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Grade VIIIScience

Exam Preparation Booklet

Chapter wise Important Questions & Solutions

Grade 8 Science

Important Questions

Chapter 1: Crop Production and Management

Topic: Basic Practises of Crop Production

1. Why is rice cultivated in the rainy season?

2. Apart from soil, what are the other sources through which plants can derive nutrition?

3. Explain the method used for cultivating crops with different nutrient requirements in an efficient manner.

4. The first step of soil preparation is tilling the soil upside down. What is this step called? Explain its importance in soil preparation.

5. List some of the traditional and modern methods of irrigation.

Topic: Crop Improvement

6. How can you select healthy seeds from a packet of seeds provided to you?

7. Give any four advantages of using manure over fertilisers.

8. Agricultural practices are usually carried out in a particular sequence to get higher crop productivity. Enlist the sequence of the agricultural practices.

9. What is the role played by a cultivator?

10. Why does vermicompost serve as a better alternative for plants than urea in the long run?

11. How is seed drill useful over conventional methods of sowing?

Topic: Harvesting and Storage of Crops; Animal Husbandry

12. What do you understand by harvesting? State any three methods of harvesting.

13. What is a combine?

14. The produce, which is harvested, should be stored safely in order to reduce the loss. What are the methods adopted by farmers to store their produce?

15. How can grains be protected during storage?

Chapter 2: Microorganisms

Topic: Microorganisms - Their Habitat and Classification

1. Microorganisms show a tremendous variety with respect to their habitat. Comment on the statement.

2. How are microorganisms classified?

3. Pick the odd one out. Give reasons for your answer. (i) Tuberculosis, tetanus, athlete’s foot, leprosy (ii) Amoeba, Rhizobium, Paramecium, Euglena (iii) Rickets, night blindness, anaemia, pneumonia (iv) Nucleoli, chromosome, gene, cristae (v) Xylem, parenchyma, collenchyma, sclerenchyma

4. Microorganisms are omnipresent. Justify the statement.

5.

Identify the given organism and write a brief account on the category which this organism belongs to.

Topic: Uses of Microbes

6. What is nitrogen fixation?

7. What is the principle on which salt and sugar work for food preservation?

8. Give the economic functions of the following microorganisms: (i) Lactobacillus (ii) Clostridium (iii) Saccharomyces

(iv) Salmonella typhimurium (v) Agaricus bisporus

9. Name the microorganism which is involved in the following processes: (i) Manufacture of vinegar (ii) Retting of fibres (iii) Symbiotic nitrogen fixation (iv) Flavouring of cheese (v) Biological war

10. Draw an illustration to represent the process by which nitrogen is recycled between biotic and abiotic components of the environment.

Topic: Microbes as Harmful Agents

11. What are pathogens? Give any four modes, with an example of each, through which pathogens spread.

12. How can diseases such as cholera and typhoid be prevented?

13. Fill in the blanks to complete the table.

Disease Characteristic Feature

Malaria Causative agent − _________

_________ Immune system gets damaged

_________ Viral disease which causes watery blisters on the skin

Ringworm Caused by _________

Pneumonia _________ borne disease

_________ Caused by Trypanosoma

14. The causative microbe for citrus canker is _________ and it spreads through _________.

15. How do carriers aid in the spread of diseases?

Chapter 3: Synthetic Fibres and Plastics

Topic: Synthetic Fibres

1. Fill in the blanks. (i) Fibres that are made artificially are called ________. They are also known as _________. (ii) Small units that are known as monomers combine to form large molecules that are known as _________. (iii) _______ is an example of a synthetic fibre.

2. Why are synthetic fibres more popular than natural fibres?

Topic: Properties and Uses of Rayon

3. Fill in the blanks.

Rayon is mixed with to manufacture bed sheets and with

to manufacture carpets.

4. Explain why rayon is not considered a true synthetic fibre.

Topic: Properties and Uses of Nylon

5. Why is nylon used for making the ropes of parachutes?

6. What are the characteristics of nylon fibres?

Topic: Polyester and Acrylic

7. What is polyester? Give examples.

8. Why are clothes made from acrylic sometimes preferred over clothes made from wool?

Topic: Types of Plastics

9. What are thermosetting plastics and thermoplastics? Give two examples of each and write their uses.

10. Plastic is a material of choice. Why?

11. The polythene bags that we generally use fall under which category of plastics? Explain.

Topic: Properties and Uses of Plastics

12. What is the characteristic property of plastics due to which it is used for making containers for storing different materials?

13. Which of the given objects is generally made of thermoplastic?

Topic: Plastic and the Environment

14. How do plastics harm our environment?

15. What does 4 R represent in the 4 R principle?

Chapter 4: Materials, Metals and Non-Metals

Topic: Physical Properties of Metals and Non-Metals

1. State whether True or False. (i) Non-metals can be beaten into thin sheets. (ii) If you touch the hot metallic part of a pan, you will not get hurt. (iii) Aluminium and copper can be drawn into thin wires. (iv) Coal pieces make a ringing sound when struck hard.

Topic: Reactions of Metals and Non-Metals with Oxygen and Water

2. Which of the given elements would yield a basic oxide? Sulphur, phosphorus, calcium and silicon

3. Name two metals which react violently with cold water. Write any three observations you would make when such a metal is dropped into water.

Topic: Reactions of Metals and Non-Metals with Acids and Bases

4. What will be observed when dilute hydrochloric acid reacts with: (i) Magnesium (ii) Copper

5. Food items such as curd and juices of citrus fruits should not be stored in metallic containers. Why?

Topic: Displacement Reactions

6. A little amount of iron filings is added to a solution of aluminium sulphate. Predict whether a reaction will take place or not. Give reasons for your answer.

