is earth wire. It is a safety device to protect the operator fromelectric shock in case of current leakage from the body of the appliance.

13.5

1. Tungsten

2. Mica

3. It reflects the heat falling on it and sends it in forward direction.

4. No

5. Electrical energy into mechanical energy


GLOSSARY

Earth wire: Green-coloured wire, which at one end is connected to the metallic bodyof an appliance and the other end is connected to a copper plate, buried deep in the ground.

Electric fuse: A short piece of thin wire made of lead-tin alloy, placed in series withthe main supply on the live wire, that acts as a safety device.

Electric generator: A device that converts mechanical energy into electrical energy.

Electric motor: A device that converts electrical energy into mechanical energy.

Electromagnet: A current-carrying solenoid with a soft core inside it.

Electromagnetic induction: Phenomenon of producing electric current in a coil bychanging the magnetic field associated with it.

Live wire: Red-coloured wire that carries electricity at high potential of 220V.

Neutral wire: A wire that is at ground potential of zero volts may be of any colourexcept red or green.

Step-down transformer: A device, which decreases the magnitude of alternatingvoltage.

Step-up transformer: A device, which increases the magnitude of alternating voltage.

Transformer: A device which can increase or decrease the magnitude of alternatingvoltage.

14

Chemical and Nuclear EnergyYou have studied in the previous lessons that energy is an essential part of our life. We allrequire energy in our daily life in the form of food, fuel, electricity etc. It is also needed forcooking food, running the transport system and industries. The conventional energy sourcessuch as coal, petroleum and natural gas are being increasingly used. But we have onlylimited resources of conventional sources of energy (coal, petroleum, etc.) and they aredepleting at a very fast rate. Therefore, scientists all over the world are trying to developalternate sources of energy. Of the various sources of energy that would serve as alternativeto conventional sources, the main sources are nuclear and solar energies. The most commonforms of energy are heat, light and electricity. Other forms of energy are chemical andnuclear energy.

In this lesson we shall study in detail about chemical and nuclear energies and theirvarious sources. We will also study about the process of combustion and the conditionsnecessary for it. This knowledge is useful in finding ways and means of controlling firewhich some times proves to be destructive and dangerous. We will also study the types offuels, change of chemical energy into electrical energy, and about nuclear energy.

OBJECTIVES

After completing this lesson, you will be able to :

• differentiate between chemical and nuclear energy;• define various fossil fuels, such as coal, petroleum, biomass;• list the important compounds of petroleum and their uses;• define combustion and calorific value of fuel and solve problems related to calorific

value;• state the conditions necessary for combustion and describe the functioning of soda-

acid fire extinguisher;• highlight the importance of food as body fuel;• explain the functioning of voltaic cell, its weaknesses and necessary modifications;• explain the terms radioactivity, radioisotopes, fission and fusion;• describe the functioning of nuclear reactor and generation of electricity therefrom;• compare nuclear power plant with a thermal power plant;• list some uses of nuclear energy and hazards involved in its production.

: 256 : Chemical and Nuclear Energy

14.1 CHEMICAL ENERGY

You must have noticed that at the time of whitewashing, when water is added to quicklime, there is loud hissing sound, and the mixture almost starts boiling.

Do you know why does this happen? In this case, a chemical reaction between quicklime (CaO) and water (H

2O) takes place in which large amount of heat is liberated as

follows:

CaO + H2O Ca(OH)

2 + Heat liberated

This means, there must be some stored energy in the chemicals involved in the reactions,which comes out as heat. This energy is chemical energy. Thus, chemical energy is thatform of energy, which the substances have by virtue of their composition and nature.Chemical energy becomes apparent during chemical reactions when chemical energy of asubstance changes into other forms such as heat, light and electricity, etc.

We shall now consider those reactions where chemical energy is converted into heatenergy or vice versa. Such reactions are called thermochemical reactions. These reactionscan be divided into two types – exothermic reactions and endothermic reactions.

14.1.1 Exothermic reactions

You know that burning of coal gives out large amount of heat.

The reaction can be represented as:

C + O2 CO

2 + Heat

Such chemical reactions in which heat is given out are called exothermic reactions.

ACTIVITY 14.1

Aim : Heat change in chemical reactions

What to do?

Take a small amount of baking soda in a test tube. Add a few drops of vinegar orlemon juice to it.


You will observe that a brisk effervescence takes place and a colourless gas isevolved? Touch the bottom of the test tube. What do you feel? Does the test tubebecome hot? You will find that the test tube becomes hot. This shows that heat isevolved during this reaction. Hence, it is an exothermic reaction.

Similarly, addition of water to quick lime is also an exothermic reaction. Canyou think of some other examples of exothermic reactions?

14.1.2 Endothermic reactions

You must have noticed that evaporation of water on a hot day is faster. Here the followingtransformation takes place.

H2O(l) + Heat H

2O(g)

Such reactions where heat is absorbed are called endothermic reactions.

Chemical and Nuclear Energy : 257 :

Similarly, the decomposition of mercuric oxide (HgO) is also an endothermic reaction.

2HgO + Heat Hg(l) + O2(g)


1. When carbon is burnt in the presence of oxygen what type of energy is evolved?2. When Uranium-235 is bombarded with neutrons which type of energy will be evolved?3. The amount of energy liberated in a chemical reaction is large or small?4. The ability to do work is known as ____________

14.2 FUELS

You know that for cooking food we require heat energy that we get by burning wood, coal,kerosene, or liquid petroleum gas (LPG). To run vehicles we need petrol or diesel. Allthese provide substances that generate energy are known as fuel.

Fuels are chemicals, which react with an oxidizing agent. Usually the oxidizing agentis oxygen itself. Energy is released during the reaction and new chemicals are formed. Anysubstance, which reacts with oxygen, or other oxidizing agent, could be used as fuel. Thebest known fuels include petrol, diesel, coal and natural gas. All these fuels burn in thepresence of oxygen.

Scientists have developed other fuels for special purposes. For example, a chemicalcalled hydrazine is used as a rocket fuel. It is not burnt in oxygen, but it is oxidized byconcentrated nitric acid. Other examples are the splitting of uranium in nuclear fuel reactorsand the conversion of chemical energy into electrical energy in electrochemical cells. Theover all changes, which take place when a fuel burns, are shown below:

Fuel Useful energy released+ reaction +

Oxidizer (burning) Chemical products(such as oxygen)

But, you should remember that every chemical, which burns is not necessarily a goodfuel. The fuel must release plenty of energy when it is burnt. But there are many otherimportant points to be considered. For most people it is probably convenience and costthat seem to be important. We prefer fuels which are safe to use and which do not produceunpleasant gases and smoke when they burn.

14.2.1 Classification of fuels

On the basis of physical states the fuels are classified into three categories:

a) Solid: coke, coal, wood and charcoalb) Liquid: petrol, alcohol, diesel and kerosenec) Gas: liquefied petroleum gas (LPG), coal gas and petrol gas

Liquid and gaseous fuels are better, as compared to solid fuels because:• these can flow through pipes,• can be lighted at a moment’s notice,• no ash is left,• have high heat content, and• their supply and distribution is easier.


14.2.2 Fossil fuels

Coal and petroleum are major fuels that are being used in large amount at present. Theseare known as fossil fuels. The fossil fuels are carbon-containing substances that wereformed from the remains of the marine organisms that lived millions of years ago, underthe influence of high temperature and pressure in the interior of earth. We have limitedamount of fossil fuels. According to some estimates we would run out of fossil fuelsbefore the middle of the twenty first century. Fossil fuels are therefore, called depeletableor non-renewable source of energy.

In this subsection, we will learn about the types of fossil fuels. The fossil fuels can bedivided into two categories – coal and petroleum.

a) CoalCoal may be defined as a sedimentary rock that burns. Coal deposits were formed long agoby decomposition of plant matter buried under theground. It is a complex mixture of compounds ofcarbon, hydrogen and oxygen and some free carbon.It also contains small quantity of nitrogen and sulphur.Coal is important because it can also be used as asource of other fuels like coal gas and synthetic petrol.

We know that wood is the starting material forcoal. Depending on the extent of carbonization, we get different varieties of coal. Theseforms are different in carbon contents as listed in Table 14.1.

When coal is heated strongly to a temperature of about 1273K to 1373K, in the absenceof air, it decomposes into coal gas, coke, ammoniacal liquor and coal tar. This process isknown as destructive distillation. Let us study more about these components of coal.

i) Coal gas: One of the most promising methods for making coal more efficient andcleaner fuel involves the conversion of coal to a gaseous form, i.e. coal gas. Thisprocess is called coal gasification. Coal gas is a mixture of hydrogen, methane andcarbon monoxide. All the gases present in coal gas can burn to provide heat. Due tothis, coal gas is an excellent fuel having high calorific value. It is used as a cookinggas. In the past it was used as illuminant also for lightning homes, factories and streets.

ii) Coke: It is used as a reducing agent in blast furnaces to extract iron from its ores. It isalso used as a source of carbon in the chemical industry and as a fuel.

iii) Ammoniacal liquor: It is converted into ammonium sulphate by absorbing in dilutesulphuric acid. The ammonium sulphate is used as a fertilizer.

iv) Coal tar: It was earlier considered to be a nuisance. Even its disposal was a problem.Subsequently, it was used for surfacing roads. It has now been found to be a richsource of aromatic hydrocarbons.

b) PetroleumThe name petroleum means rock oil (petra: rocks; oleum: oil). It is called petroleum becauseit is found in the crust of earth trapped in rocks. It is used to describe a broad range of fossilhydrocarbons that are found as gasses, liquids and solids beneath the earth surface. Thetwo common forms of petroleum are crude oil and natural gas.

Table 14.1 : Types of coal

Type Carbon content

Anthracite 90%Bituminous 80%Lignite 70%Peat 60%


i) Crude oil: It is a complex mixture of alkane hydrocarbons with water and earthparticles. The final stage of refining involves the removal of impurities such as sulphurcompounds. When a fuel containing sulphur is burnt, the sulphur in it turns into sulphurdioxide, an acidic gas. So it is to be purified or refined before it can be used for specificpurposes. The process of separating crude petroleum oil into more useful fractions iscalled refining. The refining of petrol is done by the process of fractional distillation.Refining is needed to make sure that all the oil is turned into useful products. Cracking

also occurs during the refining process of petroleum. The process of breaking biggerhydrocarbon molecules into smaller hydrocarbons molecules by heating in the presenceof a catalyst is called cracking.

The refining of petroleum or separation of petroleum into different components isbased on the fact that the different compounds of crude oil have different boiling pointsranges. The fraction of petroleum having highest boiling point range is collected in thelowest part of the fractionating tower (Fig. 14.1). The fraction having lowest boiling pointrange is collected in the topmost part of the tower. Such a process of separation of differentfractions of petroleum from crude oil is called fractional distillation. The various fractionsobtained by the fractional distillation of crude petroleum oil and their uses are given inTable 14.2.

Fig. 14.1 Fractionating tower

crude oilstorage

crude oilstorage

pre-heating furnace

section ofbubble cap

273 – 303 K

303 – 360 K

363 – 513 K

623 – 773 K

513 – 623 K

C1 – C

4

C5 – C

7

C7 – C

13

C13

– C18

Over 500 °C

Boiling pointrange


Table 14.2 : Fractions obtained by fractional distillation of petroleum

Fraction Approximatecomposition

Boiling range Uses

Gaseoushydrocarbons

Crude naphthaPetroleum ether

Petrol/gasolineBenzene

Kerosene oil

Fuel oilGas oilDiesel oilFurnace oil

Lubricant oilMedicinal oilMotor oilGrease

Paraffin waxPetroleum jellyPetroleum waxPetroleum coke

Heavy fuel oiland bitumen

C1-C4

C5-C7

C7-C9

C9-C10

C10-C13

C13-C18

C15-C18

C18-C30

C30 onward

Up to 303K

303-363K

363-393K393-423K

423-513K

513-623K

Above 543K

673K Upward

Forms residue

As fuel gas after liquefaction, ascarbon black.

As solvent in varnish and rubberindustries, for dry cleaningAs motor fuel, for dry cleaningFor dry cleaning

Fuel for stoves, manufacture of oilgas, as an illuminant

Fuel for diesel engine and tractors,cracking stock for gasoline

Paint oil, transformer oil, forlubrication etc.

Ointments, candles, paraffin wax,for matches, paints, water proofing,as solid fuel, protecting paints

Paints, road surfacing

(ii) Natural gas: Natural gas is a mixture of lightweight alkanes. The composition ofnatural gas depends upon the source, but a typical sample contains 80% methane, 7%ethane, 6% propane and 4% butane. Natural gas occurs deep under the crust of theearth alone or along with the petroleum deposits. Therefore, some wells dug into theearth produce only natural gas, whereas others produce natural gas as well as petroleum.In the later case, natural gas is a byproduct of petroleum. The propane and butaneseparated from the natural gas are usually liquefied under pressure and called as liquefiedpetroleum gas (LPG). It is used as domestic and industrial fuel. Compressed NaturalGas (CNG) is also used as fuel for transport as well as in industries.


1. Name any two constituents of coal gas.2. Write two examples of fossil fuels that you use in your daily life.


3. What are two main varieties of coal?4. The boiling point of water, methyl alcohol and kerosene are 373K, 313K and 543K,

respectively. If a mixture of these three liquids is separated by fractional distillationcolumn, which component of the mixture will be collected near the bottom of thecolumn?

5. Name any two products of the petroleum.6. State any two uses of petroleum products.7. Name any one hydrocarbon fraction obtained during fractional distillation of petroleum

which is used as domestic fuel.

14.3 COMBUSTION

You would have seen that when coal is burnt in an angithi or chulah, it turns red hot. Aftersome time when the chulah cools down, we find no coal but the ash is left. What hashappened to the coal?

