chemistry 9th notes punjab text book board

Name --------------------------------------------------------------

Section --------------------------------------------------------------

Roll Number --------------------------------------------------------------

SOLVED

Test Yourself questions,

Exercise questions

and Numerical Problems.

Muhammad Fahid Asif

MSc Chemistry B.Ed.

03003023885

CHEMISTRY 9

Talent Science School, A project of Jajja Academy Grw.

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FUNDAMENTALS OF CHEMISTRY Chapter 1

1. In Which branch of chemistry behavior of gases and liquids is studied?

Ans. Behavior of gases and liquids is studied in physical chemistry.

2. Define biochemistry.

Ans. It is the branch of chemistry in which we study the structure, composition and

chemical reactions found in living organisms.

3. Which branch of chemistry deals with preparation of paints and paper?

Ans. Industrial deals with preparation of paints and paper.

4. In which branch of chemistry are the metabolic processes of carbohydrates and

proteins studied?

Ans. The metabolic processes of carbohydrates and proteins are studied in biochemistry.

5. Which branch of chemistry deals with atomic energy and its uses in daily life?

Ans. Nuclear chemistry deals with atomic energy and its uses in daily life.

6. Which branch of chemistry deals with the structure and properties of naturally

occurring molecules?

Ans. Organic chemistry deals with the structure and properties of naturally occurring molecules.

1. Can you identify mixture, element or compound out of the

followings:

Coca cola, petroleum, sugar, table salt, blood, gun powder, urine,

aluminum, silicon, tin, lime and ice cream.

Ans: Mixtures: Coca cola, petroleum, blood, urine, gun powder, ice-cream.

Elements: Aluminum, Silicon, Tin

Compounds: Sugar, table salt, Lime.

2. How can you justify that air is homogeneous mixture. Identify its components.

Ans. Air has uniform composition throughout so it is a homogeneous mixture. Air is the mixture of

Nitrogen (78%), oxygen (21%), carbon dioxide (0.04%), noble gases and water vapours.

3. Name the elements represented by the following symbols: Hg, Au, Fe, Ni, Co, W, Sn, Na, Ba, Br, Bi. Ans. Hg(Hydragyrum) Au(Aurum) Fe(Ferrum) Ni(Nickel) Co(Cobalt) W(Wolframium)

Sn(Stannum) Na(Natrium) Ba(Barium) Br(Bromine) Bi(Bismuth).

4. Name a solid a liquid and a gaseous element that exists at the room temperature.

Ans. Iron (Ferrum-Fe) exists in solid state, Mercury (Hydragyrum-Hg) exists in liquid state while

Oxygen (O2) exists in gaseous form at room temperature.

5. What elements do the following compounds contain?

Sugar, common salt, lime water and chalk.

Ans. Sugar (C12H22O11):- Carbon, Hydrogen and oxygen. Common salt (NaCl):- Sodium and

Chlorine.

Lime water (Ca(OH)2):- Calcium, Oxygen and Hydrogen. Chalk (CaCO3):- Calcium, carbon

and Oxygen.


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1. How many amu 1 g of a substance has?

Ans. 1.66 X 10-24 g = 1 amu

So, 1g = 1/ 1.66 X 10-24

amu

1g = 6.02 X 1023 amu

2 Is atomic mass unit a SI unit of an atomic mass?

Ans. The CIPM (International Committee for Weights and Measures) have categorized it as a "non-SI unit whose values in SI units must be obtained experimentally.” SI unit of mass is gram.

3. What is the relationship between atomic number and atomic mass?

Ans. The atomic number is the number of protons in the nucleus of an atom. It tells us which

element it is. Atomic mass is calculated by adding up the masses of all the protons,

neutrons, and electrons in the atom. As electrons weigh less than 1/1840th of what a proton

weighs, they contribute only the smallest amount to the weight of the atom. These weights

are expressed in atomic mass units.

4. Define relative atomic mass.

Ans. The relative atomic mass of an element is the average masses of atoms of that element as

compared to 1/12th the mass of an atom of carbon-12 isotope. Its unit is amu.

5. Why atomic mass of an atom is defined as relative atomic mass?

Ans. We know that the mass of an atom is too small to be determined practically. However,

certain instruments enable us to determine the ratio of the atomic masses of various

elements to that of carbon-12 atoms. This ratio is known as the relative atomic mass of the

element.

1. What is the relationship between empirical formula and formula unit?

Ans. Empirical formula is the simplest whole number ratio of atoms present in a compound. Empirical formula of benzene (C6H6) is CH. Formula unit is the simplest whole number ratio of ions, as present in the ionic compound. Formula unit of sodium chloride is NaCl.

2. How can you differentiate between molecular formula and empirical

formula?

Ans. Empirical formula is the simplest whole number ratio of atoms present in a compound. Empirical formula of benzene (C6H6) is CH.

Molecular formula shows actual number of atoms of each element present in a molecule of that compound. Molecular formula of benzene is C6H6.

3. Identify the following formulae as formulas or unit molecular formulae:

H2O2 , CH4 , C6H12O6 , C12H22O11 , BaCO3 , KBr

Ans. Formulas: H2O2, CH4, C6H12O6, C12H22O11

Unit molecular formulae: BaCO3, KBr

4. What is empirical formula of acetic acid (CH3COOH)?

Ans. The empirical formula of acetic acid is CH2O.

5. Find out its molecular mass and Calculate the formula masses of: Na2SO4 ,

ZnSO4 and CuCO3.

Ans.


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Formula mass of Na2SO4 = 2(23) + 1(32) + 4(16)

= 46 + 32 + 64

= 142 amu

Formula mass of ZnSO4 = 1(65) + 1(32) + 4(16)

= 65 + 32 + 64

= 161 amu

Formula mass of CuCO3. = 1(63) + 1(12) + 3(16)

= 63 + 12 + 48

= 123 amu

1. Identify the followings as diatomic, triatomic or polyatomic

molecules; H2SO4, H2, CO2, HCI, CO, C6H6, H2O

Ans. Diatomic molecules: H2, HCI, CO, Triatomic molecules: CO2, H2O Polyatomic molecule: H2.SO4, C6H6

Test yourself1.5

2. Identify among the followings as cation, anion, free radical, molecular ion or

Molecule: Na+ Bro N2+ N2+ CI2 CO3

2- H- O2 O2-

Ans. Cation Na+ Anion H-, O2-, CO3

2- Free radical Br o Molecular ion N2

+ Molecule N2, CI2,

1. Which term is used to represent the mass of 1 mole of molecules of

a substance?

Ans. The term “Gram molecular mass” is used to represent the mass of 1 mole of molecules of a substance.

2. How many atoms are present in one gram atomic mass of a

substance?

Ans. There are 6.02 x 1023 atoms present in one gram atomic mass of a substance.

3. Explain the relationship between mass and mole of a substance

Ans. Mole is amount (mass) of a substance that contains 6.02 x 1023 numbers of particles. It establishes a link between mass of substance and number of particles.

Number of moles = mass of substance / molar mass of substance Mass of substance = number of moles x molar mass.


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4. Find out the mass of 3 moles of oxygen atoms.

Ans. No. of moles = Mass / molar mass Mass = No. of moles X molar mass Mass = 3 X 16 = 48 g

5. How many molecules of water are present in half mole of water?

Ans. Number of molecules = Number of moles X 6.02 x 1023

= 0.5 X 6.02 x 1023

= 3.01 x 1023 molecules

1. How many atoms of sodium are present in 3 moles of sodium and

what is the mass of it?

Ans. No. of atoms = No. of moles X 6.02 x 1023

= 3 X 6.02 x 1023 = 1.806 x 1024

Mass = No. of moles X Molar mass = 3X 23 = 69g

2. How many atoms are in 1 amu and 1 g of hydrogen (H)?

Ans. We know that: 1 amu = 1.66 x 10-24 g No. of atoms = mass X NA / molar mass No. of atoms = 1.66 x 10-24 X 6.02 x 1023 / 1.008 = 0.991 or 1 atom While in 1 g of Hydrogen: No. of atoms = mass X NA / molar mass No. of atoms = 1 X 6.02 x 10

23 / 1

= 6.02 x 1023 atoms

3. How many atoms are present in 16 g of O and 8g of S?

Ans. No. of atoms in 16 g O = 16 X 6.02 x 1023 / 16 = 6.02 x 1023 atoms No. of atoms in 8 g of S = 8 X 6.02 x 1023 / 32 = 1.505 x 1023 atoms

4. Is the mass of 1 mole of O and 1 mole of S same?

Ans. Mass of 1 mole of O = 16 g Mass of 1 Mole of S= 32 g Their molar masses are not equal.

5. What do you mean by 1 atom of C and 1 gram atom of C?

Ans. 1 atom of C means number of atom which has mass 12 amu.1 gram atom of carbon means the amount of Carbon which contains 6.02 x 1023 atoms.

6. If 16 g of oxygen contains 1 mole of oxygen atoms calculate the mass of one atom

of oxygen in grams.

Ans. No. of atoms = mass X NA / molar mass Mass= No. of atoms X molar mass / NA Mass = 1 X 16 / 6.02 x 1023 = 2.65 x 10-23 g


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7. How many times is 1 mole of oxygen atom heavier than 1 mole of hydrogen atom?

Ans. Mass of 1 mole of O = 16g Mass of 1 mole of H= 1g So, 1 mole of O is 16 times heavier than 1 mole of H.

8. Why does 10 g nitrogen gas contain the same number of molecules as 10 g of

carbon monoxide?

Ans. Molar masses of both the gases are equal i.e. 28 g, so they have equal number of molecules.

No. of molecules = mass X NA / molar mass = 10 X 6.02 x 1023 / 28 = 2.15 x 1023 molecules

SHORT ANSWER QUESTIONS

1 Define industrial chemistry and analytical chemistry.

Ans. Industrial Chemistry The branch of chemistry that deals with the manufacturing of chemical compounds on

commercial scale is called industrial chemistry. Analytical Chemistry Analytical chemistry is the branch of chemistry that deals with separation and analysis of a

sample to identify its components.

2 How can you differentiate between organic and inorganic chemistry?

Ans. Organic Chemistry is the study of covalent compounds of carbon and hydrogen (hydrocarbons) and their derivatives while Inorganic chemistry deals with the study of all elements and their compounds except those of compounds of carbon and hydrogen (hydrocarbons) and their derivatives.

3 Give the scope of biochemistry.

Ans. It covers all chemical processes taking place in living organisms, such as synthesis and metabolism of biomolecules like carbohydrates, proteins and fats.

4 How does homogeneous mixture differ from heterogeneous mixture?

Ans. Mixtures that have uniform composition throughout are called homogeneous mixtures e.g. air, gasoline, ice cream. Heterogeneous mixtures are those in which composition is not uniform throughout e.g. soil, rock and wood.

5 What is the relative atomic mass? How is it related to gram?

Ans. The relative atomic mass of an element is the average mass of the atoms of that element as compared to 1/12th (one-twelfth) the mass of an atom of carbon- 12 isotope.

When this atomic mass is expressed in grams, it is: 1 amu = 1.66 x 10-24g

6 Define empirical formula with an example.

Ans. Empirical formula is the simplest whole number ratio of atoms present in a compound. Glucose has simplest ratio 1:2:1 of carbon, hydrogen and oxygen, respectively. Hence, its empirical formula is CH2O


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7 State three reasons why do you think air is a mixture and water a compound?

Ans. 1. Water is formed by chemical combination of H and O whereas air is formed by mixing of atmospheric gases.

2. The components of water cannot be separated by physical means but the components of air can be separated by physical methods.

3. Water has sharp and fixed melting and boiling points while air has not.

8. Explain why are hydrogen and oxygen considered elements whereas water as a

compound.

Ans. Hydrogen and Oxygen are elements because each of these has same type of atoms of same atomic number while water is a compound made up by the chemical combination of Hydrogen and Oxygen.

9. What is the significance of the symbol of an element?

Ans. 1. Symbol represents the name of an element. 2. Symbol represents one atom of the element. 3. Symbol makes the representation of an element easy in a chemical equation.

10. State the reasons: soft drink is a mixture and water is a compound.

Ans. Water is a compound because it is made from Oxygen and Hydrogen chemically bonded together to form a water molecule (H2O). With water one cannot separate the Oxygen from the Hydrogen by physical means. A soft drink is a mixture because it is made from: Water, Sugar, Flavorings and Coloring agents. It is possible to separate these things out of mixture using physical means.

11. Classify the following into element, compound or mixture:

i. He and H2 ii. CO and Co iii. Water and milk iv. Gold and brass v. Iron and steel

Ans. Elements: He, H2 , Co, Gold, Iron Compounds: CO, Water Mixtures: Milk, Brass, Steel

12. Define atomic mass unit. Why is it needed?

Ans. The unit for relative atomic masses is called atomic mass unit, with symbol ‘amu’. One atomic mass unit is 1/12th the mass of one atom of carbon-12th. Relative atomic mass is expressed in atomic mass unit because mass of an atom is too small to be expressed in grams.