7. Two test tubes X and Y were taken and some chemicals were added to them as shown in the given figure.

In which of the above test tubes will the reaction take place?

Topic: Uses of Metals, Non-Metals and Alloys

8. Why is aluminium used to make parts of an aircraft?

9. Write the name of an alloy that is used for (i) making coins (ii) constructing railway tracks

10. Name the described non-metals. (i) Essential for our life as it is required by all living beings for breathing (ii) Makes up 78% of the earth’s atmosphere and is used in manufacturing fertilizers (iii) Used in the process of water purification

Chapter 5: Coal and Petroleum

Topic: Renewable and Non-Renewable Sources of Energy

1. (i) What is a ‘solar cell’? (ii) Why is biogas considered an ideal fuel for domestic use?

Topic: Disadvantages of Fossil Fuels

2. Economic growth and ecological conservation should go hand-in-hand. Explain why.

Topic: Coal - Its Formation and Characteristics

3. Explain the process of carbonisation.

4. Explain the given figure.

5. Write five characteristic features of coal.

Topic: Products of Coal and Their Characteristics

6. Product Characteristics Uses (i) Coke ----------------- -------------- (ii) Coal tar ----------------- -------------- (iii) Coal gas ----------------- --------------

7. Fill in the blanks. (i) __________ is used for surfacing the roads. (ii) __________ , a product of the destructive distillation of coal, is used in the extraction of metals. (iii) The primary constituent of coal is ___________. (iv) ____________, ____________ and ___________ are produced by the

processing of coal.

8. A foul-smelling dark brown liquid obtained by destructive distillation of coal and used to manufacture dyes, explosives, perfumes, etc. is ____________.

Topic: Petroleum - Its Formation and Characteristics

9. How is petroleum formed?

10. Which of the following figures correctly represents the petroleum and natural gas deposits in the earth? (i)

(ii)

Topic: Petroleum, Its Products and Their Uses

11. Name the products that are obtained by the process of petroleum refining.

12. Name the constituent of petroleum used for the given purposes. (i) As a fuel for heavy motor vehicles (ii) For the lubrication of machine parts

13. Identify the petroleum products whose uses are given below. (i) It is used as a fuel for cooking at home. It is also considered a clean fuel. (ii) It is used for road surfacing. (iii) It is used as a base for ointments, candle making, vaseline, etc. (iv) It is used as a dry-cleaning agent, solvent, and fuel for automobiles.

Topic: Conservation of Petroleum

14. Write some ways to saving petrol/diesel?

Topic: Compressed Natural Gas

15. Write a short note on natural gas.

Chapter 6: Combustion and Flame

Topic: Essential Conditions Required for Combustion

1.

The wood splint in which of the given cases will produce the maximum amount of ash? Explain why.

2. Name the factors on which the rate of combustion depends. Give examples.

Topic: Methods to Control Fire

3. Explain how sodium bicarbonate is able to control fires?

4.

Which of the illustrated methods of extinguishing fires should be used in case a fire breaks out at a power house?

Topic: Types of Combustion

5. When Vijay heated a substance X, he observed that heat, light and sound

were produced. This is an example of which type of combustion?

6.

Which of the given substance will undergo spontaneous combustion?

Topic: Different Parts of a Flame

7. Draw a neat, well-labelled diagram to show the different zones of a burning candle?

Topic: Characteristics of a Good Fuel

8. What are the characteristics of a good fuel?

9. Which among the given fuels is the best? Explain with the help of reasons.

Fuel Calorific Value (kJ/kg)

Ignition Temperature (oC)

A 120 7

B 100 60

C 80 80

D 60 100

Topic: Harmful Effects of Combustion of Fuels

10. State whether the following statements are true or false. (i) The combustion of fuels such as wood and coal can cause diseases such as asthma. (ii) It is dangerous to burn coal in a closed room.

Chapter 7: Conservation of Plants and Animals

Topic: Conservation of Plants

1. What could be the possible consequences of deforestation?

2. State some of the steps that can be taken towards the conservation of forests.

3. Can a fertile land ever change into a desert? If yes, then how?

4. In the long run, the continuous cutting of trees will threaten the balance of the entire Earth. Justify.

5. How does deforestation disturb the water cycle?

Topic: Conservation of Animals

6. Differentiate between Wildlife Sanctuary and National Park.

7. What do you understand by the term endangered species? What is the significance of the Red Data Book in conserving endangered species?

8. Explain the objective behind project tiger.

9. Define the following. (i) Biodiversity (ii) Migration

10. How is migration as a mechanism useful for survival in some birds?

Chapter 8: Cell - Structure and Function

Topic: Cell - Structure and Functions

1. Differentiate between prokaryotes and eukaryotes. Write any 3 differences?

2. With the help of suitable examples, explain how some cells are capable of changing their shapes.

3. Explain the difference between a plant cell and an animal cell with the help of labelled diagrams.

4. Plant cell lacks the mechanism of locomotion. What advantages do plants have at cellular level to compensate for the loss? Give any two examples.

5. The first cells to be observed by humans were dead cells. Justify.

Topic: Cell Organelles

6. (i) Why lysosomes are called ‘suicide bags’ of the cell? (ii) What are Golgi bodies in plants called?

7. (i) State any two functions of vacuoles in the cell. (ii) What is the membrane of vacuole called? (iii) Compare the vacuole present in plant cells and animal cells.