On burning, coal changes into carbon dioxide and ash. Hence, on burning, thecomposition of a substance changes, i.e. the substance changes into other substances. Thisis called combustion. Combustion may be defined as a chemical change in the presenceof oxygen in which both heat and light are produced at the same time and the compositionof substance changes.

Burning of coal, paper, candle and hydrocarbon are the examples of combustion.Chemical equations of some combustion reactions are given below:

combustionC(s) + O

2(g) CO

2(g)

+ heat + light

CH4(g) + 2O

2(g) CO

2(g)

+ 2H

2O(l) + heat + light

It may be noted that during combustion certain chemical change should occur. If nochemical change occurs in the reaction but heat and light are produced, that reaction willnot be combustion. For example, when we switch on an electric bulb it starts glowing. Weget light from it. If we touch, we find that glowing bulb also produces heat. Do you thinkthat glowing of electric bulb is a case of combustion?

No, glowing of bulb is not combustion because no chemical change occurs, i.e. nonew substance is formed.

14.3.1 Conditions for combustionLet us look at some of our day to day experiences and find out the conditions that arenecessary for combustion. If we bring a burning matchstick near paper, kerosene, petrol oralcohol, they immediately catch fire and start burning, but in case of the substances likeglass and stone no change is observed. Such substances that can burn are called combustiblesubstances. For example, petrol, kerosene, alcohol, etc. Substances that do not burn arecalled non-combustible substances. For example, stone and glass.

Hence, we can say that for combustion a combustible substance is required.

We know that air contains oxygen, which is a good supporter of combustion. Whenwe cover burning coal with vessel the supply of air is cut off, hence, the coal fire stops. Weknow that in chulahs used in the villages for cooking food, gaps are left between the logsof wood. These gaps are left for the air to enter the chulah. Thus, a good supply of oxygenis necessary for burning.


Often we find that in order to light up a pressure stove, a burning matchstick is kept forsome time over the kerosene oil taken in a cup round the burner and the oil starts burning.

Let us perform an activity to prove that air is necessary for burning.

ACTIVITY 14.2

Aim : Air is necessary for burning

What is required?

A plastic trough, water, a candle, a glass tumbler, match box

What to do?

Take a candle about 8 cm long and fix it in a plastic trough. Pour water in the troughas shown in Fig. 14.2 a. Light the candle. Invert the glass tumbler over the candle.

Fig. 14.2 To show that air is necessary for burning


You will see that candle continues burning for a few seconds. The flame then startsflickering and finally goes off (Fig. 14.2 b).

Why does this happen?

It is because no fresh air enters in the glass tumbler to support combustion. Thus, theactivity clearly proves that air is necessary for combustion.

It is also seen that to burn coal in an angithi sufficient amount of heat is supplied byburning waste paper or cloth soaked in kerosene oil. Why is it so?

Whenever a substance is heated, its temperature increases till it become equal to atemperature, at which the substance starts burning. This temperature is called the ignitiontemperature. A substance cannot catch fire or burn as long as its temperature is lowerthan its ignition temperature. The ignition temperature is the lowest temperature at whicha substance catches fire and starts burning.

The ignition temperatures of different substances are different. Lower the value ofignition temperature of a substance, lesser the amount of heat required to burn it.

The ignition temperature of kerosene is higher than that of petrol. So petrol catchesfire immediately whereas kerosene requires more heat to start burn. Similarly the ignitiontemperature of coal is very high. It requires more heat to start burning.

Can you give reasons as to why a matchstick does not catch fire on its own?

Burning candle Plastic trough

Glass tumbler

Candle goes off

Water

(a) Immediately after setting up (b) After some time


Room temperature is much lower than the ignition temperature of matchstick therefore,it does not catch fire. On rubbing of matchstick against the side of the box, heat is produceddue to friction. This heat raises the temperature of chemicals present on the matchstickhead to its ignition temperature. Thus, the matchstick starts burning.

Hence, we find that a substance can not catch fire or burn as long as its temperature islower than its ignition temperature. Here is a simple activity which prove that ignitiontemperature is necessary for combustion

ACTIVITY 14.3

Aim : Ignition temperature is necessary for combustion

What is required?

A paper cup, water and a spirit lamp

What to do?

Take a paper cup, pour water in the cup. Heat the paper cup.


You will see that we can boil the water in a paper cup without burning the paper.

Why is it so?

It can be explained on the basis of ignition temperature of paper cup. When weheat water in a cup, the heat supplied to the paper cup is quickly transferred fromthe paper cup to the water, the temperature of paper cup does not reach its ignitiontemperature, and hence it does not burn.

Now we can say that three conditions are necessaryfor combustion:

• presence of a combustible substance (that burnseasily e.g. fuel),

• presence of supporter of combustion (oxygen fromair), and

• attainment of ignition temperature i.e. heating.Unless all of these three conditions are fulfilled,

combustion cannot take place.


1. Why does the coke not burn in air at room temperature?2. It is said that oxygen is essential for burning. From where does this oxygen come?3. State any one condition necessary for combustion.4. Petrol catches fire immediately whereas kerosene does not why?

14.4 FIRE EXTINGUISHERS

We know that fire is very useful in our day to day life. However, some times it proves to bedestructive, especially when it becomes uncontrollable. Therefore, it is necessary to learnthe ways and means of controlling fire.

Fig. 14.3 The fire triangle

Oxy

gen H

eating

HeatingO

xyge

n

Fuel Fuel


As you are aware that small fire can be extinguished by covering it with a lid. Forexample, coal fire or fire in frying pan is extinguished by covering it with a lid. Similarly,you would have seen that when a person catches fire than we cover him with a thickblanket and make him to roll on the ground. We often see that whenever fire spreads overa vast area, pouring water or sand puts it off. How is the fire put off by covering or pouringthe water?

As you have learnt one condition of supporting combustion is air (oxygen). If we cutoff the supply of air by covering fire with lid, the fire is extinguished.

The apparatus used to extinguish fire is called fire extinguisher. You would have seenfire extinguishers in petrol pumps, big buildings, cinema halls and other public places.

14.4.1 Principle of fire extinguisher

The principle of working of fire extinguisher is based on either of the following threeconditions:

• cooling the combustible material below its ignition temperature, or• cutting off the supply of air, or• cooling the combustible material and at the same time cutting the supply of air.

The different types of fire extinguishers, their working principle and the nature of firefor which they are used are listed in Table 14.3.

Table 14.3 : Working principle and uses of different types of fire extinguishers

Type of fire extinguisher Working principle Nature of fire for which used

Dry powder extinguisher Cuts off supply of air All types of fire(sand and baking soda)Baking soda sulphuric acid Cuts off supply of air All types of fire except due toextinguisher (soda acid) electrical and inflammable

liquidsFoam type extinguisher Cuts off supply of air Fire due to inflammable liquidsWater Cools the substance All types except due to

below the ignition electricity and oiltemperature

Carbon tetrachloride Cuts off supply of air Fire due to electricityextinguisher

14.4.2 Soda acid fire extinguisher

The most common fire extinguisher is soda acid.The carbon dioxide is liberated by the action of acidon baking soda. It increases the percentage of carbondioxide in air (CO

2 is non supporter of combustion).

How does this happen?

This type of fire extinguisher contains a bottleof sulphuric acid supported by a metallic containerfilled with a baking soda solution (Fig. 14.3). Whenthe cylinder is inverted and knob struck, against the Fig. 14.4 Soda acid fire extinguisher

Solution of

Ampule with H2SO

4

Trigger

Iron grating

sodium bicarbonate


ground, the acid bottle breaks and the acid comes in the contact with the backing soda.

2NaHCO3

+ H2SO

4Na

2SO

4+ 2H

2O + 2CO

2baking sulphuric sodium water carbonsoda acid sulphate dioxide

As a result carbon dioxide is liberated. This increases the percentage of carbon dioxidein air. Due to this the supply of air is cut off and there, fire is extinguished. These types ofextinguishers are used in cinema halls, multistorey buildings, etc.


1. Name the chemicals present in a soda acid fire extinguisher.

2. Why is fire of frying pan extinguished, when it is covered with lid?

3. If fire is due to the electricity, can we use to water as fire extinguisher?

4. Name the gas evolved in soda-acid fire extinguisher.

5. Give any one condition on which principle and working of fire extinguisher is based.

14.5 BIOFUELSThe organic waste, such as wood, agricultural residues and cattle dung, are called biomass.Biomass contains carbon compounds and it is the oldest source of heat energy for domesticpurposes.

Biofuels, such as firewood, dung cakes and agricultural wastes, constitute main sourceof energy in rural areas. A cause of concern in recent years has been the excessiveconsumption of firewood, which is not sustainable for long at present level of consumption.Deforestation and desertification are taking place, adversely affecting the ecology. Secondly,the age-old practice of burning dung cakes and agricultural wastes is depriving the landsof much needed humus and consequently causing loss of soil fertility. Moreover, inefficientburning of biofuels in traditional chulhas causes air pollution.

14.5.1 Smokeless chulahIn order to overcome the problems of lower energy and smoke hazards of conventionalchulahs, smokeless chulahs designed scientifically are now available for use. These chulahsare designed in such a way that less amount of heat is lost to the surroundings. Thus, thesechulah consume less fuels and hence, more efficient than the conventional chulahs. Theseare provided with a tall chimney, which help the smoke to escape into the upper atmosphere.

14.5.2 Biomass as fuelBiomass can be used as fuel in two ways:

• By burning dry biomass like wood and cattle dung directly to produce heat.

• By converting biomass into more useful fuels, for example, wood can be convertedinto charcoal, which is a better fuel as compare to coal. Similarly cattle dung can beconverted into biogas, which is better fuel than cattle dung.

14.6 CALORIFIC VALUE (FUEL VALUE) OF FUELSYou know heat is produced on burning of fuels. Different, fuels on burning produce differentamount of heat. Various fuels have different composition and hence, different energycontents. The usefulness of the fuels is measured in terms of Calorific values. Higher thecalorific value, better is the fuel.


Calorific value may be defined as the

amount of heat liberated by the completecombustion of a unit mass of fuel. The unit ofmass usually taken, for measuring thecalorific value of fuel, is gram. Therefore,calorific value may be defined as the amountof heat produced by burning completely onegram of fuel. For example, by burning of onegram carbon (charcoal) produces 8137calories of heat (or 34013 joules). Therefore,calorific value of carbon is 34013 Jg-1. Thecalorific value is expressed in kilojoule/gram(kJ g-1) because Joule is very small unit ofenergy. So the calorific value of carbon is 34kJ g-1.

1 cal = 4.18 J

1000 J = 1 kJ

Calorific value of some fuels is listed in table 14.4.

From table 14.4 we know that the calorific value of petrol is 50 kJ g-1. This means thatif one gram of petrol is burnt completely, then it will produce 50 kiloJoule of heat energy.

a) Hydrogen as fuel: Why hydrogen is not commonly used as a fuel even though itscalorific value is high. Hydrogen gas has highest calorific value, but it is not usedcommonly as a domestic or industrial fuel. There are two problems in using hydrogenas a fuel. Firstly, its handling is difficult and secondly, it burns with an explosion.

b) Hydrocarbons as fuels: Hydrocarbons contain carbon and hydrogen and are used asfuels. Since hydrogen has the highest calorific value therefore, the fuel containinghigher percentage of hydrogen will have a higher calorific value than that which havea lower percentage of hydrogen in it. The calorific value of methane (CH

4) is higher

than that of butane (C4H

10) because percentage of hydrogen in CH

4 (25%) is higher

than that in C4H

10 (17%). The calorific value of CH

4 is 55 kJ g-1 whereas for butane it

is 50 kJ g-1.c) Wood as fuel: Cellulose, i.e. (C

6H

10O

5)

n is the chief constituent of wood. The percentage

of oxygen in wood is quite high. The oxygen supports the combustion but does notproduce heat. Therefore, wood has lower calorific value.Out of CH

4, C

2H

6 and

C

12H

22O

11, the lowest calorific value is of sugar C

12H

22O

11 because

it has lower percentage of hydrogen due to the presence of oxygen. Whereas, out of CH4,

C2H

6, C

3H

8 and H

2 the lowest calorific value is of C

3H

8 because it has lower percentage of

hydrogen.

In compound A, each carbon in a molecule is bonded with three hydrogen atoms. Inthe molecule of another compound B, each carbon atom is bonded with one hydrogen andone oxygen atom. Can you tell which compound will have higher calorific value? CompoundA will have higher calorific value because of more percentage of hydrogen.

Table 14.4 : Calorific value of some fuelsType of fuel Calorific value

WoodCharcoalDung cakeCoalGasolineKeroseneNatural gasPetrolBiogasL.P.GMethaneHydrogen gas

18 kJ g-1

35 kJ g-1

8 kJ g-1

30 kJ g-1

34 kJ g-1

37 kJ g-1

50 kJ g-1

50 kJ g-1

40 kJ g-1

50 kJ g-1

55 kJ g-1

150 kJ g-1


Example 14.1 : Calorific value of LPG is 55k Jg-1. Calculate the energy consumed by afamily in one month if it required a cylinder containing 14.5 kg of LPG.

Solution: The calorific value of LPG is 55 k Jg-1. It means one gram of LPG on burning

will produce 55 kJ heat energy.

1 kg = 1000 g and 14.5 kg = 1000 x 14.5 = 14500 g

Heat energy produced by 1 g LPG = 55 kJ

So, 14500 g LPG will produce heat energy = 55 x 14500 = 797500 kJ

14.6.1 Food as fuel

We have already discussed the types of fuelrequired for cooking food, transport andindustry. Energy is also necessary for ourbody to carry on the various life processes.The food, which we eat, is a kind of fuelfor our body that supplies us the energy.