13. State the nature and name of the substance formed by combining the following:

i. Zinc + Copper ii. Water + Sugar

iii. Aluminium + Sulphur

iv. Iron + Chromium + Nickel

Ans. i. Brass (Mixture) ii. Sugar solution (Mixture) iii. Aluminum sulphide (Compound) iv. Nichrome (Mixture)


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14. Differentiate between molecular mass and formula mass, which of the followings

have molecular formula? H2O, NaCl, KI, H2SO4

Ans. The sum of atomic masses of all the atoms present in one molecule of a molecular substance is its molecular mass while Formula mass is the sum of atomic masses of all the atoms present in one formula unit of a substance.

H2O and H2SO4 are molecular formulae whereas NaCl and KI are the formula units.

15. Which one has more atoms: 10 g of Al or 10 g of Fe?

Ans. No. of atoms of Al= 10 X 6.02 x 1023 /27 = 2.22 x 1023 atoms No. of atoms of Fe = 10 X 6.02 x 1023 /56 = 1.39 x 1023 atoms Hence 10g of Al contains more atoms.

16. Which one has more molecules: 9 g of water or 9 g of sugar (C12H22O11)?

Ans. No. of molecules of water = 9 X 6.02 x 1023 /18 = 3.01 x 1023 molecules No. of molecules C12H22O11 = 9 X 6.02 x 1023 /342 = 1.5 x 1022 molecules Hence 9g of water contains more molecules.

17. Which one has more formula units: 1 g of NaCl or 1 g of KCl?

Ans. No. of formula units of NaCl = 1 X 6.02 x 1023 /36.5 = 10.2 x 1023 formula units No. of formula units of KCl = 1 X 6.02 x 1023 /74.5 = 8.08 x 1023 formula units. Hence 1g of NaCl has more formula units.

18. Differentiate between homoatomic and heteroatomic molecules with examples.

Ans. A Molecule containing same type of atoms is called homoatomic molecule. For example: hydrogen (H2), ozone (O3), sulphur (S8) and phosphorus (P4) are the examples of molecules formed by the same type of atoms.

When a molecule consists of different kinds of atoms, it is called heteroatomic molecule. For example: CO2, H2O and NH3.

19. In which one of the followings the number of hydrogen atoms is more? 2 moles of

HC1 or 1 mole of NH3

(Hint: 1 mole of a substance contains as much number of moles of atoms as are in

1 molecule of a substance)

Ans. No. of molecules of 2 moles HCl = 2X6.02 x 1023 = 12.04 x 1023 molecules No. of atoms of H in 2 moles of HCl = 1 X 12.04 x 1023 = 12.04 x 1023 atoms No. of molecules of 1 mole NH3 = 1 X 6.02 x 1023 = 6.02 x 1023 molecules No. of atoms of H in 1 mole NH3 = 3 X 6.02 x 1023 = 18.06 x 1023 atoms So, one mole of NH3 has more hydrogen atoms than 2 moles HCl.


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OBJECTIVE KEYS

1. C) On commercial scale 2. A) Mixture 3. A) Oxygen 4. A) Oxygen

5. D) Argon 6. B) 1.66 X 10-24 g 7. A) H2 8. A) 18 amu

9. A) 98 g 10. A) 32 amu 11. B) 0.18

12. C) ½ mole of NaCl and 1/3 mole of MgCl2 13. A) 1 mole of CO and 1 mole of N2

NUMERICAL PROBLEMS


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1. Sulphuric acid is the king of chemicals. If you need 5 moles of sulphuric acid for reaction, how many grams of it will you weigh?

Ans.

Given data:

Compound = H2SO4 Molecular mass = 1 ×2+32+16 ×4 = 98g /mol No. of moles = 5

To find: Mass in grams = ?

Solution :

no. of moles = ��

= �

Mass = 98 x 5 = 490g

2. Calcium carbonate is insoluble in water. If you have 40 g of it; how many Ca2+

and CO32–

ions are present in it?

Ans.

Give Data: Compound = CaCO3 Mass = 40g

To find: No. of ions of Ca+2 and CO3

-2 Solution=?

Solution: � . � = �� × ��

= × . ×

=2.408 × 1023 Molecules

No. of Ca+2 in 2.408×1023=2.408×1023 × 1 =2.41 × 1023 ions No. of CO3

-2 in 2.408×1023=2.408×1023×1 =2.41 × 1023 ions

3. If you have 6.02 x 1023 ions of aluminium;

how many sulphate ions will be required to prepare Al2(SO4)3 ?

Ans.

Given Data :

No. of ions of Al = 6.02 ×1023 Compound = Al2(SO4)3

To find: Number of SO4 ions=?

Solution: Al3+ : SO4

2-

2 : 3 1 : 3/2

6.02 ×1023 : × . ×

= 9.03 × 1023 ions

4. Calculate the number of molecules in the following compounds: a. 16 g of H2CO3

Ans. (a)

Given Data: Mass = 16g Compound = H2CO3 Molar mass = 2 + 12 + 48= 62 g/ mol

To find:

No. of molecules

Solution:

no of molecules = � � � � × ��

= × . ×

= 1.55 ×1023 molecules


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4 (b). 20 g of HNO3

Given Data: Mass = 20 g Compound = HNO3 Molar mass =1 + 14 + 16 × 3 = 15 +48= 63 g/ mol

To find: No. of molecules

Solution:


= × . ×

= 1.91× molecules

4 (c). 30 g of C6H12O6

Given Data: Mass = 30g Compound = C6H12O6

Molar mass = 12 × 6 + 1 × 12 + 16 × 6 = 72 + 12 + 96 = 180 g/mol

To find: No. of molecules

Solution:


= × . ×

= . × �

5. Calculate the number of ions in the following compounds:

(a). 10 g of AlCl3 Given Data: Mass = 10g Compound = AlCl3

Molar mass = 27 + 35.5 × 3= 133.5 g / mol

To find: No. of ions

Solution:


= . × . ×

= . × � No. of ions in . × molecules

= . × × = . × �

5 (b). 30 g of BaCl2

Given Data: Mass = 30g Compound = BaCl2 Molar mass = 137+ 35.5 ×2=208 g / mol


Solution:


= × . ×

= . × � No. of ions in one BaCl2 molecule = 3 No. of ions in . × BaCl2 molecules = 3× . × = . × �

5 (c). 58 g of H2SO4(aq)

Given Data: Mass = 58 g Compound = H2SO4 = 1 × 2 + 32 + 16 × 4 = 98 g / mol


Solution:

no. of molecules = � � � � × ��

6. What will be the mass of 2.05xl016

molecules of H2SO4

Ans.

Given Data

No. of molecules = 2.05 × Compound = H2SO4 Molar Mass = 98 g / mol

To find:

Mass

Solution:


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= × . ×

= . × � . No. of ions in 1 H2SO4 molecule= 3 No. of ions in . × H2SO4 molecules = × . × = . × �

No. of molecules = � � � � × ��

. × = � × . ×

Mass = . × × 6. ×

= . × − = . × −

7. How many atoms are required to prepare 60 g of HNO3?

Given Data: Compound = HNO3

Molar mass = 1 + 14 + 16 ×3 = 63 g / mol Mass = 60 g

To find: No. of atoms

Solution:


= × . ×

= . × � No. of atoms in 1 HNO3 molecule = 5 No. of atoms in . × HNO3 molecules = . × × = . × = . × �

8. How many ions of Na+ and Cl- will be present

in 30 g of NaCl?

Given Data: Compound = NaCl Molar mass = 23 + 35.5 = 58.5 g /mol Mass = 30 g

To find : No of ions of Na+ and Cl-1

Solution:


= . × . ×

= . × formula units No. of ions in 1 NaCl unit = 2 No. of ions in . × NaCl units = 2 ×3.087 × = . × �

9. How many molecules of HCl will be required to have 10 grams of it?

Given Data: Compound = HCl Mass = 10 g Molar mass =1 + 35.5 = 36.5 g/ mol No. of molecules Solution:


= . × . ×

= . × molecules

10. How many grams of Mg will have the same number of atoms as 6 grams of C have?

Ans.

STRUCTURE OF ATOM Chapter 2

Talent Science School, a project of Jajja Academy

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1. Do you know any element having no neutrons in its atoms?

Ans. Hydrogen (Protium) has no neutron in its atom. There is only one proton in its nucleus around which an electron revolves.

2. Who discovered an electron a proton and a neutron?

Ans. Protons were discovered by Ernest Rutherford in 1918 while neutrons were discovered by James Chadwick in 1932. Electrons were discovered by Sir Joseph John Thomson, a British physicist.

3. How does electron differ from a neutron?

Ans. Electrons have a charge of -1. Neutrons have no charge. Neutrons are located in the nucleus, while electrons are located in the electron cloud surrounding the nucleus.

Neutron mass = 1.675 x 10-27 kg Electron mass = 9.1x10-31 kg

4. Explain, how anode rays are formed from the gas present in the discharge tube?

Ans. These rays do not originate from the anode. In fact these rays are produced when the cathode rays or electrons collide with the residual gas molecules present in the discharge tube and ionize them as follows:

1. How was it proved that the whole mass of an atom is located at its

centre?

Ans. Keeping in view the experiment, Rutherford concluded following results: i. Except electrons, all other fundamental particles that lie within the nucleus are known as nucleons.

ii. The complete rebounce of a few particles show that the nucleus is very dense and hard.

The above results prove that the whole mass of an atom is located at its centre.

2. How was it shown that atomic nuclei are positively charged?

Ans. Rutherford performed ‘Gold Foil’ experiment. He bombarded alpha particles (He2+) on a 0.00004 cm thick gold foil. The deflection of a few particles proved that there is a ‘center of positive charges’ in an atom, which is called ‘nucleus’ of an atom.

3. Name the particles which determine the mass of an atom.

Ans. Protons and neutrons (nucleons) determine the mass of an atom. Electrons and other subatomic particles are very light weight as compared to protons and neutrons. So there masses are negligible.

4. What is the classical theory of radiation? How does it differ from quantum theory?

Ans. According to classical theory of radiation, the charged particles should release or emit energy continuously. Max Planck (1858-1947) presented quantum theory. Quantum means fixed energy. It is smallest amount of energy that can b emitted or absorbed as electromagnetic radiation.

5. How can you prove that angular momentum is quantized?

Ans. Angular momentum of 1st orbit

mvr = nh2π



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= 1 X 6.63 X 10 -34 / 2 X 3.14 = 1.0 X 10-34 kg m2 s-1

Angular momentum of 2nd orbit

mvr = nh2π

= 2 X 6.63 X 10 -34 / 2 X 3.14 = 2.0 X 10-34 kg m2 s-1

Similarly the angular momentums of next orbits increase by same sequence as

calculated above. So, angular momentum is quantized.

1. How many the maximum number of electrons can be

accommodated in a p-sub shell?

Ans. 6 electrons can be accommodated in a p-sub shell.

↑ ↓ ↑ ↓ ↑ ↓ px py pz

2. How many sub shells are there in second shell?

Ans. There are two sub shells in second shell i.e., 2s, 2p.

3. Why does an electron first fill 2p orbital and then 3s orbital?

Ans. According to the Aufbau rule, the electrons must fill lower energy level first. 2p orbital has less energy than 3s orbital, so electrons first fill 2p orbital.

4. If both K and L shells of an atom are completely filled; how many total number of

electrons are present in them?

Ans. [Total electrons in K shell = 2] + [Total electrons in L shell = 8] = 10 electrons

5. How many electrons can be accommodated in M shell?

Ans. 18 electrons can be accommodated in M shell. (2n2, M is third shell so, 2(3)2 = 2 X9 = 18)

6. What is the electronic configuration of a hydrogen atom?

Ans. (H = 1) E.C = 1s1

7. What is atomic number of phosphorus? Write down its electronic configuration.

Ans. (P = 15) E.C. = 1s2, 2s2, 2p6, 3s2, 3p3

8. If an element has atomic number 13 and atomic mass 27; how many electrons are

there in each atom of the element?

Ans. Atomic number (Z) is the number of protons in nucleus. In neutral atom, number of protons and electrons is equal. So, an element with atomic number 13 has 13 electrons in each of its neutral atom.

9. How many electrons will be in M shell of an atom having atomic number 15?

Ans. Atomic number = 15; K= 2 L= 8 M = 5 There will be 5 electrons in M shell.



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10. What is maximum capacity of a shell?

Ans. Maximum capacity of a shell means the number of electrons that can accommodate in

that shell. It is determined by 2n2 formula. According to this formula, maximum

capacities of K, L, M, N shells are 2, 8, 18 and 32 respectively.

1. Why do the isotopes of an element have different atomic masses?

Ans. Atomic mass is the mass of neutrons and protons mainly as electrons have negligible mass as compared to protons and neutrons. Isotopes of an element have different number of neutrons so they have different atomic masses.

2. How many neutrons are present in C-12 and C-13?

Ans. There are 6 neutrons in C-12 and 7 neutrons in C-13.

3. Which of the isotopes of hydrogen contains greater number of neutrons?

Ans. Tritium (13H) contains greater number of neutrons i.e. 2.

4. Give one example each of the use of radioactive isotope in medicine and

radiotherapy.