8. Write the functions of the following organelles. (i) Nucleus (ii) Chloroplast

9. Name the types of plastids. Which plastid is known as kitchen of the cell and why?

10. Explain the structure of mitochondria.

Chapter 9: Reproduction In Animals

Topic: Modes of Reproduction

1. Name the process that enables the continuous existence of an organism on earth.

2. What is sericulture? How is silk fibre obtained from the cocoon of silkworms?

3. Describe the process of embryo formation in egg laying animals.

4. Which category do reptiles belong to − oviparous or viviparous? Why?

5. Differentiate between external and internal fertilisation.

Topic: Sexual Reproduction In Humans

6. Define the term “foetus”.

7. Define the term menarche and menopause

8. Answer the following questions. (i) Draw a well-labelled diagram of human sperm. (ii) Describe the process of embryo development in humans.

9. Which organ is analogous to ovaries in males?

10. State the three major organs included in the male reproductive system. Write the function of each.

11. How does the embryo receive nutrition in the uterus? Explain with the help of a diagram.

Topic: Reproduction and Development in Animals

12. Explain the lifecycle of a silkworm.

13. What is the importance of thyroxine in the life cycle of a frog? What will happen if this hormone is not released?

14. With the help of a suitable example, explain how development occurs in oviparous animals.

15. Trace the events that lead to the formation of an adult frog from eggs. Represent the same with the help of a diagram as well.

Chapter 10: Reaching The Age Of Adolescence

Topic: Puberty, Adolescence and Secondary Sexual Characteristics

1. Define the following. (i) Adolescence (ii) Puberty

2. Answer the following questions. (i) Name the male and female sex hormones. (ii) What are secondary sexual characters?

3. It is observed that a lot of teenagers face the problem of acne or pimples. Why?

4. What are the changes that occur in females during puberty?

5. How can personal health, hygiene, and nutrition be maintained during adolescence?

Topic: Chemical Coordination in Humans

6. Name the hormone that helps in regulating the level of sugar in our blood. Name the gland that secretes it.

7. Mention four characteristics of hormones.

8. Write a short note on abnormalities related to the functioning of hormones.

9. Hormones are target-specific. Justify this statement with the help of a suitable example.

10. Which hormone is responsible for the development of low-pitched voice in human beings?

11. Name any one gland which acts as both endocrine and exocrine in nature.

Topic: Reproductive Phase and Sex Determination in Humans

12. What are chromosomes? How many chromosomes do human beings have?

13. Give reasons for the following statement. Adam’s apple protrudes and becomes prominent in boys during puberty.

14. How is sex determined in humans?

15. Environmental factors determine the sex of some animals. How?

Chapter 11: Force and Pressure

Topic: Force

1. Give an example in which force appears as a push on an object.

2. Name a device used for measuring the force acting on an object. How is it used for measuring force?

3. What are the changes possible when force is applied to a body?

4. What are contact forces and non-contact forces? Give examples.

5. Which contact force is a result of interlocking between two surfaces?

6. If you stop rowing a boat, you will observe that the boat comes to rest after a while. Name the force involved and state its direction. Is the force a contact or a non-contact force?

7. What happens to the speed of an object when force is applied in the direction opposite to its direction of motion?

8. Rohan is pushing a big box towards the left with a force of magnitude 60 N. His sister Naina is applying a force in the same direction. The force applied by her is half of that applied by her brother. Rohan’s friend Nikhil is applying a force in the opposite direction (i.e., towards the right). This force is double of that applied by Rohan. What is the resultant force on the box?

Topic: Pressure

9. Why do porters place a round piece of cloth on their heads while carrying heavy load?

10. (i) Define the term pressure. (ii) Write three characteristics of pressure exerted by liquids.

11. When a box of base area 5 units is left on the surface of a table, the table exerts a force of 25 units on the box. Calculate the magnitude of pressure exerted by the table on the box.

12. With the help of an example, explain how liquids exert equal pressure at the same depth.

13. Though atmospheric pressure is as high as 1000 kg, we do not get crushed under it. Why?

14. Why is there a loud noise when a bicycle tube bursts?

15. When a rubber sucker is pressed onto a smooth wooden surface, why it becomes difficult to pull the sucker?

Chapter 12: Friction

Topic: Friction

1. In which direction does the force of friction act?

2. What happens when a ball is thrown on a frictionless surface?

3. Friction is a necessary evil. Explain this statement using an example.

4. List some methods by which friction can be reduced?

5. What is static friction?

6. Why do vehicles moving through air and water have specially designed bodies?

7. Why is the force of friction greater on rough surfaces as compared to smooth surfaces?

8. Give three advantages and three disadvantages of friction.

9. Friction produces heat. Explain this statement by giving two examples.

10. Differentiate between static friction and sliding friction.

Chapter 13: Sound

Topic: Production of Sound and Propagation

1. How is human voice produced?

2. How do we hear sound?

3. Which characteristic of the human body is responsible for the difference in the voices of males and females?

4. Lightning is usually seen a little before thunder is heard. Explain.

5. Why can’t sound travel through vacuum?

Topic: Characteristic of Sound

6. What is the hearing range of a normal human ear?

7. In order to alert dogs of the presence of thieves, policemen blow whistles at a frequency of 25893 Hz. Would a thief be able to listen to the sound produced by the whistle? Give a reason for your answer.