The food that we eat is broken downinto smaller molecules of glucose duringdigestion. Glucose so formed is absorbedin the blood and taken to the cellsthroughout our body. When we breathe inair, then oxygen of the air is also absorbedby the blood and carried to all cells. Thisoxygen to produce CO

2 and H

2O oxidizes

the glucose C6H

12O

6 slowly and gives us

energy.

When energy is released from food,some of it is transferred to a special molecule found in cells called ATP* (adenosinetriphosphate). Thus, ATP is the energy-storing molecule in the body. To release energyATP is converted into ADP (adenosine diphosphate).

ATP ADP + phosphate + energy

Calorific value of some foods is given below Table 14.5.


1. Which of the following fuels has lowest calorific value?C

2H

6, C

2H

5OH, C

2H

4, H

2

2. Which of the following fuels has highest calorific value?C

2H

6, C

2H

5OH, C

2H

4, H

2

3. Hydrogen compounds are abundantly available on earth and it has a high calorificvalue but why this gas is not commonly used as a domestic fuel?

4. Why do the fuels like wood and alcohol have lower calorific values as compared toLPG and biogas?

*You will learn more about ATP and ADP in lesson 24 of this course.

Table 14.5 : Calorific value of some foodsType of food Calorific valueCarbohydrateFatProteinApplesCurdBreadCheeseMilkEggWheatMeatButterHoneyHamburgerPeanutsPotato

17 kJ g-1

39 kJ g-1

18 kJ g-1

2.5 kJ g-1

2.5 kJ g-1

1.8 kJ g-1

12 kJ g-1

20 kJ g-1

3 kJ g-1

6.0 kJ g-1

12 kJ g-1

34 kJ g-1

13.3 kJ g-1

15 kJ g-1

23 kJ g-1

3 kJ g-1


5. How do we get energy in our body from the food?6. Which food has higher calorific value – carbohydrate, egg, butter, peanuts and curd?

14.7 VOLTAIC AND DRY CELLS

Now we will learn about electrochemical cells. You know that cars and other automobilesare started with the help of battery. We use cells in torches, transistors and watches etc.The chemicals present in cell and batteries react to generate the electric current. The deviceused to generate electricity through chemical reaction is called an electrochemical cell. Letus learn about some of the commonly used electrochemical cells.

14.7.1 Voltaic cell

The first electrochemical cell was constructed by Volta in 1796. It is called Voltaic cell. Inthis cell, a strip of zinc is placed in zinc sulphatesolution and a copper strip is placed in copper sulphatesolution. Both the solutions are separated by a porouspartition which allows the ions to pass through it, butdoes not allow the mixing of the two solutions. Thezinc plate acts as an anode (negative electrode), whilecopper plate acts as cathode (positive electrode).

It is to be kept in mind that the signs of the electrodesin an electrochemical cell are opposite to that of anelectrolytic cell.

Working of a voltaic cell

When both the electrode terminals are connected by a wire then there is a flow of electrons(electric current) from zinc to copper terminal (Fig. 14.4).

Zinc metal is more reactive than copper, so it has a greater tendency to lose electron.

Zn Zn 2+ + 2e– (Oxidation)These electrons flow through the wire to the copper cathode. The reaction that occurs atthe copper cathode is

Cu2+ + 2 e– Cu (Reduction)

14.7.2 Daniel cell

An improvement over the voltaic cell was Daniel cell. Here, the zinc sulphate solution iskept in a porous pot that is suspended in a solution of copper sulphate in a copper vessel.This cell gives a more steady current. The voltage of cell is 1.1 volt.

14.7.3 Dry cell

The cell used in torch and transistor etc is called dry cell. The most common dry cell, thatis, the Leclanche cell, is used in flashlight and transistor radios. The anode of the dry cellconsists of zinc container which is in contact with manganese dioxide (MnO

2) and an

electrolyte. The electrolyte consists of ammonium chloride and zinc chloride in water towhich starch is added to thicken the solution to a paste like consistency so that it is lesslikely to leak (Fig. 14.6). A carbon rod serves as cathode, which is immersed in the electrolytein the centre of cell.

Fig. 14.5 (a) Voltaic cell

Flow of electrons

Copperplate

Zincplate

Porous partition

CuSO4

solutionZnSO

4

solution


The cell reactions are:

Anode : Zn(s) Zn2+(aq) + 2e–

Cathode : 2NH4+(aq) + 2MnO

2(s) + 2e– Mn

2O

3(s) + 2NH

3(aq) + H

2O(l)

_________________________________________________________________________________________________

Overall : Zn(s) + 2NH4+(aq) + 2MnO

2(s) Zn2+(aq) + 2NH

3(aq) + H

2O(l) +

Mn2O

3(s)

Actually this equation is over simplification of the complete process. The voltageproduced by dry cell is about 1.5V.


1. Name the materials used to make the electrodes of a voltaic cell.2. At which electrode in a cell does reduction take place?3. At which electrode in a cell does oxidation take place?4. The conversion of Zn to Zn2+ is oxidation or reduction?5. In which cell does chemical energy change into electrical energy – electrochemical

cell or electrolytic cell?6. Dry cell is also known as ______________7. Name the materials used to make cathode of a dry cell.8. Name the electrolytes used in dry cell.9. How much voltage is produced in dry cell?10. What precaution is adopted to prevent over mixing of the solutions of electrolytes?

14.8 NUCLEAR ENERGY – ENERGY FROM THE ATOM

You have seen that chemical reactions are accompanied by energy changes. In a chemicalreaction, the composition of the nucleus of an atom does not change. But there are somereactions in which the composition of the nucleus of an atom changes. Such reactions arecalled nuclear reactions and the energy released during such reaction is known as nuclearenergy. To understand the difference between chemical and nuclear energy, study thefollowing table (14.6).

Fig. 14.5 (b) Daniel cell Fig. 14.6 Construction of a dry cell

CuSO4

solution

ZnSO4

solution

Coppervessel

Cathode

Zinc Anode

Central carbon rodsurrounded by MnO

2 paste

Steel cover

Expansionchamber

Porousseparator(paper)

Zinc canPaper cover

ElectrolyteNH

4Cl and

ZnCl2 paste

Insulatingwasher


Table 14.7: Characteristics of various types of radiations/particles

Characteristicsand properties

Rays of alphaparticles

Rays of betaparticles

Gamma rays

Nature

ChargePenetrating effect

Ionization effect

Each particle consistsof 2 protons and 2neutrons, i.e. they aredoubly-charged heliumions.PositiveStopped by thick sheetof paper or skin

High ionization power

They are electrons

NegativeStopped by a fewmillimeters thicksheet of aluminiumMedium ionizationpower

Electromagneticwaves similar toX-rays

No chargeNot stopped

Weak ionizationpower

Table 14.6 : Differences between chemical and nuclear energy

Chemical energy Nuclear energyChemical energy is released or absorbeddue to the influence in the bond energies ofthe bonds in the reactants and products.Chemical energy is obtained when achemical reaction takes place.The amount of energy evolved is verysmall.No harmful radiation is emitted.

Nuclear energy is released due to thechange in the composition of the nucleusof an atom.Nuclear energy is obtained when nuclearchanges take place.The amount of energy evolved is verylarge.Radiation emitted during nuclear changesis harmful.

Energy stored in the nucleus of an atom is known as nuclear energy. In a nuclear reaction,when the nucleus of an atom is bombarded with neutrons, it undergoes a change to formsmaller fragments of new atoms. In this process, a tremendous amount of energy is evolvedin the form of heat. For example, when Uranium-235 nucleus is bombarded with neutrons,it splits into two smaller nuclei and a large amount of energy is released in the form ofheat.

14.8.1 RadioactivityIt has been observed that the atoms of some elements, such as radium and uranium,spontaneously emit radiations. Such a process is called radioactivity. It is a spontaneousprocess in which the nucleus of the atom disintegrates and the energy bearing particles orrays are emitted. Radioactivity is a spontaneous process of disintegration or breaking upof the nucleus of an atom accompanied by the emission of energy bearing particles orrays.

The materials which give off energy bearing rays or particles or both are calledradioactive materials. The three main types of radiations emitted are: Alpha particles,Beta particles, Gamma rays

The characteristics and the properties of these radiations are given in Table 14.7.


Apart from the above properties, all three types of radioactive radiation can(a) affect photographic plate,(b) cause fluorescent materials like ZnS to glow,(c) have pronounced physiological effects like

• power to kill plants seeds and human tissues,• cause cancerous growths,• destroy bacteria,• can cure skin cancer and other diseases if used in controlled quantities.

14.8.2 Nuclear fission

The splitting of the nucleus of an atom into fragments that are roughly equal in mass alongwith the release of energy is called nuclear fission.

When a neutron strikes the nucleus of a uranium atom at an appropriate speed, it getsabsorbed. Uranium nucleus on absorbing a neutron becomes highly unstable and splitsinto smaller atoms releasing huge amount of energy in the process.

235U + 1n 141Ba + 92Kr + 3 1n + energy 92 0 56 36 0

During this process three neutrons are also released. These neutrons split other nuclei ofthe uranium. The reaction continues rapidly and is known as chain reaction (Fig. 14.7). Agreat deal of heat is produced in this reaction.

Fig. 14.7 Nuclear fission

If the chain reaction is uncontrolled, all the nuclei of uranium split in a fraction ofsecond and this is the case of a devastating explosion, such as that of atom bombs whichwere dropped on Hiroshima and Nagasaki.

14.8.3 Nuclear reactors

A peaceful application of nuclear fission is the generation of electricity using heat from acontrolled chain reaction in a nuclear reactor.

A nuclear reactor is an arrangement in which the energy produced (in the form ofheat) in a nuclear fission can be used in a controlled manner to produce steam, which canrun the turbine and produce electricity.

Uranium – 235

Fission reaction

Chain reaction

Neutrons


The main part of nuclear reactor is called the core as shown in Fig. 14.8. The reactorcore is made up of the following parts:

a) Nuclear fuel: It is the fissionable material used in nuclear reactors to produce energyby fission process. The nuclear fuel consists of uranium, usually in the form of itsoxide, U

3O

8. Naturally occurring uranium contains about 0.7% of uranium 235 isotope

which is too low a concentration to sustain a chain reactions. For effective operationof reactor, uranium 235 must be enriched to a concentration of 3 or 4%.

b) Moderator: An important aspect of the fission process is the speed of the neutrons.Slow neutrons hit uranium-235 nuclei more efficiently than do fast ones. Becausefission reactors are highly exothermic, the neutrons produced usually move with highvelocities. For greater collision efficiency, neutrons must be slowed down. For thispurposes a substance is used that can reduce the kinetic energy of neutrons. Such asubstance is called as a moderator. A good moderator should be a nontoxic andinexpensive substance. And it should be resist conversion into radioactive substanceby neutron bombardment. Graphite (C) or heavy water (D

2O) are commonly used as

moderators.

c) Control rods: In principle, the main difference between an atomic bomb and nuclearreactor is that the chain reaction that takes place in a nuclear reactor is kept undercontrolled conditions at all the times. The factor limiting the rate of the reaction is thenumber of neutrons present. This can be controlled by lowering cadmium or boronrods between the fuel elements.

d) Coolant: It is the substance which is circulated in pipes to absorb the heat given off bythe nuclear reactor and transfer it outside the reactor core, where it is used to producesteam to drive an electric generator. Large quantity of water is used as coolant.

e) Shield: To prevent the losses of heat and to protect the people operating the reactorfrom the radiation and heat, the entire reactor core is enclosed in a heavy steel orconcrete dome, called the shield.

Fig. 14.8 Core of nuclear reactor Fig. 14.9 Nuclear reactor

Heat exchangerControl rod

Concrete

Steel

Coolant

Moderator

Nuclearfuel

Steam

Water

Shield

To steamturbine

Water

PumpControl rodUranium fuel


A complete nuclear power plant essentially consists of the four parts: reactor core,steam generator, steam turbine, and steam condensing system (Fig. 14.9).

14.8.4 Nuclear fusion

Energy is also produced when two light nuclei such as deuterium (heavy hydrogen) fusetogether to form a heavy nucleus.

A process in which the nuclei of light atoms combine to form the nucleus of a heavieratom with the release of energy is called nuclear fusion.

Nuclear fusion requires very high temperature, say of the order of 4 million degreeCelsius (4000000oC). This is the mechanism through which the energy is produced instars, including the sun. The hydrogen bomb also relies on this kind of reaction. Enormousamounts of energy are released during nuclear fusion. It is still not possible to controlnuclear fusion to provide us with a steady supply of energy. In our country the scientistsare making attempts to understand the basic process which may in future lead to controllednuclear fusion. Some of the reactions that occur during nuclear fusion are shown below.

1H + 1H 2H + 0e + energy1 1 1 +1

2H + 1H 3He + γ + energy1 1 2

3He + 3He 4He + 1H + 1H + energy2 2 2 1 1

14.8.5 Uses of nuclear energy

The important uses of nuclear energy are as follows:

a) The heat produced in a nuclear reactor is used to boil the water to form steam. Thesteam then turns a turbine, which runs an electric generator to produce electricity.

b) Nuclear energy is now being used to run submarines and ships. Vessels driven bynuclear energy can sail for long distances without having to refill.

c) Nuclear energy in the form of bombs (atom bomb and hydrogen bomb) is used inwarfare.

d) Nuclear energy is used in making radioisotopes that are used in medicine, agricultureand research.