Ans. 1. Radiotherapy: For the treatment of skin cancer, isotopes like P-32 and Sr-90 are used.

2. Tracer for Diagnosis and Medicine: The radioactive isotopes are used as tracers in medicine to diagnose the presence of tumor in the human body. Isotopes of Iodine-131

are used for diagnosis of goiter in thyroid gland.

5. How is the goiter in thyroid gland detected?

Ans. Thyroid gland is present in neck region and needs proper amount of iodine to work properly. When iodine is not available, it swells and the condition is known as goiter. Isotopes of Iodine-131 are used for diagnosis of goiter in thyroid gland.

6. Define nuclear fission reaction.

Ans. Nuclear fission is a reaction in which a large nucleus breaks apart into two smaller nuclei, releasing a great deal of energy. For example

7. When U-235 breaks up, it produces a large amount of energy. How is this energy

used?

Ans. This energy is used to convert water into steam in boilers. The steam then drives the turbines to generate electricity. This is the peaceful use of atomic energy for development of a nation.

8. How many neutrons are produced in the fission reaction of U-235?

Ans. 3 neutrons are produced in the fission reaction of U-235.

9. U-235 fission produces two atoms of which elements?

Ans. It produces Barium and Krypton.



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SHORT QUESTIONS.

1. What is the nature of charge on cathode rays?

Ans. They are deflected towards positive plate in an electric field showing that they are negatively charged.

2. Give five characteristics of cathode rays.

Ans. 1 These rays travel in straight lines perpendicular to the cathode surface. 2 They can cast a sharp shadow of an opaque object if placed in their path.

3 They are deflected towards positive plate in an electric field showing that they are negatively charged.

4 They raise temperature of the body on which they fall. 5 Light is produced when these rays hit the walls of the discharge tube

3. The atomic symbol of a phosphorus ion is given as 3115P

3-

• How many protons, electrons and neutrons are there in the ion? • What is name of the ion? • Draw the electronic configuration of the ion.

• Name the noble gas which has the same electronic configuration as the phosphorus ion has.

Ans. 1. Protons= 15, Electrons = 18 , Neutrons= 16 2. 31

15P3- is known as phosphide ion.

3. E.C.= 1s2, 2s2, 2p6, 3s2, 3p6

4. Argon (Ar=18) has the same electronic configuration as the phosphide ion ha.

4. Differentiate between shell and subshell with examples of each.

Ans. SHELL:- An electron shell, also known as a main energy level, is a group of atomic orbitals with the same value of the principal quantum number n. Shells are identified by the letters K, L, M, N etc.

SUBSHELL:- Electron shells are made up of one or more electron subshells, or sublevels, which have two or more orbitals with the same quantum number l. Electron subshells are identified by the letters s, p, d, f etc.

5. An element has an atomic number 17. How many electrons are present in K, L and

M shells of the atom?

Ans. K shell= 2 electrons, L shell= 8 electrons, M shell = 7 electrons.

6. Write down the electronic configuration of Al3+. How many electrons are present in

its outermost shell?

Ans. Number of electrons in Al= 13 While in Al3+ = 10 E.C. of Al3+ = 1s2, 2s2, 2p6 (K=2, L=10) 8 electrons are present in outermost shell.



Page | 16

7. Magnesium has electronic configuration 2, 8, 2, how many electrons are in the

outermost shell? In which sub shell of the outermost shell electrons are present?

Why magnesium tends to lose electrons?

Ans. There are 2 electrons in outermost shell. Outermost electrons are present in 3s (1s2, 2s2, 2p6, 3s2). It tends to lose its valence electrons to complete octet i.e. to get nearest noble gas electronic configuration.

8. What will be the nature of charge on an atom when it loses an electron or when it

gains an electron?

Ans. When an atom loses an electron, it gets positive charge. M ---- M1+ + e- When an atom gains an electron, it gets negative charge. N + e- -- N1-

9. For what purpose U-235 is used?

Ans. When U-235 is bombarded with slow moving neutrons, the uranium nucleus breaks up to produce Barium-139 and Krypton-94 and three neutrons. A large amount of energy is released which is used to convert water into steam in boilers. The steam then drives the turbines to generate electricity. This is the peaceful use of atomic energy for development of a nation.

10. A patient has goiter. How will it be detected?

Ans. Isotopes of Iodine-131 are used for diagnosis of goiter in thyroid gland.

11. Give three properties of positive rays.

Ans. 1. These rays travel in straight lines in a direction opposite to the cathode rays 2. Their deflection in electric and magnetic field proved that these are positively charged.

3. The nature of canal rays depends upon the nature of gas, present in the discharge tube

12. What are the defects of Rutherford’s atomic model?

Ans. Although Rutherford’s experiment proved that the ‘plum-pudding’ model of an atom was not correct, yet it had following defects:

1. According to classical theory of radiation, electrons being the charged particles should release or emit energy continuously and they should ultimately fall into the nucleus.

2. If the electrons emit energy continuously, they should form a continuous spectrum but in fact, line spectrum was observed.

13. As long as electron remains in an orbit, it does not emit or absorb energy. When

does it emit or absorb energy?

Ans. According to the Bohr’s atomic theory, as long as electron remains in a particular orbit, it does not radiate or absorb energy. The energy is emitted or absorbed only when an electron jumps from one orbit to another.

OBJECTIVE KEYS



Page | 17

1. b) canal rays 2. c) neutrons 3. c) Bohr 4. d) M shell 5. b) iodine-131 6. b) neutrons 7. c) 3 subshells 8. b) heavy water 9. d) None 10. a) Goldstein

ELECTRONIC CONFIGURATION METHOD s for school, p s for public school, d p s for divisional public school, f d p s for federal

divisional public school. Write abbreviation of each of above schools twice. Do not rearrange it by your own. Start numbering from s first, then p and so on. s will start with number 1, p from 2, d from

3 and f from 4. Subshells are ready to fill electrons. s, p, d, f can accommodate 2, 6, 10 and 14 electrons respectively.

s , s , p , s , p , s , d , p , s , d , p , s , f , d , p , s , f , d , p , s Numbering; 1s , 2s , p ,3 s , p ,4 s , d , p ,5 s , d , p ,6 s , f , d , p ,7 s , f , d , p ,8 s 1s , 2s , 2p , 3s , 3p , 4s , d , 4p , 5s , d , 5p , 6s , f , d , 6p ,7 s , f , d , 7p , 8s 1s , 2s , 2p , 3s , 3p , 4s , 3d , 4p , 5s , 4d , 5p , 6s , f , 5d , 6p , 7s , f , 6d , 7p , 8s 1s , 2s , 2p , 3s , 3p , 4s , 3d , 4p , 5s , 4d , 5p , 6s , 4f , 5d , 6p , 7s , 5f , 6d , 7p , 8s Electronic configuration of some elements.

Element Symbol Atomic Number

Electronic Configuration

Aluminium Al 13 1s2 , 2s2 , 2p6, 3s2 , 3p1

Silicon Si 14 1s2 , 2s2 , 2p6, 3s2 , 3p2

Phosphorus P 15 1s2 , 2s2 , 2p6, 3s2 , 3p3

Sulphur S 16 1s2 , 2s2 , 2p6, 3s2 , 3p4

Chlorine Cl 17 1s2 , 2s2 , 2p6, 3s2 , 3p5

Argon Ar 18 1s2 , 2s2 , 2p6, 3s2 , 3p6

PERIODIC TABLE AND PERIODICITY OF PROPERTIES Chapter 3

Talent Science School, A project of Jajja Academy

Page | 18

1: What was the contribution of Dobereiner towards classification of

elements?

Ans: A German chemist Dobereiner observed relationship between atomic

masse of several groups of three elements called triads. In these groups, the

central or middle element had atomic mass average of the other two elements.

One triad group example is that of calcium (40), strontium (88) and barium

(137).The atomic mass of strontium is the average of the atomic masses of

calcium and barium.

2: How Newlands arranged the elements?

Ans: In 1864 British chemist Newlands put forward his observations in the form of ‘law of octaves’. He noted that there was a repetition in chemical properties of every eighth

element if they were arranged by their increasing atomic masses.

3: Who introduced the name Periodic table?

Ans: Mendeleev (1834-1907) was a Russian chemist and inventor. He was the creator of first

version of periodic table of elements. With help of the table, he predicted the properties of

elements yet to be discovered.

4: Why the improvements in Mendeleev’s periodic table were made?

Ans: Although Mendeleev periodic table was the first ever attempt to arrange the elements, yet

it has a few demerits in it. His failure to explain the position of isotopes and wrong order of

the atomic masses of some elements suggested that atomic mass of an element cannot

serve as the basis for the arrangement of elements. So improvements in Mendeleev’s periodic table were made.

5: State Mendeleev’s Periodic law. Ans: Properties of elements are periodic function of their atomic masses.

6: Why and how elements are arranged in a period?

Ans: Elements that have same number of electronic shells are arranged in a period by

increasing order of their atomic numbers.

1: How the properties of elements repeat after regular intervals?

Ans: Elements with similar properties occur at regular interval when the

elements are arranged according to their atomic number. For example, the

elements with similar chemical properties are present in same group.

2: In which pattern modern periodic table was arranged?

Ans: Modern periodic table was arranged according to increasing order of atomic numbers.

3: How many elements are in first period and what are their names and symbols?

Ans: There are 2 elements in first period. These are:

Hydrogen (H), Helium (He)

4: How many elements are placed in 4th period?

Ans: There are 18 elements in 4th period. It start from K-19 (Kalium) and ends at Kr-36

(Krypton).



Page | 19

5: From which element lanthanide series starts?

Ans: Lanthanide series start from La-57 (Lanthanum). Lanthanide series is present in 6th

period.

6: From which element actanides series starts?

Ans: Lanthanide series start from Ac-89 (Actinum). Actanides series is present in 7th period.

7: How many elements are in 3rd period, write their names and symbols.

Answ: There are 8 elements in 3rd period. These are:

Lithium(Li), Beryllium(Be), Boron(B), Carbon(C), Nitrogen(N), Oxygen(O), Fluorine(F),

Neon(Ne)

8: How many periods are considered normal periods?

Ans: 2nd and 3rd periods are considered normal periods. Each of them consists of 8 elements.

9: What do you mean by a group in periodic table?

Ans: There are 18 vertical columns in the periodic table numbered 1 to 18 from left to right,

which are called groups.

10: What is the reason of arranging elements in a group?

Ans: The elements of a group show similar chemical properties. That is why elements of same

chemical properties are placed in same group.

11: What do you mean by periodic function?

Ans: Periodic function in chemistry means the property of elements in periodic table changes

with position in table of element and they follow a pattern in periodic table. For example

elements in the same group have similar chemical properties.

12: Why the elements are called s or p block elements?

Ans: Elements are classified into four blocks depending upon the type of the sub shell which

gets the last electron. In s and p block elements, s and p sub shells get the last electron(s)

respectively.

13: Write down the names of elements of group 1 with their symbols.

Ans: H (Hydrogen), Li (Lithium), Na (Natrium), K (Kalium), Rb (Rubidium), Cs (Cesium), Fr

(Francium).

14: How many members are in group 17 is there any liquid, what is its name?

Ans: There are 7 elements in group 17 out of which 6 are naturally occurring while 7th is

artificially prepared. These are:

Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The artificially created

element 117 (ununseptium) may also be a halogen.

Bromine (Br) is liquid at room temperature.



Page | 20

1: Define atomic radius?

Ans: Half of distance between the nuclei of two covalently bonded atoms is

referred as atomic radius. For example, the distance between the nuclei of two

carbon atoms is 154pm. So, the atomic radius of carbon is 77pm.

2: What are SI units of atomic radius?

Ans: The most commonly used SI unit for atomic measurements is the pico meter (pm).

3: Why size of atoms decreases in a period?

Ans: In any one period, the outer electrons lie in the same valence shell. As atomic number

increases, effective nuclear charge increases, so there is an increased attraction of

electrons to the nucleus. So the atomic radius decreases.

4: Define ionization energy.

Ans: The ionization energy is the amount of energy required to remove the most loosely bound

electron from the valence shell of an isolated gaseous atom. For example, the ionization

energy of sodium atom is + 496 kJmol-1.

5: Why the 2nd ionization energy of an element is higher than first one?

Ans: When an atom loses one electron to become a cation, its nuclear charge increases and it

shrinks as compared to its atomic size. It becomes difficult to remove an extra electron.

So, more ionization energy is required to remove 2nd electron as compared to 1st

ionization energy for the removal of first electron.

6: What is the trend of ionization energy in a group?

Ans: In a group, ionization energy decreases from top to bottom, it is because:

1. Atomic size increases

2. Shielding effect increases

7: Why the ionization energy of sodium atom is less than magnesium atom?

Ans: Ionization energy increases from left to right in a period because atomic size decreases

gradually. Sodium atom is bigger than magnesium atom, Hence ionization energy of

sodium is less than magnesium.

8: Why is it difficult to remove electron from halogens?

Ans: Halogens have highest ionization energies after noble gases. Secondly these are highly

electro negative. That is why it is difficult to remove electron from halogens.