8. Which property of sound is determined by frequency? Explain its variation with frequency with the help of examples.

9. A pendulum oscillates 400 times in 40 seconds. Find its frequency and time period.

10. What is noise pollution? Give two sources of noise pollution.

Chapter 14: Chemical Effects of Electric Current

Topic: Conductivity of Liquids

1. During an experiment, Reeta takes three beakers X, Y and Z, each filled with a different liquid. The liquids used by her are nitric acid solution, pure water and vinegar. She connects each beaker one by one to an electric circuit.

Her observations are as follows: In case of beaker X, the bulb does not glow. In case of beaker Y, the bulb glows very dimly. In case of beaker Z, the bulb glows brightly. Identify the liquid contained in each beaker. Explain your answer.

2. A teacher asked her students to construct a tester in order to test the conductivity of liquid. What are the necessary requirements for constructing this tester? Draw a diagram to show the construction of the circuit?

3. Differentiate between good and poor conductors of electricity. Give examples of each.

Topic: Chemical Effect of Current

4. Mention any two visible changes that a solution can undergo when electric current is passed through it.

5.

Which effect of current is exhibited by the given activity?

6. Two electrodes are immersed in impure water such that electricity is generated. Which gases would we obtain at the two electrodes?

Topic: Electroplating

7. What is electroplating?

8. (i) Name a process used in industries for coating metallic objects with a thin layer of a different metal. (ii) Why is food stored in tin cans and not in iron cans?

9. In the process of electroplating, if the two electrodes are interchanged and the entire process is repeated, then what would be observed?

10. Write two properties of chromium that make it a good electroplating element.

Chapter 15: Some Natural Phenomena

Topic: Electrical Charge

1. What happens when a charged body is brought close to an uncharged body? What is this process called?

2. If you rub two inflated balloons against a woollen cloth, and then bring them close to each other, then what will you observe? Explain your answer.

Topic: Interaction between Charges

3. Two thin aluminium strips, P and Q, are attached to a wooden block, as shown in the given figure.

A negatively charged rod is brought in contact with strip Q, as shown in the given figure. What is the nature of the force acting between the strips?

4. A ball pen refill is first rubbed vigorously against polythene and then brought near some small pieces of paper. It is observed that it attracts the small pieces of paper. Explain.

Topic: Construction and Working of an Electroscope

5. Name an instrument that can be used for detecting a charged body. Describe its functioning with the help of a diagram.

Topic: Charging by Conduction

6. Why are buildings earthed?

Topic: Lightening

7. How is lightning produced?

8. In case lightning strikes, what is the safest position for the body to be in?

Topic: Earthquakes

9. Draw the structure of the earth and label its layers. Name the layer on which earthquakes occur.

Topic: Richter Scale

10. Name the scale on which the power or intensity of an earthquake is measured. An earthquake is measured 3 on this scale in Delhi, while at the same time it is measured 7.2 in Kashmir. Which place would have higher risk of damage?

11. Richter scale is not a linear scale. What does this statement mean?

Topic: Seismograph

12. Label the various parts of the instrument shown in the given figure.

13. Fill in the blanks. (i) The outermost layer of the earth consists of some moveable pieces called _______. The boundaries of these pieces form weak zones, called ________, in earth’s crust. (ii) An earthquake is said to be destructive if the reading on the ________ scale is more than _______.

Topic: Precautions During Earthquakes

14. What safety measures should one take during an earthquake?

15. Raju was sleeping on his bed when he felt the tremors of an earthquake. What should he do in order to protect himself?

Chapter 16: Light

Topic: Laws of Reflection

1. State the laws of reflection?

2. What is the phenomenon of lateral inversion?

3. A light is incident on a plane mirror in such a way that it makes an angle of 32° with the normal. Calculate the angle made by the reflected light ray with the mirror plane.

4. What is the magnification of an image produced by a plane mirror?

5. Differentiate between regular reflection and diffused reflection.

Topic: The Human Eye

6. Draw a labelled diagram of the human eye and explain the functioning of the following parts. (i) Iris (ii) Lens (iii) Retina

7. What kind of lens is present in our eyes?

8. Explain why we cannot see objects clearly when we enter a cinema hall from a brightly lit room.

9. What is the function and location of the aqueous and vitreous humour?

10. Name four non-optical aids used by visually-challenged persons.

Chapter 17: Stars and The Solar System

Topic: Eclipses and Tides

1. If a full moon appears on a Sunday, then when will be the next full moon day? Explain your answer.

2. How does the moon appear at night even though it does not have its own light?

Topic: Surface of the Moon

3. (i) Explain the term phases of moon? (ii) Write the characteristics of the surface of the moon?

Topic: Facts about Stars

4. Why don't we see stars during the day?

Topic: Constellations

5. Which star always appears at the same position in the night sky throughout the year?

Topic: Sun and Planets

6. What is the solar system? List the inner and outer planets of the solar system.

7. List the essential environmental conditions responsible for the existence of life on planet Earth. Is there any other planet in the solar system that has all these environmental conditions?

Topic: Asteroids, Comets, Meteors

8. Answer the following questions. (i) What are asteroids, and where are they mostly observed? (ii) Why do meteors burn out when they enter the earth’s atmosphere?

Topic: Artificial Satellites

9. (i) What are geostationary satellites?

(ii) What is the role of communication satellites?