14.8.6 Hazards of producing nuclear energy

While producing nuclear energy harmful radiations may be released which can penetratehuman bodies and cause irreparable damage to cells. To prevent leakage of these dangerousand toxic radiations, nuclear reactors are covered with a thick covering of radiation absorbingsubstance such as lead. However, a minor fault in the design of reactors or a natural calamitystriking a perfectly designed reactor, could result in the release of these extremely harmfulradiations into the environment. It could pose a permanent threat to the living beings of thesurrounding areas. You may be aware of the two major accidents in the nuclear powerplants, one at Three Mile Island (USA) in 1979 and the other at Chernobyl (The SovietUnion) in 1986. The devastation caused in these two accidents by the release of nuclearradiations is yet to be fully assessed. Apart from possible accidents at the reactor site, thereis of course, the additional danger of harmful waste matter produced at various steps of


nuclear cycle, such as mining, enrichment of ore, etc. In every step of nuclear cycle anumber of substance capable of emitting nuclear radiations are generated. These substancesare called nuclear wastes. We have not yet been able to discover safe methods of dealingwith such nuclear waste generated in nuclear power plants. It is simply being stored instrong containers. Thus, the problem of its disposal is yet to be solved.

14.8.7 Radioisotopes

An isotope that spontaneously decays into an isotope of different elements is known asradioactive isotope.

The first radioisotope of O178 was produced by bombardment of alpha particles on

ordinary nitrogen N147 by Rutherford in 1919.

N147 + He4

2 O178 + H1

1

a) Production of radioisotopes

Bombarding atoms of some elements with lighter nuclei such as protons, alpha particles orneutron produces radioisotopes. Some common examples of the production of radioisotopesare given below:

40Ca + 1H 40Sc + 1n (p, n)20 1 21 0

19Fe + 4He 22Ne + 1H (α, p) 9 2 10 1

12C + 4He 15O + 1n (α, n) 6 2 8 0

35Cl + 1n 36Cl + γ (n, γ)17 0 17

37Cl + 1n 38Cl + γ (n, γ)17 0 17

Sometimes even heavier nuclei are used as bombarding material.

b) Applications of radioisotopes

Some important applications of radioisotopes are given below:

(i) In determining the age of fossils and old rocks.(ii) In determining the solubility of sparingly soluble materials.(iii) To determining the amount of an element in a sample.(iv) Isotopes exchange reactions provide information on the mechanism of certain reactions

(radioactive tracer).(v) In industry the isotope 60Co is used for γ-radiography to detect cracks/ flaws in metal

plants and pipes.(vi) The isotopes have immense use in the field of medicine. For example, 135I is used to

locate brain tumors and of disorders of the thyroid gland. 24Na is used to locate bloodclots. 99Tc is used to obtain images of organs such as heart, liver and lungs. The isotopes51Cr and 59Fe are used to determine the amount of total blood in a patient.

(vii) Radioactive isotopes are used in biological, fields and in agriculture. For example,32P is used to detect the deficiency of phosphetic fertilizers in the soil and 18O isotope


was used to determine the source of O2 in

photosynthesis.

14.9 NUCLEAR POWER PLANTS ININDIA

India has 14 operating reactors out of whichtwo are Boiling Water Reactors (BWR) and12 are Pressurized Heavy Water Reactors(PHWR). Two more reactors with capacitiesof 500 MW are under construction at Tarapurin Maharashtra and are expected to attaincriticality in 2005 and 2006, respectively. Thenuclear power generation for the year 2001- 2002 was 19193 million units.

The sun is the ultimate source of energy

The ultimate source of all the energies is sun. Plants take energy from sun throughthe process of photosynthesis. Plants serve as food for animals. Plants and animalsare fossilized to coal, petroleum and natural gas. Plants also supply wood as fuel.


1. Which isotope of uranium is used in nuclear fission?2. Name the elements produced in nuclear fission.3. Atomic bomb is based on nuclear _______________ reaction.4. Hydrogen bomb is based on nuclear _______________ reaction.5. How many neutrons are emitted in a single nuclear fission?6. What is the role of moderator in nuclear reaction?7. What is the function of cadmium rods in a nuclear reactor?8. State peaceful uses of nuclear energy.9. Name the coolant used in nuclear reaction.10. Give two examples of nuclear fuels.11. How many operative reactors are present in India?12. What do you mean by BWR and PHWR?

LET US REVISE

• Coal and petroleum are fossil fuels.• The energy related to the nature and composition (atoms/molecules) of a substance is

called the chemical energy.• The chemical energy can be converted into heat energy and vice versa during chemical

reactions.• The reaction where heat is absorbed is called endothermic where as the reaction where

heat is given out is called exothermic.• Chemical energy can be converted to electrical energy and vice versa.

Table 14.8 : Location of Atomicpower plants in India

Place Number Capacity

Tarapur 2 160 MWRajasthan 2 220 MW

2 100 MWKalapakkam 2 220 MWNarora 2 220 MWKakrapara 2 220 MWKaiga 2 220 MW


• Combustion is a chemical change in which heat and light are produced at the sametime.

• The lowest temperature at which a substance starts burning is called ignitiontemperature. Ignition temperature is different for different substance.

• Substances, which burn rapidly, are called combustible substances and those, whichdo not burn at all, are called non-combustible substances.

• The three conditions necessary for combustion are presence of combustible substanceattainment of ignition temperature continuous supply of a good supporter of combustion(generally air).

• Fire which is very useful in our daily life, is produced by combustion of substanceslike coal, petrol, etc.

• The instruments, which have been developed to extinguish fire, are called fireextinguishers. The working principle of different type of fire extinguishers is basedeither on the conditions to remove the combustible substance or to cutoff the supply ofair or to cool the burning substance below its ignition temperature.

• Radioactivity is a spontaneous process of disintegration of the nucleus of an atomaccompanied by the emission of energy bearing rays or particles.

• Fission is a process of splitting of the nucleus of a heavy atom into fragments that areroughly of equal masses with the release of huge amount of energy.

TERMINAL EXERCISES

A. Multiple choice type questions.1. Which of the following variety of coal has maximum carbon content?

(a) Anthracite(b) Bituminous(c) Lignite(d) Peat

2. Which of the following has highest calorific value?(a) Natural gas(b) LPG(c) Biogas(d) Hydrogen

3. Which of the following can provide maximum voltage?(a) Voltaic cell(b) Daniel cell(c) Dry cell(d) Distilled water

4. Which of the following food components has maximum calorific value?(a) Carbohydrates(b) Proteins(c) Fats(d) Mineral salts


B. Fill in the blanks.

1. The energy stored in a substance is known as _____________released in form of_____________ energy.

2. When water added in lime the heat released in form of _____________energy.3. When heat is released and absorbed, the reactions are called_____________and

_____________reactions, respectively.4. The energy released by bombarding Uranium-235 with neutrons is _____________5. Two main products of petroleum are _____________and _____________6. Soda acid fire extinguisher contains _____________and _____________7. Coal gas is a mixture of _____________and _____________8. LPG is known as _____________9. Coke does not burn at room temperature because _____________is high.10. The isotope of Uranium used in nuclear fission is _____________

C. Descriptive type questions.

1. Define chemical and nuclear energy.2. With the help of examples explain exothermic and endothermic reactions.3. Define fuel and also explain its role in every day life.4. What is biomass? For what purpose it is usually used?5. Explain why charcoal is a better fuel then wood?6. Define fossil fuel. Give suitable examples.7. Mention different types of fossil fuel.8. Name two organic compounds obtained by distillation of coal.9. Give the chemical composition of coal gas.10. Name the product obtained by the distillation of petroleum that is used for making

road surfaces.11. The boiling points of substances A, B and C are 443K, 523K and 623K, respectively.

On fractional distillation, which of the three compounds will be obtained at the bottomof fractional distillation column.

12. Name the two components of petroleum obtained by fractional distillation.13. Name any one hydrocarbon fraction obtained during fractional distillation of petroleum

which is used as domestic fuel.14. What is the full form of LPG and also gives its chemical composition?15. How does we get chemical energy in our body from the food we consume? How does

this process differ from normal burning process?16. What are three conditions necessary for combustion? Pouring water on a fire which

condition of combustion is not satisfied?17. It is very difficult to burn a heap of fresh green leaves but it catches fire easily once the

leaves dry up. Why?18. How is it possible that water can be boiled in a paper cup without burning?19. Calorific value and ignition temperature of fuel X are 75kJ g-1 and 20 oC respectively

and those for Y fuel are 50k J g-1 and 75 oC respectively. On burning the fuel Y produces


only CO2 while fuel X produces CO

2 and CO. Which of the two is a better fuel? Give

the reasons to support your answer.20. Define ignition temperature and also explain why coke does not burn in air at room

temperature?21. Compound A has each of its carbon atom bonded with four hydrogen atoms while

compound B has each carbon atom bonded with three hydrogen atoms. Which one ofthe two compounds will have higher calorific value?

22. An electric spark is struck between two electrodes placed near each other, inside aclosed tank full of petrol. Will the petrol catch fire? Explain your answer.

23. On what principle does fire extinguisher work? Explain the working of soda- acid fireextinguisher.

24. Why is water not used to extinguish fire due to electricity?25. Why is the crude oil sometimes called “Black Gold”?26. Calorific value of LPG is 55kJ g-1. Calculate the energy consumed by a family in one

month, if it requires a cylinder containing 14.5 kg of LPG.27. A burner consumes 1g of LPG in 55 seconds. If the calorific value of LPG be 55k Jg-

1, what will be the power of combustion of the burner?28. Define nuclear fission, nuclear chain reaction and critical mass.29. Which isotope can undergo nuclear fission?30. What is the function of cadmium rods in a nuclear reactor?31. Define term moderator in a nuclear reactor.32. Name the isotopes of two different elements, which can be fissioned easily.33. What is a nuclear reactor? Will the help of a labeled diagram describe how a nuclear

power plants used as nuclear reactor to generate electricity.34. Which fuel is used in nuclear reactor? Why can it not be used as a fuel in the form it

occurs in nature?35. State two peaceful uses of nuclear energy.36. Explain the nuclear wastes, what are the problems inherent in their disposal.37. Name any two type of radiations emitted during nuclear fusion. What are the measures

taken to prevent the leakage of radiations from the nuclear reactors?


14.1

1. Chemical energy2. Nuclear energy3. Small4. Energy

14.2

1. Methane and carbon monoxide.2. Coal and petroleum3. Lignite and bituminous4. Kerosene


5. Petroleum kerosene6. Fuels, lubricants, solvents for organic compounds7. Petroleum gas

14.3

1. The ignition temperature of coal is high2. From air3. Attainment of ignition temperature4. Petrol has lower ignition temperature as compared to kerosene

14.4

1. Baking soda and sulphuric acid2. It stops the supply of oxygen.3. No4. Carbon dioxide5. Any one of the following:

(a) Cooling below ignition temperature or(b) Cutting the supply of air or(c) Cooling the fire. and also cutting the supply of air.

14.5

1. C2H

5OH

2. H2

3. Problem in handling and also burn with explosion4. Because they have higher percentage of oxygen which is a supporter of

combustion5. Metabolism (by burning the food)6. Butter

14.6

1. Zn and Cu rods2. Cu electrode (Cathode)3. Zn electrode (Anode)4. Oxidation5. Electrochemical cell6. Leclanche cell7. Carbon rod.8. NH

4Cl + ZnCl

2

9. 1.5 V10. By using porous pot

14.7

1. 235U 92


2. 141Ba and 92Kr 56 36

3. Fission4. Fusion5. Three6. Graphite and D

2O

7. Slow down the speed of neutrons.8. Nuclear reactor (to generate electricity)9. Water10. U

3O

8 enriched with 235U

92

11. 1412. BWR: Boiling water reactor; PHWR: Pressurized heavy water reactor

GLOSSARY

Chemical energy: Energy stored within the structural unit of a substance.

Combustion: Chemical change in the presence of oxygen in which both heat andlight are produced at the same time and the composition of the substance changes.

Critical mass: The minimum mass of fissionable material required generating a self-sustaining nuclear chain reaction.

Daniel cell: A galvanic cell utilizing the reduction of Cu2+ ion by zinc.

Dry cell: A chemical galvanic cell with a zinc anode and a graphite cathode surroundedby solid MnO

2. The electrolyte is a moist paste of NH

4Cl, ZnCl

2 and some inert filler.

Energy: The capacity to work or produce change.

Fission: The splitting of a heavy nucleus (mass number 7200) into lighter fragmentswith the release of energy. Most fission processes are initiating by bombarding the heavynucleus with thermal electrons.

Fuel: Any substance that produces energy in the form that can be used for practicalpurposes.

Fusion: The process in which two light nuclei combine to produce a heavier nucleuswith mass number A less than 60, with the release of energy.

Galvanic cell: A device for converting chemical energy into electrical energy.

Ignition temperature: It is the latest temperature at which a substance catches fireand starts burning.

Nuclear chain reaction: A self-starting sequence of nuclear fission reactions.

Nuclear energy: Energy stored in a nucleus of an atom.

Radioactive isotopes: An isotope that spontaneously decays to become an isotope ofdifferent elements.

Radioactivity: The spontaneous break down of an atom by emission of particles and/or radiations.

16

The Earth – A Living PlanetMother Earth has every thing to fulfill man’s needs but not his greeds.

- Mahatma GandhiYou have studied in the previous lesson that the Earth is the third planet of the solar systemas counted from the sun. The Earth also came into existence almost the same time as theother members of the solar system, i.e. around 4.5 billion years ago. This is the planet welive on and where multitude of diverse life forms have evolved.

In this lesson we will study the brief story of the 4.5 billion years of Earth’s life - itschanging structure and evolving life support system. We will also study that variouscomponents of our environment are in fine balance but facing a threat due to varioushuman activities. This should put us on alert to save our planet.