9: What is shielding effect?

Ans: The electrons present between the nucleus and the outermost shell of an atom, reduce

the nuclear charge felt by the electrons present in the outermost shell. It means that the

electrons present in the inner shells screen or shield the force of attraction of nucleus felt

by the valence shell electrons. This is called shielding effect.

10: How does shielding effect decrease the force of attractions between nucleus and

outer most electrons?

Ans: The electrons present between the nucleus and the outermost shell of an atom, reduce

the nuclear charge felt by the electrons present in the outermost shell. It means that the



Page | 21

electrons present in the inner shells screen or shield the force of attraction of nucleus felt

by the valence shell electrons.

11: Why do the bigger size atoms have more shielding effect?

Ans: In bigger sized atoms, more electrons lie between the nucleus and valance electrons. The

electrons present between the nucleus and the outermost shell of an atom, reduce the

nuclear charge felt by the electrons present in the outermost shell. So bigger size atoms

have more shielding effects.

12: Why does the trend of electron affinity and electro negativity is same in a period?

Ans: Both the factors are somewhat same in the sense that these are abilities to attract the

electrons, either from electropositive atom or shared pair respectively. So, both have

same trends in periods and groups.

13: Which element has the highest electro negativity?

Ans: Fluorine has highest electro negativity i.e. 4 according to Pauling scale.

EXERCISE

1. Why are noble gases not reactive?

Ans: Noble gases have their valence shells completely filled. Their atoms do not have vacant

space in their valence shell to accommodate extra electrons. Therefore, noble gases do

not react with other elements.

2. Why Cesium (at.no.55) requires little energy to release its one electron present in the

outermost shell?

Ans: Cesium has largest atomic radius i.e. 265 pm in its group. Secondly 6 shells lie between

the outermost electron and nucleus, which means shielding effect plays major role in

decreasing Zeff. Thus it becomes easy to remove outer most electrons with little ionization

energy.

3. How is periodicity of properties dependent upon number of protons inan atom?

Ans: The arrangement of elements according to increasing atomic number (Number of protons)

shows the periodicity in the electronic configuration of elements that leads to periodicity in

their properties.

4. Why shielding effect of electrons makes cation formation easy?

Ans: The attractions of outer electrons towards nucleus are partially reduced because of presence of inner electrons. As a result valence electron experiences less nuclear charge than that of actual charge. So it becomes easy to make cation by removing valence electron.

5. What is difference between Mendeleev’s periodic law and Modern periodic law?

Ans: Mendeleev’s Periodic law: Properties of the elements are periodic function of their atomic masses.

Modern Periodic law: Properties of the elements are periodic function of their atomic numbers.



Page | 22

6. What do you mean by groups and periods in the periodic table?

Ans: Groups: The vertical columns in the periodic table are called groups. There are 18 groups in Modern periodic table. Periods: The horizontal rows in periodic table are called periods. There are 7 periods in Modern periodic table.

7. Why and how are elements arranged in 4th period?

Ans: Elements which have 4 atomic shells in their atoms are arranged in 4th period according to increasing atomic number. There are 18 such elements.

8. Why the size of an atom does not decrease regularly in a period?

Ans: As we move from left to right in a period, number of shells remain same so atomic radii decrease but in irregular pattern because of addition of new sub shells in each block of periodic table.

9. Give the trends of ionization energy in a period.

Ans: In Periods: Ionization energy increases in a period from left to right because atomic size decreases and cation formation become difficult. In Groups: Ionization energy decreases in a group from top to bottom because atomic size increases and cation formation become easy.

PERIODICITY OF PROPERTIES (TREND)

In Periods In Groups

Atomic Radius Decreases Increases

Shielding effect Decreases Increases

Ionization energy Increases Decreases

Electron affinity Increases Decreases

Electro negativity Increases Decreases

STRUCTURE OF MOLECULES Chapter 4


Page | 23

Q1. Why does sodium form a chemical bond with chlorine?

Ans: Sodium has only one electron and chlorine has seven electrons in their valence shells. Sodium being electropositive element has the tendency to lose electron and chlorine being an electronegative element has the

tendency to gain electron. Therefore, they form positive and negative ions by losing and gaining electrons, respectively. They attain electronic configuration to the nearest noble gases by forming chemical bond.

Test yourself 4.1

Q2. Why does sodium lose an electron and attains +1 charge?

Ans: Sodium has only one electron in its valence shell. Sodium being electropositive element

has the tendency to lose electron. Therefore, sodium lose an electron and attains +1

charge.

Q3. How do atoms follow octet rule?

Ans: An atom can accommodate 8 electrons (follow octet rule) in its valence shell in three

ways:

i. By giving valence shell electrons (if they are less than three) to other atoms.

ii. By gaining electrons from other atoms (if the valence shell has five or more electrons

in it).

iii. By sharing valence electrons with other atoms.

Q4. Which electrons are involved in chemical bonding?

Ans: Only valence shell electrons are involved in chemical bonding.

Q5. Why do group 1 elements prefer to combine with group 17 elements?

Ans: The elements of Group-1being metals have the tendency to lose their valence electrons

forming positively charged ions whereas, non-metals of Group-17 have the tendency to

gain or accept electrons. They are electronegative elements with high electron affinities.

If atoms belonging to these two different groups, metals and non-metals, are allowed to

react, chemical bond is formed.

Q6. Why chlorine can accept only 1 electron?

Ans: Chlorine has seven electrons in its valence shell. In order to complete octet, chlorine can

accept only one electron. Therefore, chlorine accepts an electron and attains -1 charge.

Q1. Give the electronic configuration of carbon atom.

Ans: C6 = 1s2 2s2 2p2

Q2. What type of elements have tendency of sharing of electrons?

Ans: The elements of Group-13 to Group-17 when allowed to react with each other, they form a chemical bond by mutual sharing of their valence shell electrons. These elements have comparable values of electro Test

Yourself 4.2 negativities.

Q3. If repulsive forces dominate to attractive forces will a covalent bond form?

Ans: If repulsive forces dominate to attractive forces, a covalent bond will not be formed.



Page | 24

Q4. Considering the electronic configuration of nitrogen atom, how many electrons are

involved in bond formation and what type of covalent bond is formed?

Ans: N7 = 1s2 2s2 2p3

Three electrons are involved in bonding and triple covalent bond is formed.

Q5. Point out the type of covalent bonds in the following molecules

CH4, C2 H4, H2, N2, and O2 Ans: CH4= four single bonds around carbon atom with four hydrogen atoms separately.

C2H4 = a double bond between two carbon atoms.

H2 = single bond between two hydrogen bonds.

N2 = Triple bond between two nitrogen atoms.

Q6. What is a lone pair? How many lone pairs of electrons are present on nitrogen in

ammonia?

Ans: The non-bonded electron pair available on an atom is called a lone pair. There is one

lone pair present on nitrogen in ammonia.

Q7. Why is the BF3 electron deficient?

Ans: In the formation of BF3 (boron tri fluoride) molecule three valence electrons of boron atom

(Z= 5) pair up with three electrons, one from each three fluorine atoms. The boron atom

even after this sharing of electrons remains short or deficit of two electrons in its

outermost shell.

Q8. What types of electron pairs make a molecule good donor?

Ans: Presence of lone pair makes a molecule good conductor. For example, NH3 is good

donor due to presence of lone pair on nitrogen.

Q9. What is difference between bonded and lone pair of electron and how many bonded

pairs of electrons are present in NH3 molecule?

Ans: The electrons that pair up to form a chemical bond are called ‘bond pair’ electrons.

The non-bonded electron pair available on an atom is called a lone pair.

There are three bond pairs and one lone pairs present in NH3.

Q10. What do you mean by delta sign and why it develops?

Ans: Delta sign is the sign of polarity. It develops due to unequal sharing of electrons between

to two atoms which have electro negativity differences.

Q11. Why does oxygen molecule not form a polar covalent bond?

Ans: Oxygen molecule is formed by the sharing of electrons between two oxygen atoms

having same electro negativity values. Polar covalent bond is formed between two

different atoms. That is why oxygen molecule is non polar.

Q12. Why have water polar covalent bonds?

Ans: Water is formed by bonding between oxygen atom and two hydrogen atoms. Oxygen

being more electro negative than hydrogen draws shared pair of electrons more towards



Page | 25

itself thus developing partial negative charge. Similarly hydrogen attains partial positive

charge. So water molecule has polar covalent bonds.

Q1. What type of elements form metallic bonds?

Ans: Electropositive elements (Metals) form metallic bonds.

Q2. Why is the hold of nucleus over the outermost electrons in metals

weak?

Ans: Metals have large sized atoms and thus more shielding effect. So is the

hold of nucleus over the outermost electrons in metals is weak.

Test Yourself 4.3

Q3. Why the electrons move freely in metals?

Ans: The hold of nucleus over the outermost electrons in metals is weak. Outermost electrons

are released easily to attain complete octet by metallic atoms. So electrons move freely

in metals.

Q4. Which types of electrons are responsible for holdings the atoms together in metals?

Ans: Free electrons removed from outer most shells by metal atoms are responsible for

holding atoms together in metals.

Q5. Why a dipole develops in a molecule?

Ans: In polar molecules, the opposite poles have partial charges, negative and positive, due to

unequal sharing of electrons. So dipole develops in such molecules. For example Water

molecule is polar one so it develops strong dipole.

Q6. What do you mean by induced dipole?

Ans: A dipole-induced dipole attraction is a weak attraction that results when a polar

molecule induces a dipole in an atom or in a non polar molecule by disturbing the

arrangement of electrons in the non polar species.

Q7. Why are dipole forces of attraction not found in halogen molecules?

Ans: Halogen molecules e.g., F2, Cl2, Br2 etc are non polar. So dipoles are not found in

halogen atoms.

Q8. What types of attractive forces exist between HCl molecules?

Ans: Dipole-dipole attractive forces exist between HCl molecules.

Q9. Define intermolecular forces; show these forces among HCl molecule.

Ans: the forces that hold atoms in a compound are chemical bonds. In addition to these strong

bonding forces, relatively weak forces also exist in between the molecules, which are



Page | 26

called intermolecular forces. The bonding and intermolecular forces of hydrochloric acid

are shown below:

Q1. Why the ionic compounds have high melting and boiling points?

Ans: Ionic compounds have high melting and boiling points. For example, sodium chloride has melting point 800 °C and a boiling point 1413 °C. As ionic compounds are made up of positive and negative ions, there exist strong electrostatic forces of attraction between oppositely charged ions. So, a great amount of energy is required to break these forces.

Test Yourself 4.4

Q2. What do you mean by malleability?

Ans: Malleability is the property by virtue of which a metal can be rolled into sheets. Gold is

most malleable metal.

Q3. Why are ionic compounds easily soluble in water?

Ans: They dissolve easily in polar solvents like water. Water has high dielectric constant that

weakens the attraction between ions and dissolution occurs.

Q4. What type of bond exists in sodium chloride?

Ans: Ionic bond exists in sodium chloride. Both of these atoms are now changed into

oppositely charged ions. They stabilize themselves by combining with each other due to

electrostatic force of attraction between them such as:

Na+ + Cl- NaCl

Q5. Why the covalent compounds of bigger size molecules have high melting points?

Ans: Large molecules with three dimensional bonding form covalent crystals which are very

stable and hard. They have very high melting and boiling points.

Q6. (a):What is the electro negativity difference between the following pair of elements

(atoms). Predict the nature of the bond between them?

(a) H and CI (b)H and Na (c) Na and I (d) K and CI

(b): Comparing the electro negativity differences, arrange these compounds in

increasing ionic strength.

Ans: (a) H and Cl

H= 2.20 Cl= 3.16 Difference= 0.96 Nature of bond= Covalent



Page | 27

(b) H and Na

H= 2.20 Na= 0.93 Difference= 1.27 Nature of bond= Covalent with varying

degrees of ionic character

(c) Na and I

I= 2.66 Na= 0.93 Difference= 1.73 Nature of bond= Ionic bond

(d) K and Cl

K= 0.82 Cl= 3.16 Difference= 2.32 Nature of bond= Ionic

(b) H and Cl < H and Na < Na and I < K and Cl

EXERCISE

1. Why do atoms react?

Ans: Atoms react with each others to attain stability. They do so by attaining 8 (Octet) or 2

(Duplet) electrons in their valence shells.

2. Why is the bond between an electropositive and an electronegative atom ionic in

nature?

Ans: Electropositive elements tend to lose electron(s) from valence shell while electronegative

elements tend to accept electron(s). So, the complete transfer of electrons results in

formation of ionic bond.

3. Ionic compounds are solids. Justify.

Ans: Ionic compounds are made up of oppositely charged ions which are held by strong

electrostatic forces of attraction. So, ionic compounds are solids.

4. More electronegative elements can form bonds between themselves. Justify

Ans: More electronegative elements have large affinity for electrons. These elements can

share their valence electrons with each other to form covalent bond. For example O=O

(O2)

5. Metals are good conductors of electricity. Why?

Ans: In metals, there is a cloud of freely moving electrons present in spaces between cations.

These mobile electrons make metals good conductors of heat and electricity.