10. What are the uses of remote sensing satellites?

Chapter 18: Pollution of Air and Water

Topic: Sources of Air Pollution

1. Name a source of air pollution that pollutes the air by releasing tiny particles of ash into the atmosphere.

Topic: Effects of Air Pollution on Living Organisms

2. Write two harmful effects of air pollution.

Topic: Effects of Air Pollution on Non-Living Objects

3. The white marble used in the Taj Mahal is turning pale. Why?

Topic: Initiatives to Reduce Air Pollution and Global Warming

4. Why is CNG considered to be a cleaner fuel?

5. All public transport vehicles in Delhi have to mandatorily use a particular type of fuel. Name this fuel.

Topic: Greenhouse Effect and Global Warming

6. Fill-in the blanks. (i) __________ (CFC/CO2) is primarily responsible for the depletion of the ozone layer. (ii) __________ (Water vapour/Sulphur dioxide) is produced by the burning of coal. (iii) __________ (O3/O4) is the molecular formula of ozone. (iv) An increase in earth’s temperature by a layer of gases is known as __________ (greenhouse effect/global warming).

7. The global average temperature change is shown in the given graph.

How will this change in temperature affect humans?

Topic: Sources of Water Pollution: A Case Study of River Ganga

8. What is the Ganga Action Plan aimed at?

Topic: Water Pollution

9. In a city, the waste from factories is directly disposed into a river. The given graph shows the reduction in the quantity of fish caught (in quintals) from the river in the past 25 years. Instances of food poisoning, because of consumption of fish, have also been on the rise in the city.

What could be the possible reasons behind the trend shown in the given bar graph?

Topic: Conservation and Purification of Water

10. Tap water sometimes appears milky. What is the reason for this phenomenon? Is this water safe for drinking?

Grade 8 Science Solutions

Chapter 1: Crop Production and Management

Topic: Basic Practises of Crop Production

1. Rice requires a large quantity of water for its growth. Hence, growing it in the rainy season fulfils its water requirement.

2. Apart from soil, plants can obtain nutrients from manures and fertilisers, which are added externally. Manures are organic matter such as cattle dung, oil cakes, vegetable wastes etc.which are obtained from plant or animal wastes. These animal or plant wastes are decomposed by microorganisms such as bacteria and fungi. This decomposed matter is used as manure. Fertilisers are substances that are rich in inorganic nutrients. They provide specific nutrients such as nitrogen, phosphorus and potassium to the soil.

3. Different crops have different nutrient requirements. Therefore, to replenish the lost nutrients of soil, a new crop with different nutrient requirements (instead of the same crop) can be grown in the next season or year. For example, in a field where legumes (as fodder crop) are grown in one season, wheat can be grown in the next season. Legumes contain Rhizobium bacteria in their root nodules, which fixes the atmospheric nitrogen in the soil. Legumes enrich the soil with nitrogen. This practise of growing alternative crops is known as crop rotation.

4. The first step of soil preparation is tilling the soil upside down. This step is called ploughing. The importance of ploughing is stated as follows: (i) It helps loosen the soil. Hence, it improves air circulation in soil. (ii) It helps in the retention of moisture. (iii) It helps in uprooting weeds from soil. (iv) It enhances the water-retaining-capacity of soil.

5. Traditional methods of irrigation include: (i) Dhekli (ii) Rahat (iii) Moat (iv) Chain pump Modern methods of irrigation include: (i) Sprinkler system

(ii) Drip irrigation

Topic: Crop Improvement

6. Healthy seeds can be selected by the sorting and floatation method. For selecting healthy seeds, put them in a container filled with water. Shake the container well and leave it aside. You will observe that while some seeds start floating, others settle at the bottom. Usually, the floating seeds are spoiled or eaten by pests, while the healthy seeds sink to the bottom of the container because they are heavy.

7. Some of the advantages of using manure over fertilisers are − (i) It is a natural substance. So, it does not cause any damage to the soil and plants. (ii) The addition of manure makes the soil porous. (iii) It increases the humus content of the soil. (iv) It also increases the water-retaining capacity of the soil.

8. The sequence of agricultural practices as carried out by farmers is as follows: (i) Preparation of soil (ii) Sowing (iii) Adding manures and fertilisers (iv) Irrigation (v) Protecting crops from weeds (vi) Harvesting (vii) Storage

9. The cultivator is a tractor-driven machine that helps in loosening and upturning of the soil (soil preparation) before the sowing of seeds.

10. Urea is a chemical fertiliser, while vermicompost is manure. In the long run, manures are more effective as compared to fertilisers because the latter decrease the fertility of the soil when used for a longer period of time, whereas no such threat is posed in the case of manures.

11. Seed drill allows the sowing of seeds at an appropriate depth. Seeds sowed at a certain depth cannot be picked by birds. This reduces overcrowding and results in uniform growth, thereby providing sufficient nutrients, water and sunlight to all plants.

Topic: Harvesting and Storage of Crops; Animal Husbandry

12. The process of gathering mature crops from crop fields is known as harvesting. Different crops have different maturing time. Therefore, they also yield at different times. Harvesting methods may vary depending on resources and crops. They can be differentiated into the following types. (i) Manual harvesting with the help of a sickle. (ii) Use of tractor-driven machines called harvesters in the case of wheat and paddy.

(iii) Use of plucking in the case of fruits and vegetables.

13. Combine is a modern tool which performs dual function of harvesting and threshing of the produce.

14. The produce, which is harvested, should be stored safely to prevent it from getting eaten by pests and rodents. Farmers employ a number of storage methods to protect their produce and prevent it from getting spoilt. Some of these methods are listed as follows: Farmers sun-dry their seeds before storing them to reduce moisture in them. This protects the grains from insect pests, bacteria, and fungi. Farmers store grains in jute bags or metallic bins. Granaries and silos are used for large-scale storing of grains. This protects the grains from pests such as insects and rodents. Dried neem leaves are also used for storing grains at home. On a larger scale, certain chemicals are used to protect the grains from pests and microorganisms.