OBJECTIVES

After completing this lesson, you will be able to :

! explain why life evolved on the Earth, stating conditions necessary for it;! explain the differentiation of Earth and evolution of atmosphere and hydrosphere in

its present form;! describe the life supporting systems on the Earth, i.e. the biosphere comprising the

lithosphere, the atmosphere and the hydrosphere;! justify that the sun is the ultimate source of all energy on Earth except nuclear energy

and geothermal energy;! explain how the solar energy is cycled in nature and utilised by living beings;! explain the origin and evolution of life on Earth and suggest some measures to protect

the Earth and its life support systems.

16.1 WHY ONLY EARTH HAS LIFE?

You can easily identify living things around you. Earth has life on its surface. Scientists aretrying hard to find out whether life exists elsewhere also in the universe, but, till date theycould not find any. Let us analyse the possibility of life in our solar system.

16.1.1 Physical conditions necessary for life

For life to exist on a celestial body the following conditions seem to be necessary :

: 300 : The Earth – A Living Planet

(i) Presence of some elements such as carbon (C), oxygen (O2), Nitrogen (N

2) and

hydrogen (H2) which are involved in the basic structures of complex molecules forming

living cells.(ii) Suitable temperature range on its surface for sustenance of life. Most of the living

organisms cannot survive at too high (>700C) or too low (<O0C) temperatures because,life processes cannot be carried out at very high and very low temperatures.

(iii) Presence of a liquid medium, like water, which is a must for transporting nutrientsinside a living body.

(iv) Presence of a protective atmosphere having a protective layer like ozone layer, toprevent harmful radiations to reach its surface.On the Earth all these conditions are satisfied and hence, we have life on it.

16.1.2 Possibility of life on other planets of the solar system

You might be wondering whether there is possibility of any life on any other planets of thesolar system. Let’s check it. Planet mercury is so close to the sun that it is too hot to sustainlife. On the other hand planets Jupiter Saturn, Urenus, Neptune and Pluto are so far awayfrom the sun that due to extremely low temperatures on their surfaces life is impossible.

The narrow belt containing Venus, Earth and Mars seem to be at suitable distancefrom the sun so that temperature of a planet in the region could possibly allow life todevelop on it, if no other phenomenon like green house effect alters its temperature.

The table given below discusses the planets Venus, Earth and Mars and the four physicalconditions necessary for life.

Table 16.1 : Conditions necesary for life on different planets

S. Planet Presence of Correct temp. Presence of ProtectiveNo. C, N

2, O

2, N

2range water blanket

1. Venus Yes No No Yes

2. Earth Yes Yes Yes Yes

3. Mars Yes No Yes No

This shows that no other planet, except Earth, fulfils the conditions necessary for lifeand hence, Earth is the only planet in Solar system where life has originated, evolved andflourished. Thus, the Earth is a unique planet.

16.1.3 Are we alone in the universe?

This is the next obvious question you will ask. The way the universe came into existenceand the way various gallaxies and solar systems are formed, suggest that there is a veryhigh probability of inhabited worlds. In fact, scientists expect millions of such worlds toexist. But all our efferts to contact the extra terriestrial beings have failed by now.


1. Name two planets of solar system which have a protective layer in their atmosphere.2. Give one reason to explain why there is no life on Jupiter.3. Why is the presence of a liquid medium necessary for life to exist on a celestial body ?

The Earth – A Living Planet : 301 :

4. Mention the region of space in solar system where life may be possible ?5. Give one reason to explain why planet mercury does not have any atmosphere ?

16.2 WHAT MADE EARTH A SPECIAL PLANET?

What is special with the Earth that has made it an abode of life? The following threefactors, it seems, have contributed in this regard.

(i) Right distance from the Sun : The Earth stays at the right distance from the sun in analmost circular orbit. Therefore, it receives just appropriate amount of energy from thesun, so that, the temperature range on its surface is suitable for the origin and evolutionof life.

(ii) Appropriate mass and size : The Earth has appropriate mass and radius so that itcould provide gravitational field sufficient enough to hold atmosphere.

(iii) Occurence of some natural events on Earth at right time and in desirable sequenceso that a life supporting system (called Biosphere) could evolve on its surface.

16.2.1 Birth of the Earth

About 5 billion years ago, when the sun was formed, the leftover gases surrounding itstarted getting condensed into small chunks of matter called planetesimals. Theplanetesimals as they revolved around the sun aggregated into bigger masses–planets,satellites, asteroids etc. by forces of mutual attraction. The Earth also came into existencethe same way around 4.5 billion years ago. When born, it was a cool, condensed aggregateof planetesimals. The primitive Earth then melted because of the following two processesnearly 3.7 billion years ago and assumed its present structure.

(i) Planetesimals were still colliding with it and imparting their kinetic energy and massto it.

(ii) There were radioactive elements like Uranium (U), Thorium (Th) etc. present in theEarth which released energy as they decayed which was absorbed by the Earth.The energy gained by these processes increased the temperature of Earth and it melted.

This resulted into the differentiation of the Earth.

16.2.2 Differentiation of the Earth

As the Earth melted it acquired a sphericalshape. The heavier elements from its surfacesank towards its centre and formed a centralregion called the core. The lighter materialsrose to the outer region. Of these materials,whatever remained on the surface of the Earthas liquid, cooled and solidified to form crust.The gases and water vapours trapped withinthe Earth's material were released from itssurface and formed atmosphere. Thus, theEarth re-organised itself into different layersof varying densities. The process of re-organisation of the Earth in different layersof varying densities is called differentiation.

Fig. 16.1 Different layers of earth

Inner coreOuter core

Mantle

Crust

Atmosphere

Dense rocks

Molteniron

Solidiron


Due to differentiation, the mass of the Earth got distributed in four different layers! The core! The mantle! The crust! The atmosphereFigure 16.1 shows the various layers of Earth (not on the scale).

Some important characterstics of each layer are given in the table 16.2.

16.2.3 Evidence of differentiation of Earth

The theory of evolution of solar system suggests that the Earth also came into being atthe same time and in the same manner as the other members of the solar system. Scientistshave developed a techinique of finding the age of a rocks. This technique is called uraniumdating. Using the technique when we find the age of a meteorite, it comes out to be 4.5billion years. When we determine the age of oldest rock found in Greenland region of theEarth, it comes out to be only 3.7 billion years. What does this mean? This means that forthe first 800 million years the temperature of Earth was increasing due to which it gotmelted and differentiated during that period. That is why all signs of Earth for theevolutionary history of this period are wiped out.

Table 16.2 : Characterstics of different layers of Earth

Characterstic Core Mantle Crust

Location Innermost part Middle part Outermost part.Constituents Iron and some Nickel Silicates of iron and A mixture of large

magnesium number of minerals likesilica, alumina etc.

Temperature very hot ~ 4000 0Cat the centre

Size Radius of the core is The thickness Very thin, only aroundabout 3400 km. between core and 10km under the oceans

crust is about . and 35-60 km2900 km below the land mass

Density About 18 gcm–3 4-6 gcm-3 3 gcm–3

Pressure About 3.7 millionatmosphere at thecentre.

State Inner core though Mostly solid only a Solidat higher temperature, thin outer layer ofis solid on account of tar like viscous fluidhigh pressure. Outer of molten rocks.core is liquid.



1. What material the inner core of the Earth is made of ?2. What is the thickness of the crust of the Earth ?3. When did differentiation of Earth took place ?4. What is the importance of right mass and right size of Earth ?5. Explain how the inner core of the Earth is solid though its temperature is (about 40000C)

higher than outer core which is liquid ?

16.3 THE LIFE SUPPORT SYSTEMS

Life on Earth is found in a nearly 20 km thick spherical shell near its surface, calledbiosphere. Living beings are found to interact with each other and with their environmentin the biosphere.

(i) Lithosphere, (ii) Hydrosphere, and (iii) Atmosphere.

These three parts of the biosphere form the life supporting systems of the Earth. Thestory of the evolution of life is intimately associated with the evolution of the biosphere.

Let us study the three parts of the biosphere one by one.

16.3.1 Lithosphere

The word lithosphere literally means a layer of rocky materials. Itconsists of the Earth’s crust and the small upper solid part of mantle.Presently, about three fourths of the surface of the lithosphere iscovered with water in the form of oceans, and the remaining onefourth is a land mass devided in seven continets, namely – NorthAmerica, South America, Antarctica, Australia, Asia, Africa andEurope. All these seven continents form six separate land masses,seperated by water bodies – Europe and Asia forming one bigentity.

As we look at the present world map we find that these sixland masses appear as a jig-saw puzzle and may be adjusted atone place to from a one big land mass. In 1912, German geologistAlfred Lother Wegener suggested that in the begining of Earth’shistory, the continents were a single piece of landmass calledpangaea (meaning all Earth). Then at about 225 million years agothe pangea fractured and started drifting apart and graduallyassumed its present shape. The position of continents at variouseras of Earth’s history is given in Fig. 16.2.

Through their studies on Earthquakes, volcanoes and formationof mountain, geologists have acquired a lot of knowledge aboutthe interior of the Earth. They have come to the conclusion thatthe entire land and water bodies of Earth in fact, stay divided intoeight large and some smaller pieces called the Lithospheric plates.These plates are rigid but they float over coal-tar like molten rocksof mantle, called magma. Due to temperature and pressure

Fig. 16.2 Position ofcontinents in various

eras of the earth’s history

(a) 225 million years ago

(b) 200 million years ago

(c) 135 million years ago

(d) 65 million years ago

(e) Present


difference between the core and the upper part of the mantle, convection currents are setup in magma because of which the Lithospheric plates drift slowly.

The scientists have estimated that the continents are drifling even today at an averagepace of 15 cm per year or so.

16.3.2 Hydrosphere

The water bearing component of biosphere is called hydrosphere-most of it is in the formof oceans (97%) and the rest as Polar ice caps (2.5%) and atmospheric vapours.

The huge water body surrounding the continents is divided into five parts called oceans.The five oceans are :

(i) Pacific ocean(ii) Atlantic ocean(iii) Indian ocean(iv) Arctic ocean(v) Antarctic ocean

Oceanologists have explored the floor ofthe oceans using Ultrasonic echo devices likeSONAR and found that the ocean floors areuneven. Like lands, they also have planes, hills,valleys and plateaus. The researches show thatthe average depth of oceans is about 4 km.Though at some places they may be more than10 km deep.

You know that sea water is salty.Weathering and erosion of rocks makes these salts available for winds and waters whichtake them to oceans and make them salty. But the percentage of salts in sea water is almosta constant for the duration of a life time. Winds and oceans are a big support for life. Someimportant functions of oceans are listed below.

(i) They regulate the global temperature.(ii) The primitive life form originated in the oceans.(iii) They dissolve atmospheric carbon dioxide and thus help in keeping the biosphere in

equilibrium.(iv) They provide good resources for fossil fuels, minerals, salts and sea foods.(v) They act as medium for transporting men and materials using ships, boats etc.

16.3.3 Atmosphere

Surrounding the Earth there is a few hundred kilometer thick envelope of air calledatmosphere. As we go up in atmosphere the air thins out, so much so that 90% of the air isfound within 20 km of height from the ground.

The main constituents of air are nitrogen and oxygen and it is upto a height of 12 kmfrom the Earth’s surface that cloud formation and weather changes take place. Betweenthe altitude of 10 - 50 km lies the ozone (O

3) layer which protects the Earth from harmful

ultraviolet radiations of sun.

Fig. 16.3 Convection currents in magmadrifted lithospheric plates

Convectioncurrentsin outermantle

Equator

Land

Ocean

1. Solid inner core. 2. Liquid outer core. 3. Solidinner mantle. 4. Tar like liquid outer mantle.

5. Crust. 6. Surface of earth – land and oceans.


Atmosphere also is a crucial life support system. It has the following importantfunctions.

(i) It is because of the atmosphere that the radiations from the sun do not straight wayreach the surface of the Earth. Thus the atmosphere prevents the Earth from gettingtoo hot.

(ii) Nitrogen and oxygen in the atmosphere are in correct proportion due to which livingbeings can breathe and controlled burning of fuels becomes possible.

(iii) Billions of meteors entering into Earth’s atmosphere burn out due to air friction. Inabsence of atmosphere they will reach Earth’s surface and hit it with great force.

(iv) The water vapours present in atmosphere provides for rains which is vital for agriculture.

(v) The ozone layer of atmosphere protects us from the harmful effects of ultra-voilet raysof sun-light.

(vi) Carbon dioxide though present in the atmosphere in a very small amount (0.033%) itis important for two reasons -

! Green plants absorb carbon dioxide from atmosphere in preparing their foodthrough photosynthesis. (Details of photosynthesis in lesson 25).

! Carbon dioxide being a greenhouse gas traps infrared (heat) radiations and makesnights warmer and more comfortable.


1. How do scientists study the interior of the Earth?

2. Give two important functions of carbon dioxide in atmosphere?

3. Give two important functions of atmosphere?

4. Give two important functions of oceans?

16.4 EVOLUTION OF LIFE ON EARTH

A very precise sequence of events gave rise to the life supporting systems mentioned in theprevious section. Then came the first ever self multiplying organisms on the scene, theblue-green algae. These components of the biosphere then interacted with each other. Theygrew in complexities and modified each other’s properties and composition. Finally, anequilibrium condition was obtained where every thing was in a fine balance supportingeach other. All this did not happen in a day. It took more than 3.7 billion years to reach tothe presennt stage of evolution of the biosphere.Let us have a broad look at the majorevents in the process of evolution of life on Earth.

16.4.1 Formation of primitive atmosphere

To begin with when the differentiation of Earth started, there was no atmosphere. In theprocess of differentiation, as the light elements rose up, the very light elements, whichwere present in gaseous form bubbled out of the surface of molten Earth and formed theprimitive atmosphere. The primitive atmosphere was very thin and had only nitrogen,hydrogen, carbon dioxide and water vapour. It had no oxygen in the beginning.