6. Ionic compounds conduct electricity in solution or molten form. Why?

Ans: In solid form, oppositely charged ions in ioic compounds are held strongly and cannot

change their positions. Hence, electricity cannot pass through ionic solids. However in

solution form or in molten form, ionic compounds ionize and produce mobile ions which

make them excellent conductor of electricity.

7. What type of covalent bond is formed in nitrogen molecule?

Ans: Triple covalent bond is formed between two atoms of nitrogen in nitrogen molecule.

N≡ N



Page | 28

8. Differentiate between bond pair and lone pair.

Ans: The non-bonded electron available on an atom, like one available on nitrogen in

ammonia (NH3) is called lone pair. The electron pair formed by the sharing of electrons

by bonded atoms is called bond pair.

H-N-H

H

9. Describe at least two necessary conditions for the formation of a covalent bond.

Ans: 1. The bonded atoms should have comparable electro negativities i.e. less than 1.7

difference.

2. Bothe the atoms should be electronegative i.e. non metals.

10. Why HCl has dipole-dipole forces of attraction?

Ans: HCl is polar molecule. H+ of one HCl molecule attracts the Cl- of next HCl molecule

and vice versa. This phenomenon extends to all available HCl molecules in a

container and dipole-dipole attraction develops in HCl.

11. What is a triple covalent bond, explain with an example?

Ans: When bonded atoms share three electrons each and form three bond pairs, this bond is

called triple covalent bond

N≡ N

12. What is difference between polar and non polar covalent bond? Give examples.

Ans: The covalent bond formed between two similar atoms is non-polar covalent bond e.g. H-

H (H2), Cl-Cl (Cl2).

The covalent bond formed between two different atoms is polar bond e.g. H+-Cl- (HCl)

13. Why a covalent bond becomes polar?

Ans: When bond is formed between two different atoms, more electronegative atom attracts

shared pair of electrons more towards itself. This unequal sharing of electron makes the

bond polar.

H+ :Cl-

14. What is relationship between electronegativity and polarity?

Ans: Polarity of covalent bond depends upon electro negativity of bounded atoms. If

electronegative values of bonded atoms are same, the bond will be non-polar and if there

is difference between their electro negativities, the bond will be polar.

15. Why does ice float on water?

Ans: Ice is less dense (0.917 gcm-3) at 0oC than water (1 gcm-3) at 0oC due to hydrogen

bonding in ice. That is why ice floats on water.

16. Give characteristic properties of ionic compounds.

Ans:



Page | 29

Ionic compounds are made up of oppositely charged ions.

They have high melting and boiling points.

They are crystalline solids.

Ionic compounds are non conductor in solid state but good conductors of

electricity in solution or in molten form

17. What characteristic properties do the covalent compounds have?

Ans:

Covalent compounds have low melting and boiling points.

They are usually bad conductors of electricity. But polar compounds dissolved in

polar solvents conduct electricity.

Non-polar compounds are insoluble in water but polar compounds are soluble.

Bigger molecules with 3-D bonding form covalent crystals which are very stable.

PHYSICAL STATES OF MATTER Chapter 5


Page | 30

1. Why the rate of diffusion of gases is rapid than that of liquids?

Ans: Rate of diffusion of gases is rapid than that of liquids because gas

molecules have higher kinetic energy than liquid molecules. Gas molecules

move fast and randomly. Hence their rate of diffusion is higher.

2. Why are the gases compressible?

Ans: There are large empty spaces between gas molecules so gases are highly

compressible.

3. What do you mean by Pascal? How many Pascals are equal to 1 atm?

Ans: SI unit of pressure is Nm-2. It is also called Pascal (Pa).

101325 pa= 1atm

4. Why the density of a gas increases on cooling?

Ans: On cooling, gas molecules come close to each other due to decrease in kinetic energy

and increase in attractive forces. So density decreases on cooling.

5. Why is the density of gas measured in g dm-3 while that of a liquid in g cm-3?

Ans: Liquids and solids are 1000 times denser than gases. So, density of gas is measured in

g dm-3.

6. Convert the following

• 70 cm Hg to atm

• 3.5 atm to torr

• 1.5 atm to Pa

Ans :

(a) 1 cm Hg = 10 mm Hg so 70 cm Hg = 700 mm Hg

760 m Hg = 1 atm

1mm Hg =

700 mm Hg = × 7 = .9 atm

(b) 1 atm = 760 tor

3.5 atm = 760 ×3.5 = 2660 torr

(c) 1 atm = 1013215 Pa

1.5 atm = 10325 × 1.5 = 151987.5 Pa

1. Is the Boyle’s law applicable to liquids?



Page | 31

Ans: Boyle’s law is only applicable to gases.

2. Is the Boyle’s law valid at very high temperature?

Ans: No, the Boyle’s law is not valid at very high temperature because at high temperature,

the kinetic energy of gas molecules increases, that increases the mobility of molecules;

so they exert more pressure that may vary the volume of gases.

3. What will happen if the pressure on a sample of gas is raised three times and its

temperature is kept constant?

Ans: If the pressure on gas is raised three times, its volume will be decreased three times.

Let Pressure = 3P and Volume= V

According to Boyle’s law: V ∝ P

So V ∝ 3P

OR P ∝ 3V

1. Which variables are kept constant in Charles’s law?

Ans: Pressure is kept constant in Charles law.

2. Why volume of a gas decreases with increase of pressure?

Ans: According to Boyle’s law, pressure and volume of a gas are inversely proportional. When pressure increases, volume decreases and vice

versa.

P ∝ V

3. What is absolute zero?

Ans: It is the temperature at which an ideal gas would have zero volume i.e. gas ceases to

exist. It is represented by K and is equal to -273.15 0C.

4. Does Kelvin scale show a negative temperature?

Ans: No, the minimum positive temperature that Kelvin shows is OK which is also

hypothetical so, it cannot show negative value.

5. When a gas is allowed to expand, what will be its effect on its temperature?

Ans: Expansion means increase in volume. Increase in volume causes increase in

temperature of gas but the temperature of surrounding will be decreased.

6. Can you cool a gas by increasing its volume?

Ans: Temperature and volume are directly proportional to each other. By increasing volume,

one cannot cool the gas.



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1. Why does evaporation increase with the increase of temperature?

Ans: Increase in temperature provides more heat to liquid molecules hence

molecules move more randomly due to increase in kinetic energy.

Resultantly the rate of evaporation increases.

2. What do you mean by condensation?

Ans: Condensation is opposite to evaporation. It is the conversion of vapours

into liquid.

3. Why is vapour pressure higher at high temperature?

Ans: High temperature provides more kinetic energy to vapours. Vapours in this case strike

more powerfully with walls of container, imparting more pressure.

4. Why is the boiling point of water higher than that of alcohol?

Ans: Water (100oC) has strong hydrogen bonding between its molecules. Secondly it is more

polar than alcohol (78oC). So, it has high boiling point.

5. What do you mean by dynamic equilibrium?

Ans: The state at which evaporation and condensation are equal, but in opposite direction is

called dynamic equilibrium.

Liquid ↔ Vapuors

6. Why are the rates of diffusion in liquids slower than that of gases?

Ans: Gas molecules move more randomly than liquid molecules. Secondly have high kinetic

energy, so diffuse faster than liquids.

7. Why does rate of diffusion increase with increase of temperature?

Ans: Temperature provides kinetic energy to molecules. Increase in temperature causes the

molecule to move more randomly. So, rate of diffusion increases.

8. Why are the liquids mobile?

Ans: Firstly liquid has no definite volume. Secondly, the intermolecular forces are not enough

to bind the molecules in fixed position. So, molecules slip on each other making the

liquid mobile.

1. Which form of sulphur exists at room temperature?

Ans. At room temperature, rhombic sulphur (s8) is found.

2. Why is white tin available at room temperature?

Ans. White tin is a stable allotrope of tin. So, found at room temperature.

3. Why the melting point of a solid is considered its ‘identification’ characteristic?

Ans. Melting point suggests stiffness of solid, hardness, stability and many of physical

properties. So, it is also identification character of solids.



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4. Why amorphous solids do not have sharp melting points while crystalline solids do

have?

Ans. Amorphous solids do not have specific molecular arrangement. On heating molecules

leave their positions easily. Crystalline solids have specific molecular arrangements in

the form of crystals. So, it is difficult to melt them.

5. Which is lighter one: aluminium or gold?

Ans. Gold (9.3g cm-3) has high density than aluminium (2.70g cm-3) so aluminium is lighter.

6. Write the molecular formula of a sulphur molecule?

Ans. Molecular sulphur is found in octa-atomic form i.e. S8.

7. Which allotropic form of carbon is stable at room temperature (25 °C)?

Ans. Carbon has many stable allotropes at room temperature. i.e. diamond, graphite bucky

balls, carbon black.

8. State whether allotropy is shown by elements or compounds or both?

Ans. Occurrence of an element in more than one form in same physical sate is called

allotropy. So, allotropy is the characteristic of elements only.

EXERCISE

1. What is diffusion, explain with an example?

Ans. Movement of molecules from the area of higher concentration to an area of lower

concentration is called diffusion. For example dissolution of ink drops into a cup of water

without shaking.

2. Define standard atmospheric pressure. What are its units? How it is related to

Pascal?

Ans. It is defined as the pressure exerted by a mercury column of 760m height at sea level.

1 atm= 760 mmHg = 760 torr = 101325 Nm-2 = 101325 Pa

3. Why are the densities of gases lower than that of liquids?

Ans. Negligible forces of attraction between gas molecules are the cause of their lower

densities. Molecules lie far away from each other making them less dense. On the other

hand, liquids have definite volume and fair intermolecular forces.

4. What do you mean by evaporation how it is affected by surface area?

Ans. Conversion of liquids into vapours is called evaporation. Larger surface area increase

rate of evaporation and vice versa.

5. Define the term allotropy with examples.

Ans. The existence of an element in more than one forms in same physical state has three

isotopes i.e, rhombic, monoclinic and plastic sulphur.

6. In which form sulphur exists at 100 °C?

Ans. Transition temperature of sulphur is 96oC. At 100 0C, sulphur is found in monoclinic form

(S8).



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7. What is the relationship between evaporation and boiling point of a liquid?

Ans. Increase in temperature causes increase in rate of evaporation. In other words vapour

pressure increases. When this vapour pressure becomes equal to atmospheric

pressure, liquid, starts boiling.

OBJECTIVE KEYS

1. b) 1000 2. c) g dm-3 3. c) Liquid and solid 4. b) Vibrational motion only 5. d)glucose

6. a)101325 7. c) very high energy 8. a) Hydrogen 9. d) initial temperature of liquid

10. b) pressure is increased 11. b) Increase of temperature

NUMERICALS

1. Convert the following units: (a) 850 mm Hg to atm (b) 205000 Pa to atm (c) 560 torr to cm Hg (d) 1.25 atm to Pa Ans. (a) 760 mmHg = 1 atm 1 mmHg = 1/ 760 atm 850 mmHg = 1X850 / 760 = 1.12 atm (b) 101325 pa = 1 atm 1 pa = 1/ 101325 atm 205000 = 1X205000 / 760 = 2.02 atm (c) 760 torr = 760 mmHg 1 torr = 1 mmHg 560 torr = 560 mmHg = 56 cmHg (d) 1 atm = 101325 pa 1.25 atm = 101325X 1.25= 126656 pa

2. Convert the following units: (a) 750 °C to K (b) 150 °C to K (c) 100K to°C (d) 172K to°C Ans. (a) K = 273 + Co K = 273 + 750 = 1023 K (b) K = 273 + Co K = 273 + 150 = 423 K (c) Co = 273 - K K = 273 - 100 = 173o C (d) Co = 273 - K K = 273 - 172 = 101o C

3. A gas at pressure 912 mm of Hg has volume 450cm3. What will be its volume at 0.4 atm. Ans. Given Data: P1 = 912 mmHg(912/760=1.2atm),P2 = 0.4atm V1 = 450 cm3 V2 =? Solution: P1V1 = P2V2 So, 1.2 X 450 = 0.4 X V2 V2 = 1.2 X 450 / 0.4 = 1350 cm3

4. A gas occupies a volume of 800 cm3 at 1 atm, when it is allowed to expand up to 1200 cm3 what will be its pressure in mm of Hg Ans. Given Data: P1 =1 atm P2 =? (in mmHg) V1 = 800 cm3 V2 =1200 cm3

Solution: We know that P1V1 = P2V2

So, 1 X 800 = P2 X 1200 P2 = 1 X 800 / 1200 = 0.667 atm Or = 0.667 X 760 = 506 mmHg

5. It is desired to increase the volume of a fixed amount of gas from 87.5 to 118 cm3 while holding the pressure constant. What would be the final temperature if the initial temperature is 23 °C. Ans. Given Data:

6. A sample of gas is cooled at constant pressure from 30 °C to 10 °C. Comment: a. Will the volume of the gas decrease to one third of its original volume? b. If not, then by what ratio will the volume decrease? Ans.