15. Farmers sun-dry their grains before storing them in order to reduce their moisture content. This protects grains from insects, pests, bacteria and fungi. Dried neem leaves are also used for storing grains at home. On a larger-scale, certain chemicals are used for protecting grains from pests and microorganisms.

Chapter 2: Microorganisms

Topic: Microorganisms - Their Habitat and Classification

1. Microorganisms are present everywhere in water, air, and soil. In fact, microorganisms can survive under all types of environment. They are found in different environments, ranging from icy cold mountains to hot springs and from deserts to marshy lands. Some can survive in highly saline or acidic conditions. Some microorganisms are also found living on or inside the body of other organisms including humans.

2. Microorganisms are classified into the following groups. (i) Bacteria (ii) Fungi (iii) Protozoa (iv) Algae (v) Viruses

3. (i) Athlete's foot − Because it is a fungal disease while rest of the diseases are caused by bacteria (ii) Rhizobium − Because Rhizobium is a bacterium while others are protozoa (iii) Pneumonia − Because pneumonia is a disease caused by bacteria while the other alternatives are nutrient deficiency diseases (iv) Cristae − Because it is present in mitochondria while other alternatives given are parts of nucleus (v) Xylem − Because xylem is a complex permanent tissue while others are simple permanent tissues

4. Microorganisms are the living organisms which are not visible with naked eye. Microorganisms are present everywhere in water, air, and soil. In fact, microorganisms can survive under all types of environment. Therefore, they are found in different environments, ranging from icy cold mountains to hot springs and from deserts to marshy lands. Some microorganisms are also found living on or inside the body of other organisms including humans. Hence, it is correct to say that microorganisms are omnipresent.

5. The organism in the given figure is a virus. Viruses are also microorganisms, but they cannot reproduce on their own and require a host cell to complete their life cycle. They do not have cellular apparatus of their own and hence require the cellular machinery of the host to survive. They essentially are parasitic and hence are responsible for causing a number of diseases in humans such as polio and chickenpox.

Topic: Uses of Microbes

6. Nitrogen fixation refers to conversion of atmospheric free nitrogen into nitrogen

compounds, which can be utilized by plants. The fixation of nitrogen can occur in two ways- Rhizobium bacteria present in the root nodules of leguminous plants help in the fixation of atmospheric free nitrogen into compounds of nitrogen in soil. The other way by which nitrogen gets fixed is by the action of lightning.

7. Salt and sugar help in preservation of food by reducing the moisture content of the food. Microbes need conditions with plenty of moisture for their growth. Hence, their growth is restricted in the absence of moisture.

8. (i) Production of curd (ii) Production of butyl alcohol and acetone (iii) Making of breads and cakes (iv) Causes food poisoning (v) Edible mushroom

9. (i) Acetobacter aceti (ii) Clostridium butylicum (iii) Rhizobium (iv) Penicillium (v) Bacillus anthracis

10. The process by which nitrogen is recycled between biotic and abiotic components of the environment is:

Topic: Microbes as Harmful Agents

11. Disease-causing microorganisms are called pathogens. Four modes through which pathogens spread from a diseased-person to a healthy person are as follows: (i) Through air. Example: Tuberculosis bacteria, common cold virus (ii) Through water. Example: Cholera bacteria, hepatitis virus (iii) Through contact. Example: Ringworm fungus, swine flu virus (iv) Through blood. Example: AIDS virus

12. Diseases such as cholera and typhoid are water-borne diseases. They can be controlled by (i) ensuring proper disposal of sewage (ii) ensuring supply of safe drinking water (iii) maintaining good sanitary habits (iv) drinking boiled water (v) getting vaccinated at the right time

13.

Disease Characteristic Feature

Malaria Causative agent − Plasmodium

AIDS Immune system gets damaged

Disease

Characteristic Feature

Chicken pox Viral disease which causes watery blisters on the skin

Ringworm Caused by fungi

Pneumonia Air borne disease

African sleeping sickness Caused by Trypanosoma

14. The causative microbe for citrus canker is bacteria and it spreads through air.

15. Carriers are insects or animals that carry the disease-causing microbes from a diseased person to a healthy person. Carriers such as house flies sit on garbage and sewage because of which pathogens stick to their bodies. Later, when they sit on food, they contaminate it. A healthy person falls sick on consuming contaminated food. Other carriers such as mosquitoes bite a diseased person and acquire pathogens in the saliva. When they bite a healthy person, they transfer the pathogens to the bloodstream of the healthy person.

Chapter 3: Synthetic Fibres and Plastics

Topic: Synthetic Fibres

1. (i) Fibres that are made artificially are called synthetic fibres. They are also known as man-made fibres. (ii) Small units that are known as monomers combine to form large molecules that are known as polymers. (iii) Nylon/Rayon/Acrylic is an example of a synthetic fibre.

2. Synthetic fibres are more durable and affordable. It is for this reason that synthetic fibres are more popular than natural fibres.

Topic: Properties and Uses of Rayon

3. Rayon is mixed with cotton to manufacture bed sheets and with wool to manufacture carpets.

4. Rayon is prepared by chemical treatment of wood pulp. Thus, it is not considered a true synthetic fibre.

Topic: Properties and Uses of Nylon

5. Nylon is a very strong, elastic and lightweight fibre. Hence, it is used for making the ropes of parachutes.

6. Nylon has the following characteristics: (i) It is considered to be the first fully synthetic fibre. It is quite lustrous and easy to wash. (ii) It absorbs very less water. Hence, it requires very less time to dry. (iii) It is very strong and elastic. Therefore, it is used for making ropes for mountaineering and parachutes.