16.4.2 Formation of primitive hydrosphere

As the outer surface of the molten Earth cooled and solidfied to form a thin crust, it wasrepeatedly hit and punctured by falling meteorites. Since the atmosphere at that time wasvery much rarefied, it could not offer any resistance to the falling meteorites and they hitthe crust with great force, creating vents in it. Through these vents magma flew out, gotsolidified and became part of the crust. In this process some gases were also releasedmainly due to the hydrated minerals in the molten rocks.

These gases enriched the atmosphere. The water vapour in this primitive atmospheregot condensed and formed clouds. The clouds then came down in the from of rains. Therainwater on one hand speeded up the process of cooling of Earth on the other hand itfilled up the low lying areas and formed the oceans.

16.4.3 Origin of life on Earth

As more and more gases were added to the atmosphere it became gradually denser. Theclouds, as they moved, got charged and electric discharge between them resulted inthunderstorms and lightening. The lightening fused the elements of the primitive atmosphereinto complex compounds. These compounds were then washed down by heavy rains andsent to oceans. In the oceans, under suitable circumstances more and more complexmolecules like carbohydrates and amino acids were developed. With passing time moreand more complex molecules came into existence which could utilize the energy andmaterials from their environment. Finally, in the oceans appeared a complex structurewhich could prepare its own food, using carbon dioxide from air and water from occean,in presence of sunlight. This was the blue-green algae, the first living organisms that everoriginated on Earth. It had two properties markedly different from the existing materials–self growth and self propagation. These are clearly the properties of living beings.

16.4.4 Evolution of higher life forms

When the blue green algae appeared in oceans, atmosphere had only nitrogen, hydrogen,water vapours and carbon dioxide. There was no oxygen in atmosphere. And this wasgood, because the algae could have easily got oxidised in an oxygen rich environment.Then the possibility of life on Earth would have been wiped out . But fortunately theoxygen in air was added at a very slow pace so that the growing life forms could adjustwith it.

The algae as they prepared their food through photosynthesis used carbon dioxide andevolved oxygen. But nature had a provision to remove this extra oxygen in the beginningyears of evolving life. The iron dissolved in oceans consumed the oxygen exhaled by algaeand got oxidised. 2000 to 3500 million year old deposits of iron stones at the bottom ofoceans are evidences of this proposition. The early forms of life thus survived and developedinto more advanced organisms.

During this period (2000 to 3500 million years from now) however the content ofcarbon dioxide in atmosphere decreased and that of oxygen increased gradually. Theincreasing level of oxygen accelerated the process of evolution of life in two ways.

(i) The new organisms adjusted themselves to have greater tolerence for oxygen and toutilise it for more efficient metabolic processes.


(ii) Some of the oxygen formed ozone layer to protect the living organisms from harmfulultraviolet radiations.On the other hand reduction in the level of carbon dioxide reduced the green house

effect, because of which the temperature on Earth could settle down to values morefavourable for higher forms of life.

Under these conditions more advanced forms of life evolved which could surviveeven on land. Around 600 million years ago the environmental conditions and compositionof atmosphere became almost similar to the present status. The organisms of the time alsohad reached a level of evolution to develop into more advanced life forms. The diverse lifeforms that we see around us today is a result of the evolutionary process of these 600million years.

16.4.5 Evolution of man

Man also has his place in the story of the evolution of life. Scientists now believe that apes(like Chimpanzee and gorilla) were the ancestors of modern man. On the basis of fossilremains from various excavation sites they have developed a chain of successive stages ofevolution of man.

Fig. 16.4 Stages in evolution of man


1. An oxygen free atmosphere was crucial for the survial of early life. Why?2. Which evolved earlier–atmosphere or hydrosphere? Could it be otherwise?3. What evidence do we have to believe that the oxygen evolved in early years of life was

removed by some natural process. What was this process?4. In which period of time oxygen replaced carbon dioxide from its dominant position?

How do we know this?5. What is the evidence to suggest that apes were the ancestors of man?

16.5 THE EARTH SYSTEM

The lithosphere, the hydrosphere, the atmosphere and various life forms on Earth interactwith each other and support each other. So the Earth may be visualised as a system. To run

Oldest of human ancestor(10-13 million years ago)

Human ancestor3.5 million years ago

Human ancestor1.7 million years ago

Human ancestor100000–50000 years ago Human ancestor

20000–50000 years ago


a system we need energy. On Earth all forms of energy that we use, with the exception ofnuclear energy and geothermal energy, are obtained from the sun.

16.5.1 Sun, as the source of energy

Basically, the energy we receive from the sun is in the form of heat and light. We can usethis energy directly in our solar cookers or solar water heaters. But, usually the energyfrom sun may manifest itself in various other forms on Earth. Let us consider a few examples.

1 Wind energy : Uneven heating of different regions of Earth’s surface, creates regionsof high and low pressures. Due to this, wind blows and the wind energy runs our windmills.

2. Hydel power : Water evaporated by solar energy, rises up in the atmosphere. Thiswater cools at high altitudes, forms clouds and comes down in the form of rains. Therain water as it flows down slopes may run our water-mills. It may also be collected indams and run our power plants to generate electricity.

3. Energy from food : To do work we need energy. We get this energy from food.Animals get food from plants. Plants prepare their own food through photosynthesisusing sunlight. So the energy that runs life on Earth is ultimately received from thesun.

4. Energy from fossil fuels : You have studied that fossil fuels such as coal, petroleumand natural gas are forms of biomass (dead remains of plants and animals) burrieddeep under the Earth. This biomass when alive had received its energy from the sun.So it is solar energy stored in the form of fossil fuels.

The importance of sun, as a source of energy for life on Earth, is therefore, unquestionable.The sun is a huge mass (~ 1030 kg) consisting of hydrogen (92%) and helium (7.8%). It isradiating out tremendously large amount of energy for the last 5 billion years and is expectedto do so for the next 5 billion years. The energy that it radiates is so large that we receive1.36 kW m-2 of solar power in the upper atmosphere of the Earth. However it is only 47%of this energy that reaches on the surface of the Earth.

The incredibly large amount of energy that is being released by the sun can not beproduced by the simple burning of hydrogen gas. The source of the energy of sun, assuggested by German physicist, Hans Bethe, in 1939, is nuclear fusion of hydrogen intohelium.

16.5.2 Circulation and utilization of solar energy

The flow of energy from the sun to the Earth is a unidirectional process. We receive energyfrom the sun, but return nothing back to it. However, the energy received from the sun mayeither be utilized or it may go waste and create problems for life. For example, if excessivesolar energy remains trapped in atmosphere it may melt the solar ice caps, which willresult in increased sea level and submergence of land.

The energy received on Earth is exchanged between atmosphere, hydrosphere,lithosphere and living organisms in various ways. Thus the energy circulates betweenthese components of biosphere and life to keep them active.


Fig. 16.5 Circulation of solar energy in the biosphere

The most effective and low cost method of utilizing solar energy is used by plants inthe process of photosynthesis and hence by increasing the green cover of Earth we canthink of maximum utilization of solar energy.


1. Name two forms of energy which we do not receive on the Earth from the sun.2. After a nuclear holocaust the sky some scientists say, will get covered with dense dust

clouds for several months. Can you think of one consequence of this situation ?3. The brown haze in Asian sky will result in low agricultural produce. Can you give one

explanation for this ?4. How is sun responsible for the energy we receive from a hydroelectric power plant ?5. Name the reaction responsible for the production of energy in the sun ?

16.6 BALANCE IN NATURE

By now you might have understood that the Earth system has several interacting constituentsin delicate balance. A slight disturbance in this balance may bring a big threat to the entirelife. Let us consider some of the human achivities which are disturbing the balance innature at an alarming rate.

16.6.1 Examples of threats to the balance in nature by human activities

1. In our craze for comfort and zeal for industrialization we are burning fossil fuels at avery fast rate. Thus, the carbon dioxide which was fixed for millions of years is beingreturned to the atmosphere in few hundred years. This is causing not only an energycrisis but also posing a threat of global warming.

2. The mindless use of chloro fluoro carbons (the chemicals we use as refrigerants andperfume sprays) are eating up our protective ozone layer creating a hole in it aboveAntarctica.

Lithosphere

Hydrosphere

Atmosphere

Sun


3. Deforestation for paper and wood is minimizing the effective use of available solarenergy on Earth.

4. Creation of non-biodegradable materials like polythene is making the Earth barrenand pausing threats for various life forms.

16.6.2 Our duty to protect the Earth

We must understand that the Earth is a unique planet. It is under very special circumstancesthat its life support systems have evolved. We must take special care to maintain theequilibrium between the various components of the Earth system so that life on this planetmay flourish and progress.


1. Give a consequence of deforestation.2. Name a substance responsible for creating ozone hole.3. What is meant by a biodegradable substance?4. Suggest one step to maintain balance in nature.

LET US REVISE

! The Earth was formed around 4.5 billion years ago along with the other members ofthe solar system.

! Due to the kinetic energy of the colliding planetesimals and disintegration of radioactiveelements the Earth melted and got differentiated around 3.7 billion years ago.

! Differentiation is the process of reorganization of Earth into different layers of varyingdensity

! Radiodating techniques used by scientists revealed that the oldest rock found inGreenland is only 3.7 billion years old.

! The Earth’s solid stuff differentiated into three layers (i) core (ii) mantle (ii) crust,after it melted.

! Crust of Earth along with oceans and atmosphere is the region in which living organismsare found and, therefore, they together are called biosphere.

! Biosphere has three life support systems (i) Lithosphere (ii) Hydrosphere and (iii)atmosphere

! The oxygen free atmosphere → the hydrosphere → the blue green algae → oxygencontaining → atmosphere → higher life forms → Homo sapiens (man) evolved onEarth in this particular sequence, and seltted down to an interactive system inequilibriun.

! Man’s activites are disturbing the balance of our ecosystem and posing a threat to theentire life on our unique planet.

! Let us be careful and mend our ways.

TERMINAL EXERCISES

A. Multiple choice type questions.

1. The first living organism developing on Earth was -(a) Bacteria (b) Virus


(c) Algae (d) Fungus2. How much time (approximately) did it take for the formation of fossil fuel?

(a) 2.5 x 108 years (b) 2.5 x 106 years(c) 2.5 x 104 years (d) 2.5 x 102 years

3. Which part of the Earth do we interact with the most?(a) Inner core (b) Outer core(c) Mantle (d) Crust

4. The first organism originated in(a) Ocean (b) Atmosphere(c) Marshy land (d) Desert

5. Which of the following is not the part of biosphare?(a) Lithosphere (b) Hydrosphere(c) Atmosphere (d) Mantle

B. Mark the following statements true or false.

1. If there were no atmosphere, the temperature of the Earth’s surface would havevaried over a wide range in 24 hours.

2. The oldest rock found on Earth is 4500 years old.3. Green plants trap solar energy when they are alive and release it when they die.4. The continents are fixed with respect to the Earth.5. The concentration of salt in oceans remains roughly constant over a life time.

C. Fill in the blanks.

1. __________ radiations present in sun light can cause skin cancer.2. The density of air __________ as we go up.3. Earth was born along with other members of solar system nearly __________

years ago.4. The temperature at the core of the Earth is about __________5. One of the factors due to which the primitive Earth melted was radio active

decay of elements like __________

D. Descriptive type questions.

1. What is meant by Pangaea ?2. What is the age of the Earth ?3. Up to what height do we have significant amount of water ?4. Name two major ways by which carbon dioxide present in air is consumed.5. When did the atmosphare reach a composition similar to what we have today ?6. Name the four layers the Earth is differentiated into.7. How does the atmosphere protect us from the falling meteors?8. “Deforestation may lead to melting of polar ice caps”. Explain.9. The primitive algae were prone to oxidation, even then these survived in the

oxygen exhaled. Explain how it could be possible ?10. How were complex molecules formed from the elements present in the primitive


atmosphere?11. Discuss why life could not evolve on planet Mars ?12. What is meant by the term differentiation. Describe major layers of Earth with

the help of a labelled diagram ?13. State five advantages of atmosphere.14. State the five advantages of oceans?15. What made Earth a unique planet of solar system ? Explain.16. List some activites of man which are disturbing balance of the life support

systems of the Earth.


16.1

1. Venus and Earth2. Because it is very cold3. So that nutrients may be transported to different parts of the organism4. The belt containing Venus at its inner edge and Mars on the outer edge.5. Very high range of temperature variation and low gravity to hold the atmosphere.

16.2

1. Iron2 10km under the sea floor and 35-65 km under the land3. Around 3.7 billion years ago from now4. Right mass and right size ensures right gravitational field to hold atmosphere5. because of very high pressure.

16.3

1. Studying the Earth quakes, volcanos and formation of mountains.2. (i) green plants prepare their food using CO

2

(ii) being green house gas it maintains night temperature to a comfortable value.3. (i) Regulate global temperature

(ii) Mineral and food resource4. Provide oxygen for respiration and CO

2 for photosynthesis.

16.4

1. Because the very first organism blue-green alga was prone to oxidation.2. Atmosphere evolved before hydrosphere. Had it been in reverse order the lighter

water molecules could have escaped out of Earth. No life was then possible withoutwater.

3. The red iron stones at the sea-beds ageing 2 billion years are evidence. Oxidationof iron dissolved in water removed the oxygen from the atmosphere.

4. Around 600 million years ago. Because that is the minimum age of the red ironstones found at sea beds.


5. The fossil remains of man-like creatures suggest a definite evolutionary trend.

16.5

1. Nuclear and geothermal energy2. A global winter3. Lesser light from sun reaches the Earth slowing down the process of photosynthesis.4. Solar energy evoporates water which rises to high altitudes, forms clouds, comes

down in the form of rains. It is the potential energy of this water collected in damswhich runs turbines of our power plants and generates electricity.