Page | 35

T1 = 23oC ( 273+23=296 K) , T2 =? V1 = 87.5 cm3 V2 =118 cm3

Solution: We know that V1 /T1 = V2 / T2

So, 87.5 / 296 = 118 / T2 T2 = 118 X 296 / 87.5 = 399 K Or = 399 – 273 = 126o C

Given Data: T1 = 30oC (303K) T2 =10oC (283K) Let V1 = 1 cm3 V2 =? Solution: We know that V1 /T1 = V2 /T2

So, 1 / 303 = V2 / 283 V2 = 283 X 1 / 303 = 0.93 cm3

a. The volume will not decrease to 1/3. b. 1 : 0.93

7. A balloon that contains 1.6 dm3 of air at standard temperature (0 °C) and (la tm) pressure is taken under water to a depth at which its pressure increases to 3.0 atm. Suppose that temperature remain unchanged, what would be the new volume of the balloon. Does it contract or expand? Ans. Given Data: P1 = 1 atm P2 = 3.0atm V1 = 1.6 m3 V2 =? Solution: P1V1 = P2V2 So, 1 X 1.6 = 3.0 X V2 V2 = 1 X 1.6 / 3.0 = 0.53 dm3

The gas will contract.

8. A sample of neon gas occupies a volume of 75.0 cm3 at very low pressure of 0.4 atm. Assuming temperature remain constant what would be the volume at 1.0 atm. pressure? Ans. Given Data: P1 = 0.4atm P2 = 1atm V1 = 75.0 cm3 V2 =? Solution: We know that P1V1 = P2V2

So, 0.4X 75.0 = 1 X V2 V2 = 0.4 X 75.0 / 1 = 30 cm3

9. A gas occupies a volume of 35.0 dm3 at 17 °C. If the gas temperature rises to 34°C at constant pressure, would you expect the volume to double? If not calculate the new volume. Ans. Given Data: V1 = 35.0 dm3 V2 =? T1 = 17oC (290 K) T2 =34oC (307K) Solution: We know that V1 /T1 = V2 /T2

So, 35.0 / 290 = V2 / 307 V2 = 35.0 X 307 / 290 = 37.05 dm3

10. The largest moon of Saturn, is Titan. It has atmospheric pressure of 1.6xl05 Pa. What is the atmospheric pressure in atm? Is it higher than earth’s atmospheric pressure? Ans. Given Data: Pt = 1.6 X 10 5 pa Pt = atm ? Solution: We know that 101325 pa = 1 atm

1 pa = 1 / 101325 1.6 X 10 5 pa = 1 X1.6 X 10 5 / 101325 =1.58atm It is higher that atmospheric pressure on earth.

SOLUTIONS Chapter 6


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1. Why solution is considered mixture?

Ans: A mixture does not have fixed ratio between the masses of its

components. It cannot be represented by a chemical formula similarly a

solution also does not have chemical formula and fixed ratio of its

components so, solution is considered as mixture.

Test yourself 6.1

2. Distinguish between the following pairs as compound or solution:

(a) water and salt solution (b) vinegar and benzene (c) carbonated drinks and

acetone

Ans: (a) Water: Compound, Salt solution: Solution

(b) Vinegar: Solution, Benzene: Compound

(c) Carbonated drink: Solution, Acetone: Compound

3. What is the major difference between a solution and a mixture?

Ans: A mixture is formed by the mixing up of two or more things. The components may or may

not retain their identity. On the other hand, solution is a type of mixture in which

components can’t be distinguished after mixing.

4. Why alloys are considered solutions?

Ans: Alloys are made by mixing metals by different percentages. It is a physical change, not a

chemical one. So, Alloys are solution.

5. Dead sea is so rich with salts that it forms crystals when temperature lowers in the

winter. Can you comment why is it named as “Dead Sea”?

Ans: Firstly, the high concentration of salts in it makes plant or animal life impossible.

Secondly, due to high density of water in “dead sea”, nothing can sink in it, even fishes etc cannot go under water making it inhabitable.

1. Does the percentage calculation require the chemical formula of

the solute?

Ans: Percentage = a a i ×

Above formula shows that to determine the percentage of solution, only mass of solute is required. There is no need of chemical formula.

Test yourself 6.2

2. Why is the formula of solute necessary for calculation of the molarity of the solution?

Ans: Molarity = a M. a × i

Above equation shows, that, we need Molar mass of solute to determine Molarity And we

also know that Molar mass can only be calculated by the help of chemical formula So, we

need chemical formula of solute to determine the Molarity of solution.

SOLUTIONS Chapter 6


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3. You are asked to prepare 15 percent (m/m) solution of common salt. How much

amount of water will be required to prepare this solution?

Ans: For this purpose, 15g of salt will be dissolved in 85g of water.

4 How much water should be mixed with 18 cm3 of alcohol so as to obtain 18 % (v/v)

alcohol solution?

Ans: To obtain 18% v/v solution of alcohol in water, 18g of alcohol will be dissolved in

sufficient amount of water to get 100 cm3 of total solution.

5. Calculate the concentration % (m/m) of a solution which contains 2.5 gof salt

dissolved in 50 g of water.

Ans: % (w/w) = a a i ×

= 2.52.5+50 ×

= 2.552.5 ×

= 4.76%

6. Which one of the following solutions is more concentrated: one molar or three molar

Ans: Three molar solution is more concentrates as compared to one molar solution. Because

more the molarity, more the amount of solute, hence more concentrated the solution is

1. What will happen if the solute-solute forces are stronger than those

of solute-solvent forces?

Ans: If solute solute forces are strong enough to resists overcoming of solute

solvent forces, the solute will not dissolve in solvent.

2. When solute-solute forces are weaker than those of solute-solvent

forces? Will solution form?

Test yourself 6.3

Ans If solvent solute forces are stronger than solute-solute forces. The former will overcome

the later and solution will be formed.

3. Why is iodine soluble in CCI4 and not in water?

Ans: Iodine is non-polar. The universal phenomenon of dissolution is “like dissolves like. So iodine will mix in non- polar CCl4 but not in polar water.

4. Why test tube becomes cold when KNO3 is dissolved in water

Ans: When KNO3 is dissolved in water. It absorbs heat from surroundings. So test tube

becomes cold due to transfer of its heat into the solution.

1. What is difference between colloid and suspension?

Ans: Suspension is a heterogeneous mixture of undissolved particles in a

given medium. While in colloid, the particles of solute are large but not larger

than suspension. So they dissolve and do not settle down.

SOLUTIONS Chapter 6


Page | 38

Test yourself 6.4

2. Can colloids be separated by filtration, if not why?

Ans: The particles can pass though the pores of filter paper. So, colloids cannot be separated

by filtration.

3. Why are the colloids quite stable?

Ans: The solute particles in colloid are smaller in diameter than suspension particles. The

solvent holds the particles and do not settle down. So colloids are quite stable.

4. Why does the colloid show tyndall effect?

Ans: The particles of solute in colloid scatter the path of light if passed through it. So, colloid

shows the tyndall effect.

5. What is tyndall effect and on what factors it depends?

Ans: When the light is passed through colloid. The particles of colloid scatter the beam of light.

It is called tyndall effect. It depends on the size of solute particles present in solution.

6. Identify as colloids or suspensions from the following:

Paints, milk, milk of magnesia, soap solution.

Ans: Colloids: Milk, soap solution Suspension: Milk of magnesia, paint.

7. How can you justify that milk is a colloid.

Ans: The solute particles of milk do not settle down. Secondary it shows tyndall effect. So milk

is colloid.

EXERCISE

1. Why suspensions and solutions do not show tyndall effect, while colloids do?

Ans : Particles of colloid scatter the path of light rays when passed through it. So, colloids

exhibit tyndall effect. On the other hand particles of suspensions are big enough to block

the path of light. So suspensions do not exhibit tyndall effect.

2. What is the reason for the difference between solutions, colloids and suspensions?

Ans : The reason for the difference between these lies in the size of solute particles. Solute

particles are in simplest form i.e. ions, molecules in true solution group of molecules in

colloid and very large size in molecules suspension.

3. Why the suspension does not form a homogeneous mixture?

Ans : These is no ideal interaction between solute and solvent in suspension. Solute particles

do not lose their identity and also do not mix-up in solvent. So, suspension does not form

homogeneous mixture.

4. How will you test whether given solution is a colloidal solution or not?

Ans : The best way is to pass beam of light through it. If given solution shows tyndall effect, it is

colloid, otherwise not.

SOLUTIONS Chapter 6


Page | 39

5. Classify the following into true solution and colloidal solution: Blood, starch solution,

glucose solution, toothpaste, copper sulphate solution, silver nitrate solution.

Ans:

True solution Colloid Solution Copper sulphate solution Blood Sliver nitrate solution Starch solution Glucose solution. Tooth paste

6. Why we stir paints thoroughly before using?

Ans : Paints are suspension. We know that solute particles settle down in suspensions after

some time. So, we have to stir paints before using.

7. Which of the following will scatter light and why? sugar solution, soap solution and

milk of magnesia.

Ans : Soap solution will scatter light because it is colloidal solution. Sugar solution is true

solution and milk of magnesia is suspension. Both of these do not show tyndall effect.

8. What do you mean, like dissolves like? Explain with examples

Ans : It is universal phenomena which means:-

Polar substances are soluble in each other. e.g. sugar in water.

Non polar substances are soluble in each other, e.g. benzene and ether.

Polar material is not soluble is non- polar e.g. petrol and water.

9. How does nature of attractive forces of solute-solute and solvent-solvent affect the

solubility?

Ans : Attractive forces play a vital role in solubility. If solute- solute interaction is weak, it is

most likely to be dissolved and vice versa. Similarly if solvent- solvent forces of attraction

are very strong, it may not provide space for incoming solute particles and may not

dissolve it.

10. How you can explain the solute-solvent interaction to prepare a NaCl solution?

Ans : For instance, if we are dissolving NaCl in water, it will readily dissolve. The reason is the

ideal interaction between water molecules and NaCl ions. Polar ends in water molecule

will attract respective ions i.e. Na+ or Cl-

11. Justify with an example that solubility of a salt increases with the increase in

temperature.

Ans : When salts like KNO3, NaNO3 and KCl are added in water, the test tube becomes cold. It

means during dissolution of these salts heat is absorbed. Such dissolving process is

called endothermic.

Solubility usually increases with the increase in temperature for such solutes.

SOLUTIONS Chapter 6


Page | 40

12. What do you mean by volume/volume %?

Ans : It is the volume in cm3 of a solute dissolved per 100 cm3 of the solution.

% by volume = V V ×

SOLUTIONS Chapter 6


Page | 41

ELECTROCHEMISTRY Chapter 7


Page | 42

1: How can you justify that a reaction between magnesium and oxygen

is a redox reaction while the reaction shows only addition of oxygen

(Oxidation)?

2Mg + O2 ⟶ 2MgO Ans. Above reaction is a redox reaction.

Test yourself 7.1 It can be justified by two ways:

There is transfer of electrons from Mg (Oxidation) to Oxygen (Reduction).

2Mg ⟶ 2Mg+2 + 4e-

O2 + 4e- ⟶ 2O-2

2Mg+2 + 2O-2 ⟶ 2MgO

There is increase in oxidation state of Mg (Oxidation) and decrease in oxidation state of oxygen (reduction).

So, oxidation and reduction reactions are taking place side by side in above reaction.

2: A reaction between carbon and oxygen involves only addition of oxygen (oxidation),

but, it is called redox reaction. Comment upon it.

Ans: C + O2 ⟶ CO2

At start, both the carbon and oxygen atoms have zero oxidation state. After the formation

of CO2, there is increase in oxidation state of carbon from 0 to +4 (oxidation). On the other

hand, there is decrease in oxidation state of oxygen from 0 to -2(reduction). So formation

of CO2 is a redox reaction.

Q3: Oxidation and reduction reactions proceed simultaneously. Explain with an example.

Ans: Where there is oxidation, there is reduction. This can be explained by the formation of

carbon dioxide.

C + O2 ⟶ CO2

At start, both the carbon and oxygen atoms have zero oxidation state. After the formation

of CO2, there is increase in oxidation state of carbon from 0 to +4 (oxidation). On the other

hand, there is decrease in oxidation state of oxygen from 0 to -2(reduction).So oxidation

and reduction reactions always proceed simultaneously.

Q4: Identify which of the following is oxidation or reduction reaction:

K ⟶ K+1 + 1e-1 Br + 1e-1 ⟶ Br -1 Cu ⟶ Cu+2 + 2e-2 I-1 ⟶ I + 1e-1 Fe+2 ⟶ Fe+3 + 1e-1 Ans: a. Oxidation. b. Reduction. c. Oxidation. d. Oxidation. c. Oxidation.

Q5: An element reacts with another element ‘X’ to form ‘MX’. In terms of loss and gain of

electrons, identify the element which is oxidized and which is reduced.

Ans: M + 2X ⟶ MX2



Page | 43

From above reaction, it is clear that “M” is acting as cation and “X” is acting as anion. M

has lost two electrons so “M” is being oxidized.

M ⟶ M2+ + 2e

Each of X atoms has gained one electron, “X” is being reduced.