Topic: Polyester and Acrylic

7. Polyester is a synthetic fibre made of repeating units of esters. Terylene, PET, polywool, and terrycot are common examples of polyester.

8. Wool is obtained from natural sources and is quite expensive, whereas clothes made from acrylic are relatively cheaper. These clothes are more durable and moth-resistant.

Topic: Types of Plastics

9. Plastics which get deformed easily on heating and can be bent easily are known as thermoplastics, e.g., polythene and PVC.

They are used for manufacturing toys and different types of containers. On the other hand, plastics which once moulded, cannot be bent or softened even by heating are called thermosetting plastics, e.g., bakelite and melamine. They are used for making electrical switches and floor tiles.

10. Plastics are materials of choice because of the following reasons: (i) They are non-reactive with water and air. Hence, they are used to store various materials including chemicals. (ii) They are very light, strong, durable, and can be moulded into different shapes and sizes for various purposes. (iii) They are cheaper than metals. (iv) They are poor conductors of heat and electricity. Hence, they are used for making coverings of various electrical appliances.

11. Plastics that can be easily deformed on heating are called thermoplastics, whereas those that do not get deformed on heating are known as thermosetting plastics. The polythene bags that we generally use are thermoplastics because they can be easily deformed on heating.

Topic: Properties and Uses of Plastics

12. Plastics do not react with water and air. They are, therefore, resistant to corrosion. It is for this reason that containers made of plastics are used for storing different materials.

13. Combs are generally made of thermoplastic.

Topic: Plastic and the Environment

14. Plastics are non-biodegradable substances. They do not get easily decomposed by the action of bacteria and other microorganisms. Hence, disposal of plastics is a major problem.

15. 4 R in the 4 R principle represent reduce, reuse, recycle, and recover.

Chapter 4: Materials, Metals and Non-Metals

Topic: Physical Properties of Metals and Non-Metals

1. (i) False (ii) False (iii) True (iv) False

Topic: Reactions of Metals and Non-Metals with Oxygen and Water

2. Metals react with oxygen to form basic oxides. Among the given elements, only calcium is a metal. Hence, calcium will yield a basic oxide.

3. Sodium and potassium react violently with cold water. Observations: (i) They react violently with water. (ii) Heat is evolved during the reaction (i.e., the reaction is exothermic). (iii) The evolved gas catches fire.

Topic: Reactions of Metals and Non-Metals with Acids and Bases

4. (i) Magnesium vigorously reacts with dilute hydrochloric to form magnesium chloride and hydrogen. (ii) Copper does not react with dilute hydrochloric at all.

5. Food items such as curd and juices of citrus fruits are acidic in nature. Metals react with acids to form salts and hydrogen gas. If such items are kept in metallic containers, then a reaction between the metal and the acid (present in the food item) can take place. Metals, from which containers are usually made, such as iron, aluminium and copper, are more prone to acid attack. Sometimes, these reactions result in the formation of toxic substances. Hence, acidic food substances should not be stored in metallic containers.

Topic: Displacement Reactions

6. No reaction will take place when iron filings are added to a solution of aluminium sulphate. In a single displacement reaction, a more reactive element displaces a less reactive element from a compound of the latter. But, iron is less reactive than aluminium. Thus, iron cannot displace aluminium from aluminium sulphate solution. Hence, no reaction will take place.

7. The reaction will take place only in case of test tube X where, iron nail will react with copper sulphate solution, turning it pale green from blue. A red powdery substance (solid

copper metal) will be deposited on the surface of the nail.

Topic: Uses of Metals, Non-Metals and Alloys

8. Aluminium is extremely light in weight. Therefore, it is used to make parts of an aircraft.

9. (i) Bronze is used for making coins. (ii) Steel is used for construction of railway tracks.

10. (i) Oxygen (ii) Nitrogen (iii) Chlorine/Iodine

Chapter 5: Coal and Petroleum

Topic: Renewable and Non-Renewable Sources of Energy

1. (i) A solar cell is a device which converts solar energy into electricity. It is a sandwich of a silicon-boron layer and a silicon-arsenic layer. (ii) Biogas is an ideal fuel for domestic use because it is a cheap and clean fuel, which does not leave behind any residue.

Topic: Disadvantages of Fossil Fuels

2. The protection of the environment must be ensured at all times because a majority of natural resources are limited. Human population and the utilisation of natural resources are increasing at a tremendous rate. We must also try to reduce the damage caused to the environment because of the over-exploitation and utilisation of these natural resources. Also, the waste generated from the utilisation of these resources must be disposed of sensibly.

Topic: Coal - Its Formation and Characteristics

3. Dead vegetation is converted into coal by a very slow process. As coal mainly contains carbon, this process is known as carbonisation.

4. Millions of years ago, Earth had a thick covering of vegetation and plants. This rich vegetation was mainly found in the low-lying areas. Over the course of time, because of heavy rains, these areas got flooded. Other natural phenomena such as earthquakes lead to the falling of large number of trees. Flooding caused sand and clay to deposit over these plants. With time, more sand and clay got deposited in these areas. As a result, the plants were compressed under high pressure. As their depth increased, temperature also increased. Owing to high pressure and temperature conditions, the vegetation gradually got converted into coal. Thus, the formation of coal takes millions of years. This slow process of conversion of dead plants into coal is called carbonisation.