5. Nuclear fusion.

16.6

1. Decrease in the effective use of available solar energy2. Chloro fluoro carbons3. A substance which can be broken up into simpler substances by some germs.4. Grow more trees.

GLOSSARY

Algae : Plants without true stems, roots and leaves found in water or ground.

Atmosphere : A blanket of gases surrounding the Earth.

Core : The innermost portion of the Earth.

Crust : The thin, rocky outer layer of the Earth.

Convection current : A process of transfer of heat in liquids and gases where in hotand light fluid rises up and gets cooled at higher place and heavy cool fluid sinks down.

Erosion : The weathering away of Earth’s surface by water, ice or wind.

Evolution : The process where by species of living things gradually change to adaptto their environment.

Geologist : A scientist who studies the Earth, its history and structure.

Global warming : Gradual rise in the average temperature of Earth.

Magma : Hot, molten rock formed beneath the Earth’s crust.

Mantle : The layer of the Earth that lies between crust and core.

Organism : Any living being.

Photosynthesis : The process by which green plants use sun light as an energy sourceto turn CO

2 and H

2O into sugars they need for their food.

Pressure : Amount of force acting on unit area.

Volcano : An opening in Earth’s crust through which magma erupts.

17

Our EnvironmentThe word environment means to encircle or surround. Any thing that surroundsus forms our environment. The plants, animals, air, water and land all form ourenvironment. All our activities are influenced by the environment in which welive. These activities include the functioning of our body and our interaction withother parts of our environment. Therefore, the environment is important for oursurvival. We should take care of it.

OBJECTIVESAfter completing this lesson, you will be able to:• define environment and list the biotic and abiotic components of the

environment;• discuss the different types of habitats and the adaptation of animals and plants

in these habitats;• explain the causes and consequences of alterations in habitats and the need to

conserve habitats;• define biosphere and ecosystem, and discuss the ecological significance of

these levels of organization;• explain food chain, food web and trophic levels in a biological community

and discuss how these are constituted as the pyramids of energy;• compare the carbon and nitrogen cycles within the ecosystem.

17.1 COMPONENTS OF THE ENVIRONMENTThe environment has two types of components.• Biotic components, which include living beings including humans.• Abiotic components, which include all non-living things around the organism.

These two components have an effect on each other. For example, if it does notrain for some days and the temperature is very high, the plants will dry up andanimals, including human beings, will find it difficult to live in such anenvironment.

The desert area is covered with sand all around. It rains very little in suchareas hence water is scarce. The days are very hot while the nights are cool indeserts. It is because of such adverse conditions of the abiotic components that

: 2 : Our Environment

there is very little vegetation and only a few species of animals can live in deserts.Animals, such as camels, and plants like cactus that can survive without water formany days are found in such places.

We know that a fish swims with ease in water. However, as soon as it is takenout of water, it dies. Do you know the reason? This is because the changedenvironmental condition is not suitable for the survival of the fish.

Thus, we find that biotic components depend upon the abiotic componentsfor their survival. On the other hand, the abiotic components are also affected bybiotic components. For example, if there are more trees at a place the air willcontain more moisture at the place. Also, the temperature of the place will berelatively low. The amount of dust particles in the air shall be less. Have you everfelt such difference between a place with more trees and the one with fewer trees?

The amount of fertility of the soil at a place gets affected by the water,temperature and air.


1. Classify the following into biotic and abiotic components of the environment:neem, soil, buffalo, air, rose, butterfly, light, heat, man, cow, humidity

2. Give an example to show that abiotic components of environment dependupon the biotic components.

3. Name one desert animal and one desert plant.

17.2 HABITAT AND ADAPTATION

Every living organism lives in a specific environment. A place or a set ofenvironmental conditions in which a particular organism lives is called its habitat.The habitats of different plants and animals are different, but at the same timemany plants and animals share the same habitat.

All forests are not habitats of tigers or lions. Jim Corbett National Park inUttaranchal has thick forests. It provides optimum conditions for the tigers tolive. There are streams and rivers flowing in the area that provide water. Thepresence of deer and sambhar in large number in the same habitat provide foodfor the tigers.Thus, a habitat must provide the organisms suitable climaticconditions, shelter and food.

17.2.1 Modes of life

The following modes of life have been identified for different organisms:

• Aquatic : For organisms living in water

• Terrestrial : For organisms living on land

• Aerial : For organisms that use air as a medium for their activities such aslocomotion

• Amphibious : For organisms, such as a moss plant and a frog that completetheir life cycles by living one part of their life in water and another part on land.

Our Environment : 3 :

Fig. 17.1 Different organisms live in different habitats

Organisms that live in a specific habitat have some important characteristicsthat help them to adjust and to live successfully. This adjustment is called adaptation.

The organisms adapt so that they can:i. successfully compete for food,ii. defend themselves from attack by other organisms (enemies),iii. find a mate to reproduce/find favourable conditions to reproduce, andiv. respond efficiently to the change in environment.

17.2.2 Aquatic adaptations in organisms

a) In plantsPlants that live in water are called hydrophytes (hydro: water, phyte: plant).

Look at the picture aboveand relate. Plants that live inwater have the followingcharacteristics:

i. Poorly developedroot system: Asthey can easilyabsorb water andminerals from thep l e n t i f u l l yavailable water.

ii. Thin and narrowleaves (Hydrilla)or long, flat andr ibbon- shapedl e a v e s(Vallisneria): Asthis helps towithstand watercurrents. Fig. 17.2 Some plants that live in water

Lotus

Hydrilla

Water hyacinth

Vallisneria


In a lotus plant, the leaves float on water with their broadupper surface coated with wax. This wax acts as waterrepellant.

Fig. 17.3 Some fishes that live in water

Hammer-headed shark

Ray fish

Catla

b) In animalsObserve the animals shown in the figure17.3. Vertebrates that live in water havethe following characteristics:i. The body is streamlined

(pointed at both ends) that helpsin reducing friction and allowsswift movement in water.

ii. Gills help the animal to breathein water.

iii. Fins help to swim, steer andmaintain balance. A whale (amammal) has flippers to swim.

iv. Pupil of the eyes is large ascompared to other vertebrates. Itallows more light to enter forclear vision in water.

v. Some fish have swim bladdersthat act as floats and allow theorganisms to float in water.

Some coelenterates

Lobster Echinoderms

Some sponges

Labeo

Octopus

vi. The body surface gives out some secretions, which lubricatethe scales and help the animal slip away and escape fromenemies.

There are numerous other kinds of aquatic animals with varyingadaptations, such as Hydra, water flea and some worms.

Fig. 17.4 Diversity of animal life in water

Labeo


17.2.3 Terrestrial (land) adaptations in organisms

a) In plantsSome plants live on land and require moderate (neither low nor high) supplyof water and temperature. These plants are called mesophytes (meso:moderate). Examples: neem, papaya, banyan, mango, wheat, tomato, etc.

Fig. 17.5 Mesophytic plants

Some plants live on land under extreme water scarcity and high temperatureconditions. These are called xerophytes (xeros: scarce water). Examples:cactus (Opuntia), Babool (Acacia).

Fig. 17.6 Xerophytic plants

Table 17.1 Adaptations in land plants

Part of the plant Mesophytes Xerophytes

Roots Well-developed Extensively developed todraw as much water as possible from the ground

Stem Well-developed, Flattened, fleshy and greensolid and branched to store water and function

as leavesLeaves Well-developed, numerous, Reduced (modified) into

of various shapes and spines, stoma if presentsizes and with large reduced in number tonumber of stomata prevent loss of water

WheatRice

Papaya Bamboo

Banyan

Opuntia

Casuarina


b) In animalsMost animals you see around are those living in a moderate type of a habitat.These are different types of animals. Examples of some terrestrial mammalsand reptiles are given below :• Mammals: tiger, lion, deer, bear, squirrels and many others• Reptiles: lizards and snakes

Terrestrial animals may be of different types according to their mode oflocomotion. Such animals have well-adapted toes. These are:• Runners: deer and antelopes• Climbers: monkeys and squirrels• Burrowers: rats, moles and snakes• Fliers: bats and birds

All vertebrate terrestrial animals breathe through lungs.

17.2.4 Xerophytic adaptations in organisms

Animals found in the xeric (dry) conditions show certain special types ofadaptations.• Extreme heat and scarcity of water: These animals have very scaly

skin, resistant to drying and show many adaptations to conserve water.Examples: camel, snakes, spiders and scorpions.

• Extreme cold and scarcity of water: These animals have oily hairsthat provide thick winter coat. Examples: polar bear, reindeer.

Fig.17.7 Aerial adaptations in a bird

17.2.5 Aerial adaptations in organisms

Besides insects, organisms, such as birds, mainlyuse air as a medium to fly. Birds show the followingadaptive features that help them to fly:i. streamlined body to steer through the air,ii. wings that help to fly are modified forelimbs,iii. strong flight muscles,iv. body covered with feathers, which trap air

to keep the body warm and help the bird tofly, and

v. light weight because of hollow bones alongwith reduction in the number of bones.


1. Name the type of habitat in which the following organisms are found.i) Acaciaii) Snakeiii) Bativ) Frogv) Lotusvi) Mango tree


2. Give the adaptive features of the following:i) Birds with respect to bones ( )ii) Snakes with respect to high temperature ( )iii) Neem tree with respect to the number of stomata ( )iv) Xerophytes with respect to the root system ( )v) Fish with respect to shape of the body ( )

17.3 EFFECT OF ALTERATION OF HABITAT

The survival of an organism in a habitat depends upon the way an organism isconditioned to the abiotic and the biotic components of the environment in thehabitat. Any change or alteration in the habitat can disrupt the balance in nature.

Bhopal gas tragedy

Do you know what happened on 3rd Dec 1984 in Bhopal? Leakage of methylisocyanate (MIC) gas from Union Carbide factory totally disturbed human andanimal life there. The adverse effects of that gas are seen till today.

Gujarat earthquake

What happened in Gujarat on 26th January 2001? The earthquake damaged human,animal and plant life.

In a similar way other natural calamities, like floods, volcanic eruptions andtornadoes cause so much of damage to life all around. Much more than this, akind of change in the habitat is brought about by human beings for their selfishgains. Some such activities are, deforestation, indiscriminate use of poisonousmaterials in form of pesticides and chemical repellants, industrialization andmismanagement of industrial waste, automobiles, hunting and fishing, and use ofnuclear weapons.

Natural calamities or adverse human activities have a destructive effect onthe natural habitat. As a result, the organisms may die an unnatural death, or losetheir place of shelter. Many of the species in turn many get completely wiped outfrom the world, i.e. they may become extinct.

To save the living treasure (flora and fauna) and to protect the natural habitatmany laws have come into force. These are being operated by many organizations.It is compulsory to implement the plan related to Human Resource Development(HRD) and Natural Resource Development (NRD). To conserve the natural habitat,many national parks and sanctuaries are being maintained by the government.


1. Name any one recent natural calamity that occurred in India and any onecalamity caused due to human activities.

2. List any four human activities that affect the natural habitat adversely.

17.4 BIOSPHERE

The land, water and air on the earth support living organisms. The regioncomprising water forms the hydrosphere. The soil and rocks on the earth’s surface


as well as below the oceans make up the lithosphere. The air above the earth’ssurface forms the atmosphere. These three parts act together to providesurroundings called the biosphere in which life exits. Biosphere is considered asthe largest organisational unit of the biological system.

17.4.1 Environmental levels of organization

Let us start at the level of the whole organism, such as a plant or an animal. Thisis called organismal level. Take an example of a human being as an organism.All human beings of your family, locality, city, state, country and the world formone kind of individuals, they can potentially interbreed and produce fertile youngones, thus they are one species. Individuals of a species occupying a definitespace or area at a given time constitute population. Thus there can be a populationof frogs in a pond, population of squirrels in a garden, or population of peepaltrees in a forest, etc.

Any population of individuals cannot live independently. Can we live withoutdomestic animals, crops or plants?

When you look in a pond, you may see plants like lotus, hydrilla and algae.You may also see frogs, fish, water fleas and some other insects.

There are different kinds of organisms (populations) in that area. All theseorganisms are interdependent and live together forming a community. A communityof living organisms is called biotic community.

We have learnt earlier that no biotic community can exist in the absence ofabiotic factors (water, air and light). The interdependence of the two types offactors occurs in an ecosystem.

A pond and a lake are examples of aquatic ecosystems. Examples of someterrestrial ecosystems are natural forests, crop fields, etc.

Fig. 17.8 A pond is an aquatic ecosystem.


Fig. 17.10 Various levels of organisations

Individual

Population

Community

Ecosystem

Biome

Fig. 17.9 A forest is a terrestrial ecosystem

In simple language we can say that, theliving organisms which are found in a definitegeographical region together with thephysical environment of that region form anecosystem.

All the ecosystems taken together in ageographical area form a bigger unit calledbiome. For example, in forest biomes onemay find ponds, lakes, grasslands and forests.

Organisms exist up to 8 km in the airabove sea level and up to 5 km below sealevel. These life supporting regions of theearth comprise the biosphere. Various levelsof organization and their sequence is givenbelow.

The biosphere includes the total worldof life. The living world which is made upof millions of organisms, depends upon theearth for the necessary materials that enterinto its composition and upon the sun for itsconstant need of energy to perform its vitalactivities.

17.5 FLOW OF ENERGYThe sun gives out a large amount of radiationthat consists of many different kinds of rays.Only some of these rays reach the earth’ssurface. Others are either reflected by theearth’s atmosphere or turned away by theearth’s magnetic field.


Fig. 17.11 Energy flow in a food chain

Energy source

Producer

Primaryconsumer

Secondaryconsumer

17.5.1 Food chainWe know that green plants make food during photosynthesis,taking raw material from the earth and energy from the sun.Thus, the green plants are producers in the living world.