2X ⟶ 2X-

M2+ + 2X- ⟶ MX2

Q6: How can you justify that the following reaction is not only an oxidation reaction but

also a complete redox reaction?

FeO + CO ⟶ Fe + CO2

Ans: Oxidation and reduction reactions are going side by side in above reaction as oxygen is

being added in CO to form CO2 (oxidation) and oxygen is being removed from FeO to

form Fe (Reduction). So, it is a complete redox reaction.

Q7: Explain the term oxidation on the basis of electronic configuration concept with an

example.

Ans: “Loss of electrons is called oxidation.”

Fe+2 ⟶ Fe+3 + 1e-1

Iron is being oxidized in above reaction because there is loss of electron.

Q1: Find out the oxidation numbers of the following elements marked

bold in the formulae:

Ba3(PO4)2, CaSO4, Cu(NO3)2, Al2(SO4)3

Ans: Ba3(PO4)2 CaSO4

(+2X3)+{P+(-2X4)}X2=0 +2+S+(-2X4)=0

Test Yourself 7.2 +6+2P-16=0 +2+S-8=0

2P-10=0 S-6=0

2P=+10 S=+6

P=+5

Cu(NO3)2 Al2(SO4)3

+2+{N+(-2X3)}X2=0 +3X2+{S+(-2X4)}X3=0

+2+2N-12=0 +6+3S-24=0

2N-10=0 3S-18=0

N=10/2=+5 S=18/3=+6

Q2: In a compound MX3, find out the oxidation number of “M” and “X”. Ans: Ionization of MX3 is supposed to occur as:

MX3 ⇌ M+3 + X-1



Page | 44

Above reaction suggests that the oxidation number of “M” is +3 and of “X” is -1.

Q3: Why the oxidation number of oxygen in OF2 is +2?

Ans: Fluorine is more electronegative than oxygen. So fluorine is assigned negative oxidation state and oxygen is assigned positive oxidation state.

OF2=0 => O+ (-1X2) =0 => O-2=0 => O=+2

Q4: In H2S, SO2 and H2SO4 the sulphur atom has different oxidation numbers. Find the

oxidation number of sulphur in each compound.

Ans: H2S=0 SO2=0 H2SO4=0

+1x2+S=0 S+2x-2=0 +1X2+S+4X-2=0

S=-2 S=+4 S=+6

Q5: An element has oxidation number zero. What will be its oxidation number when it

gains three electrons?

Ans: X0 + 3e-1 ⟶ X-3

The oxidation state will be -3.

Q6: An element in oxidation state +7 gains electrons to be reduced to oxidation state +2.

How many electrons did it accept?

Ans: Reduction of oxidation state from +7 to +2 requires 5 electrons to be added in that

element.

Q7: If the oxidation state of an element changes from +5 to -3, has it been reduced or

oxidized? How many electrons are involved in this process?

Ans: Decrease in oxidation state means reduction. 8 electrons are involved in this reaction.

1: In the following reactions, how can you justify that H2S is oxidized and

SO2 is reduced:

SO2 + 2H2S ⟶ 2H2O + 3S Ans: In above reaction, hydrogen is being removed from H2S, which means it oxidation is taking place. On the other hand, oxygen is being removed from SO2, which means reduction is taking place in this case.

Test Yourself 7.3

Q2: The reaction between MnO2 and HCl is a redox reaction written as balance chemical

equation.

MnO2 + 4HCl ⟶ MnCl2 + 2H2O + Cl2

Find out:

The substance oxidized. The substance reduced. The substance acts as oxidizing agent. The substance acts as reducing agent. Ans: a. HCl is oxidized.



Page | 45

b. MnO2 is reduced.

c. Mn is acting as oxidizing agent.

d. Cl is acting as reducing agent.

Q3: The following reactions are redox reactions. Find out the element which has been

reduced and the element which has been oxidized.

Zn + CuSO4 ⟶ ZnSO4+ Cu

Cu + 2AgNO3⟶ Cu(NO3)2+ 2Ag

H2S + Cl2 ⟶ 2HCl + S

Ans: Zinc is oxidized and copper is reduced.

Copper is oxidized and silver is reduced.

Sulphur is oxidized and chlorine is reduced.

Q4: Why the following reaction is both a redox reaction? Explain with reason.

NaOH + HCl ⟶ NaCl + H2O Ans: In above reaction, there is no change in oxidation sate of any element before and after

reaction. So it is not considered as redox reaction.

1: Why are strong electrolytes termed as good conductors?

Ans: Degree of ionization of strong electrolytes is much high. They conduct electricity very well in solution or molten form, thus termed as good

conductors.

2: Do non-electrolytes form ions in solutions?

Ans: Non-electrolytes do not form ions in solution form and are termed as

Test Yourself 7.4 non-conductors.

Q3: What is difference between strong electrolyte and weak electrolyte?

Ans: Strong electrolytes ionize almost completely in their aqueous solutions e.g. NaOH, while weak electrolytes ionize to a small extent when dissolved in water e.g. acetic acid.

Q4: Identify a strong or weak electrolyte among the following compounds:

CuSO4 , H2CO3 , Ca(OH)3 , HCl, AgNO3

Ans: Strong electrolytes: CuSO4 , HCl, AgNO3

Weak electrolytes: H2CO3 , Ca(OH)3

Q5: Which force derives the non-spontaneous reactions to take place?

Ans: In electrolytic cells, non-spontaneous reaction takes place with the help of electric current.

Q6: Which type of reaction takes place in electrolytic cells?

Ans: In electrolytic cells, non-spontaneous redox reaction takes place with the help of electric current.



Page | 46

Q7: What type of reaction takes place at anode of in electrolytic cell?

Ans: In electrolytic cells, oxidation (loss of electrons) takes place at anode.

Q8: Why the positively charged electrode is called anode in electrolytic cell?

Ans: At anode, oxidation (loss of electrons) takes place. In electrolytic cells, anions move towards positive electrode and discharge there by losing electrons (oxidation). Thus positive electrode in electrolytic cell is called anode.

Q9: In the electrolysis of water, towards which terminal H+ ion moves?

Ans: In the electrolysis of water, H+ moves towards negative terminal of the cell (cathode).

Q10: In the electrolysis of water, where is the oxygen produced?

Ans: In the electrolysis of water, oxygen is produced at anode.

Q11: Towards which electrode of the electrolytic cell cations move and what do they do?

Ans: In electrolytic cells, cations move towards negative electrode (cathode) and discharge there by gaining electrons (reduction).

Q12: How the half cells of galvanic cell are connected? What is the function of salt bridge?

Ans: Half cells of Galvanic cells are connected by salt bridge. A salt bridge is used to keep the solutions of two half cells neutral by providing the pathway of migration for ions.

1: Anode of Down’s cell is made of a non-metal, what is its name? What is

the function of this electrode?

Ans: Anode of Down’s cell is made of graphite. Oxidation takes place at graphite

anode.

2Cl-1 ⟶ Cl2 + 2e-1

Test yourself 7.5

Q2: Where does the sodium metal is collected in Down’s cell?

Ans: Sodium ion is reduced at cathode and is converted into sodium metal which is collected in a side tube.

Q3: What is the name of by product produced in the Down’s cell?

Ans: Chlorine is produced as by-product in Down’s cell.

Q4: Are anode of Down’s cell and Nelson’s cell made of same elements? If yes, what is its name?

Ans: In both cells, anodes are made up of graphite and cathodes are made up of iron.

Q5: What is the shape of cathode in Nelson’s cell? Why is it perforated?

Ans: A U shaped iron cathode is used in Nelson’s cell. It is made perforated so that NaOH can percolate through it and can be collected in a steel tank present below it.

Q6: Which ions are discharged at cathode in Nelson’s cell and what is produced at

cathode?

Ans: Hydrogen ions are discharged on cathode to produce hydrogen gas.



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Q1: What is difference between corrosion and rusting?

Ans: Corrosion is slow and continuous eating away of a metal by the surrounding medium. Corrosion is a general term but corrosion of iron is called rusting.

Q2: What happens to the iron in the rusting process?

Ans: Redox reaction occurs on the surface of iron during rusting process. Test Yourself 7.6 This converts metallic iron into anhydrous ferric oxide (Fe2O3.nH2O)

Q3: Rusting completes in how many redox reactions?

Ans: Process of rusting completes in two redox reactions. In first phase, iron is converted into iron cation without the formation of rust. In second phase, this cation reacts with oxygen to form rust.

Q4: Explain the role of oxygen in rusting.

Ans: In the first phase of rusting, oxygen reaches at cathode region of iron sheet. Here, reduction takes place.

O2 + 4H+ + 4e-1 ⟶ 2H2O

In second phase, oxygen reacts with Fe+2 ions to form rust.

2Fe+2 + ½ O2 + (2+n) H2O ⟶ Fe2O3.nH2O + 4H+1

Q5: State the method for the prevention of metal from corrosion.

Ans: Following methods can be taken into account for the prevention of rusting:

Removal of stains

Painting and greasing the surface of metal.

Alloying the metal with suitable noble metal.

Metallic coating.

Galvanizing and tin coating.

By electroplating.

Q6: What do you mean by galvanizing?

Ans: The process of coating a thin layer of zinc on metallic surface is called galvanizing. This process is done by dipping metal into zinc chloride solution and then heating it.

Q7: What is advantage of galvanizing?

Ans: Advantage of galvanizing is that zinc protects metallic surface against corrosion even after zinc coating is broken.

Q8: Why tin plated iron is rusted rapidly when tin layer is broken?

Ans: Once the tin coating is broken, iron is exposed to air and water, a galvanic cell is established and iron rusts rapidly.

Q9: Name the metal which is used for galvanizing iron.

Ans: Zinc is used for galvanizing iron.



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Q1: Define electroplating.

Ans: Electroplating is depositing one metal on the surface of other metal by means of electrolysis.

Q2: How electroplating of zinc is carried out?

Ans: Metal is dipped into zinc sulphate solution and current is applied which Test Yourself 7.7 results inzinc being deposited on the target metal.

Q3: Which metal is used to make cathode in electroplating?

Ans: The cathode is made up of the object that is to be electroplated like some sheet made up of iron.

Q4: Why is the anode made up of a metal to be deposited during electrolysis?

Ans: At anode, oxidation takes place which converts metal to its respective cation. This cation, being positively charged, moves toward cathode and deposits there. That is why anode is made up of a metal to be deposited.

EXERCISE

1. Define oxidation in terms of electrons. Give an example.

Ans:

Oxidation is loss of electrons by an atom or an ion. e.g.

2. Define reduction in terms of loss or gain of oxygen or hydrogen. Give an example.

Ans: Reduction is defined as addition of hydrogen or removal of oxygen during a chemical reaction.

3. What is difference between valency and oxidation state?

Ans: The unique property of an element is valency. It is combining capacity of an element with other elements. The valency of chlorine, oxygen, nitrogen and carbon is 1, 2, 3 and 4, respectively.

Oxidation state or oxidation number (O.N.) is the apparent charge assigned to an atom of an element in a molecule or in an ion. For example: in HCl, oxidation number of H is + 1 and that of CI is -1.

4. Differentiate between oxidizing and reducing agents

Ans: Reducing agent is a substance that oxidizes itself and reduces other.

Oxidizing agent is a substance that reduces itself and oxidizes other.

5. Differentiate between strong and weak electrolytes.



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Ans: The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes. Example of strong electrolytes are aqueous solutions of NaCl, NaOH and H2SO4, etc.

The electrolytes which ionize to a small extent when dissolved in water and could not produce more ions are called weak electrolytes. Acetic acid (CH3COOH) and Ca(OH)2

6. How electroplating of tin on steel is carried out?

Ans: Tin is usually electroplated on steel by placing the steel into a container containing a solution of tin salt. The steel is connected to an electrical circuit, acting as cathode. While the other electrode made of tin metal acts as anode. When an electrical current passes through the circuit, tin metal ions present in the solution deposit on steel.

7. Why steel is plated with nickel before the electroplating of chromium.

Ans: For practical convenience, the steel is usually plated first with nickel or copper and then by chromium because it does not adhere well on the steel surface. Moreover, it allows moisture to pass through it and metal is stripped off. The nickel or copper provides adhesion and then chromium deposited over the adhesive layer of copper lasts longer.

8. How can you explain, that following reaction is oxidation in terms of increase of oxidation

number?

Ans: Increase in oxidation state is called oxidation. The O.S of Al has been increased from 0 to +3, that is why above reaction is oxidation reaction.

9. How can you prove with an example that conversion of an ion to an atom is an oxidation process?

Ans: Cl- Cl + e

Cl + Cl Cl2

In this equation the Chloride ion loses an electron (oxidation) and become an atom.

10. Why does the anode carry negative charge in galvanic cell but positive charge in

electrolytic cell? Justify with comments.

Ans: The anode of an electrolytic cell is positive, since the anode attracts anions from the solution. However, the anode of a galvanic cell is negatively charged, since the spontaneous oxidation at the anode is the source of the cell's electrons or negative charge. The cathode of a galvanic cell is its positive terminal.

11. Where do the electrons flow from Zn electrode in Daniel’s cell?

Ans: The electrons flow from Zn-electrode through the external wire in a circuit to copper electrode. These electrons are gained by the copper ions of the solution and copper atoms deposit at the electrode.



Page | 50

12. Why do electrodes get their names ‘anode’ and cathode in galvanic cell?

Ans: Anode: positively charged terminal - oxidation reaction. Cathode: negatively charged terminal - reduction reaction

13. What happens at the cathode in a galvanic cell?

Ans: Cathode: negatively charged terminal - reduction reaction takes place.

14. Which solution is used as an electrolyte in Nelson’s cell?

Ans: Brine solution is used as an electrolyte in Nelson’s cell

15. Name the by-products produced in Nelson’s cell?

Ans: Hydrogen gas and chlorine gas are the by-products in Nelson’s cell.

16. Why is galvanizing done?

Ans: Galvanizing is done because zinc protects the iron against corrosion even after the coating surface is broken.

17. Why an iron grill is painted frequently?

Ans: Iron grill is painted frequently because paint protects the iron only as long as its protective layer remains intact. Once it is broken and the iron is exposed to the air and water, a galvanic cell is established and iron rusts rapidly.

18. Why is O2 necessary for rusting?

Ans: Rusting is completed in two redox reactions. Both reactions involve oxygen to produce rust.

19. In electroplating of chromium, which salt is used as an electrolyte?

Ans: Aqueous solution of chromium sulphate containing a little sulphuric acid, that acts as an electrolyte.

20. Write the redox reaction taking place during the electroplating of chromium?

Ans: Electrolyte produces the following ions:

Reactions at the electrodes are as follows:

21. In electroplating of silver, from where Ag+ ions come and where they deposit?

Ans: When the current is passed through the cell, the anode dissolves to produce Ag+ ions, that migrate towards the cathode. At cathode they are discharged and deposited on the object e.g. spoon.



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22. What is the nature of electrode used in electrolyzing of chromium?

Ans: The object to be electroplated acts as cathode while anode is made of antimonial lead.

CHEMICAL REACTIVITY Chapter 8


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1: What type of elements are metals?

Ans: The electropositive elements are metals i.e., the elements that tend to

lose electrons to attain positive charge. Out of 118 elements, 91 are metals.

2: Name a metal which exists in liquid form.

Ans: Mercury exists in liquid form.

Test Yourself 8.1

3: What is the nature of metal oxides?

Ans: Metal oxides are basic in nature. These oxides give bases when react with water.

CaO +H2O ⟶ Ca(OH)2

4: Which group of metals is highly reactive?

Ans: Alkali metals (Group I) are highly reactive. These are:

Li, Na, K , Rb, Cs, Fr

5: Why sodium metal is more reactive than magnesium metal?

Ans: Sodium has low ionization energy than magnesium, hence more electropositive. That is

why sodium is more reactive than magnesium.

6: Name a metal which can be cut with a knife.

Ans: Sodium being very soft metal can be cut with knife.

7: Name the best ductile and malleable metal.

Ans: The best ductile and malleable metal is gold.

8: Name the metal which is the poorest conductor of heat and electricity.

Ans: The least conductor of heat and electricity is lead (Pb).

9: What do you mean by malleability and ductility?

Ans: Malleability is the property by virtue of which metals can be drawn into sheets and

ductility is the property by virtue of which metals can be drawn onto wires. Gold is the

most malleable and ductile metal.

10: Why alkali metals are more reactive than alkaline earth metals?

Ans: Alkali metals have low ionization energies than alkaline earth metals. It means that they

are more electropositive. That is why alkali metals are more reactive than alkaline earth

metals.

11: What do you mean by metallic character?

Ans: Metals have the character of losing electrons to form cations. This property is called

electro positivity or metallic character.

12: Why metallic character decreases along a period and increases in a group?

Ans: Metallic character decreases along a period because size of atom decreases in a period

and loss of electrons becomes difficult. In a group, metallic character increases because

atomic size increases down the group and atoms can easily lose electron(s).

1: Give the application of silver.



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Ans: Alloys of silver with copper are widely used in making coins, silver

ware and ornaments. Compounds of silver are used in photographic

films and dental preparations. Silver is also used in mirror industry.

2: Why silver is not used in pure form?

Ans: Silver is very soft metal. So it is rarely used as such. Alloys of silver

Test Yourself 8.2 with other metals are widely used.

3: What do you mean by 24 carat gold?

Ans: Purity of gold is shown by carat that indicates the number of parts by weight

of gold that is present in 24 parts of alloy. 24 carat gold is pure.

4: Why gold is used to make jewelry?

Ans: Gold is used for making jewelry Because of its inertness in atmosphere and its luster.

5: Why platinum is used for making jewelry?

Ans: Platinum is used to make jewelry items because of its unique characteristics like colour,

beauty, strength, flexibility and resistance to tarnish.

6: Why the second ionization energy of Mg is higher than its first ionization energy?

Ans: It becomes much difficult to remove second electron from Mg+1 ion as nuclear charge

attracts remaining electrons strongly. That is why the second ionization energy is higher

than first ionization energy.

7: What is difference between steel and stainless steel?

Ans: Steel and Stainless steel are not that very far in comparison with each other. Steel is the

by-product of combining iron and carbon in the same way stainless steel is made up of

iron and chromium. Steels are generally used in railways, roads, subways, bridges, other

huge buildings and modern skyscrapers, shipbuilding, cars, armored vehicles, and

bulldozers, while stainless steel due to its low cost and high resistance for corrosion and

rust is normally used in small to medium objects like surgical instruments, cutlery, home

appliances, cookware, storage tanks and even some handguns are made of stainless

steel.

8: How platinum used as a catalyst in automobiles and what are advantages of this use?

Ans: Platinum alloyed with palladium and rhodium is used as catalyst in automobiles as

catalyst convertor. They convert most of the gases being emitted by vehicles into less

harmful carbon dioxide, nitrogen and water vapours.

1: Why valency of chlorine is 1?

Ans: Chlorine gains one electron to complete its octet. So its valency is -1.

Cl + 1e-1 ⟶ Cl-1

2: Which factor controls the non-metallic character of elements?

Ans: Non-metallic character depends upon the electron affinity. More the

Test Yourself 8.3 electron affinity of an element, more will be non-metallic character.



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3: Why fluorine is more non-metallic than chlorine?

Answ: Fluorine has more electron affinity as compared to chlorine. So fluorine is more non-

metallic in nature.

4: Iodine exists in solid form; can it be beaten with hammer to form sheets?

Ans: Malleability is the property of metals. Iodine is a non-metal and forms covalent bond. So it

cannot be drawn into sheets.

5: Can liquids and gases be brittle?

Ans: Brittle means a hard but liable to break easily. Liquids and gases are unable to break like

solids. So, they cannot be brittle.

6: Why the oxygen is non-metal?

Ans: Oxygen forms anion by losing two electrons which is the character of non-metals. So

oxygen is not metal.

7: Name two non-metals which are both brittle and ductile.

Ans: Solid non-metals are brittle but do not show ductility. So there is no such non metal which

is both brittle and ductile.

8: Name the most abundant non metal found in earth’s crust. Ans: Oxygen has the highest percentage (47%) in the formation of earth crust.

9: Give the non-metallic character in halogens.

Ans: Non-metallic character decreases down the group. Order of non-metallic character in

halogens is:

F > Cl > Br > I

10: Why do the non-metals readily accept electrons?

Ans: Non-metals have high electron affinity. Secondly they are short of electrons so readily

accept electrons to complete octet.

11: Why non-metals do not react with dilute acids while metals do react?

Ans: Non-metals are acidic in nature (oxides of non-metals) so do not react with acids. On the

other hand metals are basic in nature (oxides of metals) so readily react with acids.

12: How can we distinguish metal from a non metal by simple physical method?

Ans: Metals are usually solids with metallic luster and are conductors of heat and electricity.

On the other hand non-metals are usually gases or liquids with dull appearance and do

not conduct both heat and electricity.

13: How can we distinguish a substance is metal or non-metal with the help of an acid?

Ans: Metals react with acids and produce hydrogen gas, but non-metals do not react with

acids.

14: Why HF is a weak acid?

Ans: There are strong intermolecular and intra molecular forces in HF. It does not ionize well in

water so acts as weak acid.



Page | 56

EXERCISE

1. Why reactivity of metals increases down the group?

Ans: Reactivity of metals increases from top to bottom in a group. This is because ionization

energy of metals decreases down the group. Low the ionization energy, greater is the

reactivity of a metal.

2. State the physical properties of metals.

Ans: 1. Almost all metals are solids (except mercury)

2. They have high melting and boiling points, (except alkali metals)

3. They possess metallic luster and can be polished.

4. They are malleable (can be hammered into sheets), ductile (can be drawn into wires)

and give off a tone when hit.

5. They are good conductor of heat and electricity.

6. They have high densities.

7. They are hard (except sodium and potassium).

3. Why nitrogen forms compounds with alkaline earth metals directly?

Ans: Nitrogen forms compounds with alkaline earth metals directly. This is because Alkaline

earth metals are electropositive or less electronegative than nitrogen.

Example:-

When magnesium (alkaline earth metal) is heated with nitrogen, magnesium nitride is

formed.

3Mg + N2 → Mg3N2

4. Why the second ionization energy of magnesium is higher than the first one?

Ans: The second ionization energy of magnesium is higher than the first ionization energy.

This is because after the removal of first electron, the nuclear charge is increased which

attracts the remaining electrons strongly. As a result of this attraction, the size of

magnesium ion is reduced. Now it is very difficult to remove the second electron. That is

why the second ionization energy of magnesium is higher than the first one.

5. How oxygen reacts with group II A metals?

Ans: Group II A metals lose 2 electrons while oxygen gains 2 electrons forming oxide ion O-2.

Hence oxygen reacts with group II A metals on heating and forms metal oxides. These

oxides are basic in nature.

Example: Magnesium is a group II A metal. When it is heated in the presence of air,

magnesium oxide is formed.

2Mg + O2 Heat→ 2MgO

6. What is relationship between electro positivity and ionization energy?

Ans: The electro-positivity of an element depends upon the values of its ionization energy. The

lower is the value of ionization energy of an element; the higher is the electro positivity of

the element and vice versa.



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7. Why electro positivity decreases from left to right in a period?

Ans: Electro positivity decreases from left to right across the periodic table. This is because

size of atoms decreases due to increase of nuclear charge.

8. How electro positivity depends upon size and nuclear charge of an atom?

Ans: Electro positive character depends upon the ionization energy which in turn depends on

size and nuclear charge of the atom. Small size atoms with high nuclear charge have

high ionization energy. In this way atoms having high ionization energy are less electro

positive or metallic.

9. Why ionization energies of alkaline earth metals are higher than alkali metals?

Ans: Alkaline earth metals have low ionization energies than alkali metals due to following

reasons:

They have less atomic radii than alkali metals.

They have more nuclear charge making difficult to remove electrons.

10. Why silver and gold are least reactive?

Ans: Reactivity of a metal depends upon its tendency to lose electrons, since silver and gold

do not lose their valence electrons easily, therefore they are least reactive.

11. Can pure gold be used for making ornaments? If not why?

Ans: Pure gold cannot be used for making ornaments because it is too soft to be used as

such. For making ornaments it is always alloyed with copper, silver or some other metals.

12. Why copper is used for making electrical wires?

Ans: Copper (Cu) is used for making electric wires because it is very ductile and is the best

metallic conductor of electricity after silver which is very expensive.

13. What is the trend of variation in densities of alkali metals?

Ans: Density is defined as mass per unit volume. Thus the greater is the volume, smaller will

be the density. When we move from top to bottom in a group, the increase in the nuclear

mass of alkali metals is greater than the increase in their atomic sizes. Thus, the

densities of alkali metals increase from top to bottom in a group.

14. Which metal is used for metal work?

Ans: Copper (Cu) metal is used as metal work because it is easily workable. It has attractive

appearance and is very resistant to corrosion. It is also used in plumbing, proofing and

other building works.

15. Why magnesium is harder than sodium?

Ans: In magnesium metal, more electrons are involved in metallic bond than sodium. Secondly

atomic radius of magnesium is less than sodium. Both these factors make metallic bond

stronger in magnesium than sodium. Stronger the metallic bond, harder the metal tends

to be.

16. Why calcium is more electropositive than magnesium?

Ans: Electro positive character increases due to increase in atomic size. Alkaline earth metal

magnesium has smaller size than calcium. Thus, calcium is more electropositive than

magnesium.



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17. Why ionization energy of Na is less than Mg?

Ans: Small size atoms with high nuclear charge have high ionization energy. While the large

size atoms with less nuclear charge have lowest ionization energy. Since, Na atom has

large size and less nuclear charge, contrary to Mg atom. That is why, the ionization

energy of sodium is less than magnesium.

18. Why the ionization energy of Na is more than K?

Ans: Small size atoms with nuclear charge have high ionization energy. While the large size

atoms with less nuclear charge have lowest ionization energy. Since, Na atom has small

size and high nuclear charge, contrary to K atom. That is why ionization energy of Na is

more than K.

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