It is seen that the animals eat green plants, which in turnare the food for other animals. Hence, the food produced bygreen plants is consumed directly or indirectly by all kindsof animals, which are called consumers.

The relationship of eating and being eaten up at differentlevels in an ecosystem is represented in the form of a chaincalled food chain.

A food chain is the representation of a single energypathway from the producer to the consumer.

The study of food chains in an area or habitat helps us toknow about interactions among the different organisms andalso their interdependence.

Let us take the example of a simple food chain in grasslandin which the grass is eaten by the grasshopper that in turn iseaten by a bird Fig.17.11. In this process of eating and beingeaten, energy is passed on from one step to next in a foodchain.

The amount of sunlight, which the earth receives in the form of energy, isvery little. A portion of it is also reflected back to the earth’s surface. No animalcan use sunlight directly for its living activities.

Green plants possess chlorophyll. This chlorophyll is capable of trapping afraction of the incoming sun’s energy to make food for the plants by a process calledphotosynthesis. During photosynthesis, water and carbon dioxide are used to buildup complex carbohydrates. The absorbed light energy is thus trapped as chemicalenergy. Thus, solar energy enters into the biosphere through photosynthesis.

Only a negligible amount of solar radiation striking the plants is fixed throughphotosynthesis. The pathway along which the energy flows through the organismscan be studied in the following two ways:

• We can study the food relationship between the species and the communityby way of food chains and food webs.

• We can also find out the energy flow in terms of number of organisms andtheir biomass (i.e. weight of all organisms and calorie content.)

This energy flow can be represented in a food chain as shown in figure 17.11.

In this chain, the grass is the producer, the grasshopper, which consumes grass, isa herbivore and the bird, which consumes the grasshopper, is called the carnivore.Animals that consume both plants and animals are called omnivores. Herbivores,carnivores and omnivores are consumers. The best example of omnivores is man.


17.5.2 Food web

In a community, a large number of food chains exist. Many of these chains areinterconnected by a species, which occurs in more than one chain. Grassland canhave many food chains operating in it as shown in fig. 17.12. These interconnectedfood chains establish a network of species’ relationships called food web.

A food web is a network of species relationship formed by interconnectedfood chains.

Fig. 17.12 A food web

A food web indicates that one organism may occupy position in more thanone food chain. For example, a snake and also a hawk may consume a rat. Theorganisms representing producers and consumers in the food chain give a definitestructure of the ecosystem.

We have seen that in a food chain there are different steps and energy ispassed on from one step to the next step. Each of these steps in a food chain iscalled trophic level.

In other words, the various steps in a food chain at which energy transfertakes place are denoted as trophic levels.

Plants are producers and form the first trophic level. Herbivores, i.e. plant eaters,are the first order consumers and form the second trophic level. Carnivores, i.e.animal eaters, which feed upon the herbivores, form the third trophic level. Largecarnivores that feed upon small carnivores form the fourth trophic level, and so on.


If you compare the number of organisms living at eachtrophic level in a food chain, then you can represent the chainby a pyramid of numbers. Producers form the base of thepyramid and the apex by the last order consumers. Thepyramidal shape shows that the large carnivores at the topare fewer in numbers (Fig. 17.13)

One can also construct a pyramid of energy if it ismeasured in term of joules for each trophic level.


1. Complete the following sentences :i) Plants trap solar energy and pass it to the next trophic level in the

form of__________ energy.ii) The third trophic level in a food chain is formed by the _________iii) A food chain is the representation of single energy pathway

from_________ to__________

2. Which of the following statements are TRUE?i) One organism cannot occupy position in more than one food chain.ii) The number of organisms living at different trophic levels in a food

chain is the least for large carnivores.iii) Plants are called producers because they can produce a new plant.iv) The solar energy enters the biosphere through the process of

photosynthesis going on in the plants.

3. What is the difference between a biome and an ecosystem?

17.6 AMOUNT OF ENERGY FIXATION

Energy after being trapped by plants (producers) is passed to the animals(consumers) of the next trophic level in the form of food. Some amount of energyis lost during these transfers. Energy is also used up by the organisms at eachtrophic level to carry out various activities. Thus, the amount of energy availablegoes on decreasing during its transfer from one trophic level to the other. And, theflow of energy through various trophic levels is one-way energy transfer.

An interesting point emerges from the study of food chains. Shorter the chain,more is the energy available at each level. Maximum energy is at the plant level(producers). Nearer the eater is to the plants, greater is the energy available to it.

17.7 CYCLING OF MATERIALS

Since materials flow from non-living to the living and back to the non-living in amore or less circular path, the cycle is also known as biogeochemical cycle.

One can study the cycling of each element and have a total picture of thisproperty of the ecosystem.

17.7.1 Carbon cycle

Carbon is the main constituent of the living matter. It is found in carbohydrates,

Fig. 17.13 Pyramid of numbers

Largecarnivores

Small carnivores

Herbivores

Producers


fats, proteins and nucleic acids that makeup the living cell. It is available fromthe following three main sources –atmosphere, oceans (hydrosphere),and lime stone, coal and petroleum ofthe lithosphere.

The atmosphere contains about 0.03to 0.04% carbon dioxide in free state.Green plants use this carbon dioxide tosynthesize food by the process ofphotosynthesis. The atmospheric carbontaken in by the plants is transferred toanimals in the form of food. From bothplants and animals, it is then passed onto the decomposers after their death. Ifsuch processes of taking in and passingon continued then there would have been

Fig. 17.14 Carbon cycle in nature

ATMOSPHERICCARBON DIOXIDE GREEN PLANTS

ANIMALS

PREHISTORIC PLANTS

Photosynthesis

Decay

Respiration

DecayRespiration

Combustion

Foss

ilisa

tion

Eaten

no carbon dioxide left in the atmosphere. However, this does not happen inreality. There are processes by which carbon dioxide is returned to the atmosphereto maintain a balance. The processes by which carbon dioxide is returned to theatmosphere are as follows:• By the process of combustion, i.e. burning of fuels like wood, coal,

petroleum, etc. which takes place continuously.• By the process of respiration in plants, animals and decomposers.

17.7.2 Nitrogen cycle

Nitrogen is an essential component of the proteins and nucleic acids in livingbeings. The atmosphere is the biggest source of nitrogen. Green plants absorbnitrogen in the form of nitrites and nitrates from the soil and water in organic orinorganic form. Nitrogen cycle can be studied in five steps as given below.i. Nitrogen fixation: Free nitrogen from the atmosphere can be fixed in

following two ways:

Fig. 17.15 Nitrogen cycle in nature

NITROGEN IN SOILammonia ammonium compounds nitrates nitrates

by action of nitrifying bacteria

ORGANICREMAINS

NITROGEN INANIMALS

NITROGEN INPLANTS

LEGUMINOUSPLANTS

NITROGEN INTHE AIR

denitrifying bacteria

absorption

excretion

deca

y

death

death

eaten

root

nod

ules

nitrogen fixing bacteria in the soil


• Nitrogen and oxygen combine with each other to form oxides in theatmosphere by lightning during cloud formation. These oxides ofnitrogen dissolve in rainy water and on reaching the earth’s surfacebecome a part of soil and water.

• Some microbes like blue green algae and bacteria fix the free nitrogenin the atmosphere into nitrites and nitrates. Nitrogen fixing bacteriaare found in the soil and in root nodules of the roots of some leguminousplants like peas, gram, beans, etc. They fix the atmospheric nitrogeninto nitrates. These nitrates are released into the plants or soil.

ii. Nitrogen assimilation: Plants absorb nitrogen in the form of nitrates to prepareamino acids. This nitrogen is then taken by animals from plants in the form ofproteins (complex form of amino acids) through the food chain.

iii. Ammonification: Proteins in the body of animals are broken down in simplerform, such as urea and ammonia. These are removed from the body alongwith urine. Remains of the dead organisms are also converted into ammonia.This process is known as ammonification.

iv. Nitrification: Conversion of ammonia into nitrates is called nitrification.Some bacteria, found in the soil convert ammonia into nitrites. Some otherbacteria convert these nitrites into nitrates.

v. Denitrification: Denitrifying bacteria living in some soils like the soil ofponds and marshes change the soil nitrates into nitrogen which goes back tothe atmosphere.


1. Choose the correct answer from the following :i. Which of the following gases is essential for burning?

a) Oxygenb) Nitrogenc) Water vapourd) Carbon dioxide

ii. The process of conversion of free atmospheric nitrogen into nitritesand nitrates is calleda) nitrificationb) denitrificationc) nitrogen assimilationd) nitrogen fixation

iii. The processes by which carbon dioxide is returned to the atmosphere area) combustion and respirationb) photosynthesis and respirationc) decomposition and nutritiond) photosynthesis and digestion

2. Why do living organisms need nitrogen?


LET US REVISE

• The environment has both living and non-living parts, i.e. biotic and abiotic.

• The biotic and abiotic components depend on each other.

• A place or a set of environmental condition in which a particular organismlives is its habitat.

• A habitat provides shelter, food and climate to the organism.

• Aquatic, terrestrial, aerial and amphibious are different types of habitat.

• Different organisms are adapted to live successfully in different modes of life.

• Any alteration in the habitat because of natural or man-made calamity causesimbalance in nature.

• Efforts are being made at international and national level to maintain balancein nature.

• Living organisms have various levels of organization, which are divided intotwo main groups, i.e. lower level (up to individual) and higher level (up tobiosphere).

• Each level of organization works like a system involving both matter and energy.

• Biosphere is the highest level of organization.

• It includes all the living (biotic) and non-living (abiotic) components of theworld, i.e. all the ecosystems.

• It works like a system showing interactions and interdependence betweendifferent organisms and different physical environments.

• The living community in the biosphere has various trophic levels like greenplants (producers), animals (consumers) and microorganisms (decomposers).

• The food inter-relationships between the organisms of various trophic levelsare studied through food chains.

• In the process of eating and being eaten up, energy trapped by the green plantsis passed on through various trophic levels.

• Ultimately, the energy is released and it does not re-enter the system. Thus, acyclic flow of energy is seen in an ecosystem.

• Energy is lost at each transfer and maximum of it is available near the beginningof the food chain.

• The materials or nutrients which plants and animals require for their normalgrowth and development, cycle through the ecosystem.

• They are absorbed by plants, passed on to animals and returned to theenvironment by decomposers.

• Nature has a unique way to maintain balance in the atmospheric gases throughvarious cycles, viz. carbon and nitrogen cycles.


TERMINAL EXERCISES

A. Multiple choice type questions.1. Plants found in which of the following habitats have poorly developed

root system?

a) Aquatic b) Terrestrial

c) Xerophytic d) Amphibious

2. As a special feature of birds, their bones are

a) small-sized and jointed for flexibility.

b) porous to allow circulation of materials.

c) filled with a hard material to provide strength.

d) hollow and few in number to make the body light in weight.

3. The government is maintaining national parks and sanctuaries in order to

a) conserve natural habitat for animals.

b) identify areas where hunting and fishing can be done withoutany restriction.

c) protect small animals from other carnivores that feed on them.

d) protect animals from accidents.

4. The largest unit of the biological system is

a) population b) biome

c) biosphere d) ecosystem

5. The apex position in the pyramid of numbers is occupied by:

a) producers b) small carnivores

c) large carnivores d) herbivores

B. Descriptive type questions.

1. Differentiate between the following :

i) Ecosystem and biosphere

ii) Food chain and food web

iii) Carnivores and omnivores

iv) Producers and consumers

2. Using a simple food chain, explain the pathway along which energyflows in an ecosystem.

3. Why do we say that energy flow in the biosphere is unidirectional, i.e.in one way? Explain with an example.

4. With the help of a diagram, explain the cycling of carbon in thebiosphere.



17.1

1. Soil NeemAir BuffaloLight RoseHeat ButterflyHumidity Man

Cow

2. Presence of trees makes the air contain more moisture and keep thetemperature low.

3. Camel, Cactus

17.2

1. i) Acacia - Terrestrialii) Snake - Terrestrialiii) Bat - Aerialiv) Frog - Amphibiousv) Lotus - Aquaticvi) Mango tree - Terrestrial

2. i) Light and hollow bones ii) scaly skin iii) large number of stomataiv) root system extensively developed v) streamlined body.

17.3

1. Natural calamity: Earthquake in Gujarat on 26th Jan. 2001Calamity due to human activities: Leakage of MIC from the UnionCarbide factory in Bhopal on 3rd Dec.1984

2. Deforestation, use of pesticides and automobiles, hunting, and fishing

17.4

1. i) Chemicalii) Carnivoresiii) Producer, consumer

2. (ii), (iv) True statements3. All living organisms in a definite geographical region along with the

physical environment form an ecosystem. While, all ecosystems in ageographical area together form a biome.

17.5

1. i. (a)ii. (d)iii. (a)

2. Nitrogen is an essential component of proteins and nucleic acids inliving beings. Therefore, it is required for growth.


GLOSSARY

Habitat: A place or a set of environmental conditions in which a particularorganism lives.

Adaptation: The adjustment made by an organism that lives in a specifichabitat by acquiring certain important characteristics that helps it to adjust andlive successfully.

Biotic community: A community of living organisms in an area.

Ecosystem: Living organisms found in a definite geographical region togetherwith the physical environment of that region.

Biome: All the ecosystems taken together in a geographical area.

Food chain: The relationship of eating and being eaten up at different levelsin an ecosystem represented in the form of a chain.

Food web: A network of species relationship formed by interconnected foodchains.

high school science part i

Documents

process of measurement

precise measurement

rough measurement

b use of body parts

senses areseeing

uses of various parts

useddifferent parts

everyday life