5. Five characteristic features of coal are as follows: (i) It is black in colour. (ii) It is as hard as stone. (iii) On burning, it releases a lot of energy. (iv) It is formed from dead vegetation. (v) It takes millions of years to form.

Topic: Products of Coal and Their Characteristics

6.

Product Characteristics Uses

Coke Tough, porous, black, purest form of carbon

Manufacture of steel, extraction of several metals

Coal tar Black, thick, pungent smelling liquid and mixture of 200

substances

Manufacture of synthetic dyes, drugs, naphthalene balls, perfumes, paints etc.

Coal gas Gas obtained during conversion of coal to coke

Industrial fuel, street illumination

7. (i) Bitumen is used for surfacing the roads. (ii) Coke, a product of the destructive distillation of coal, is used in the extraction of metals. (iii) The primary constituent of coal is carbon. (iv) Coke, coal tar and coal gas are produced by the processing of coal.

8. A foul-smelling dark brown liquid obtained by destructive distillation of coal and used to manufacture dyes, explosives, perfumes, etc. is coal tar.

Topic: Petroleum - Its Formation and Characteristics

9. Petroleum or crude oil is derived from marine life forms, which got buried under the sea bed millions of years ago. Heat and pressure under the layers of soil, rocks, and water helped to bring about the chemical changes. Bacterial action also helped to change the marine life forms into oil and gas.

10. (i) Gas is the lightest of all the three. Therefore, it is at the top. Oil being heavier than gas settles beneath it and water being the heaviest settles down at the bottom.

Topic: Petroleum, Its Products and Their Uses

11. Petroleum refining separates various constituents of petroleum. Petroleum gas, petrol, kerosene, diesel, lubricating oil, paraffin wax, and bitumen are obtained from refining.

12. (i) Diesel is used as fuel for heavy motor vehicles. (ii) Lubricating oil is used for lubrication.

13. (i) L.P.G (ii) Bitumen (iii) Paraffin wax

(iv) Petrol

Topic: Conservation of Petroleum

14. Petrol/diesel can be saved by: (i) walking or using a bicycle for covering short distances, and carpooling for covering long distances (ii) driving at a constant and moderate speed (iii) properly maintaining one’s vehicle, and getting it serviced regularly (iv) not applying brakes and/or accelerating rapidly

Topic: Compressed Natural Gas

15. Natural gas is found above the petroleum deposits. It is used as a fuel and is considered as a clean fuel. This is because it releases only carbon dioxide and water vapours on burning. It can be easily transported through pipelines. It is compressed and stored under high pressure. It takes millions of years to form and is found deep inside the earth. Uses of natural gas are as follows: (i) It is used as a fuel at home and in industries (ii) It is used as a fuel for motor vehicles in the form of CNG (iii) It is used in the production of chemicals and fertilisers (because it can be used to produce hydrogen gas, which is used in manufacturing urea) Natural gas is advantageous as compared to other fuels because of the following reasons: (i) It can be transported easily through pipes. (ii) It burns without leaving any residue. (iii) It is a clean fuel.

Chapter 6: Combustion and Flame

Topic: Essential Conditions Required for Combustion

1. The maximum amount of ash will be produced by the wood splint in case A. Air is necessary for combustion. In case A, air can enter the jar from the bottom of the setup, while hot air can leave from the top. Thus, enough oxygen is available in this case for burning to continue. In case B, there is not enough space for fresh air to come in. As a result, the wood splint in this case will give out smoke and will go off after some time. In case C, air cannot come in from any side. Thus, the wood splint will go off as soon as it is covered from the top.

2. The rate of combustion depends upon the following factors: (i) Size of combustible particles For example, coal dust burns more rapidly than solid coal. (ii) Nature of combustible substances For example, inflammable substances such as petrol, burn more rapidly as compared to wood. (iii) Nature of gaseous environment For example, combustible substances burn more rapidly in the presence of oxygen. (iv) Ignition temperature of combustible substances For example, substances having low ignition temperature such as ether burn rapidly.

Topic: Methods to Control Fire

3. When heated, sodium carbonate decomposes to form carbon dioxide gas. Carbon dioxide gas does not burn, nor does it support burning. Thus, when it is released, it expands enormously in volume and then cools down. As a result, it forms a blanket around the fire, cutting off direct contact with oxygen (which is essential for combustion to take place).

4. The method shown in the figure B should be used in case a fire breaks out at a power house. In this method carbon dioxide, which poses no danger, is used for extinguishing the fire. On the other hand, water is a good conductor of electricity. It will allow the flow of current, thereby exposing firemen and the persons trapped in fire to electric shocks.

Topic: Types of Combustion

5. The combustion reaction in which substances burn to produce huge amounts of heat, light and sound is an example of the process of explosion.

6. Among the given substances, phosphorus will undergo spontaneous combustion.

Topic: Different Parts of a Flame

7.

Topic: Characteristics of a Good Fuel

8. (i) It should be cheap and easily available. (ii) It should have high calorific value and a moderate ignition temperature (neither too high nor too low). (iii) It should not leave behind any undesirable substances. (iv) It should burn easily at a moderate rate.

9. Fuel B is the best fuel among the given fuels. This is because it has a high calorific value and a moderate ignition temperature. Fuel A has high calorific value but its ignition temperature is very low i.e., 7ºC. Thus, it will catch fire at room temperature. Fuels C and D are not good fuels as their calorific values are on the lower side. Also, the values of their ignition temperatures are quite high.

Topic: Harmful Effects of Combustion of Fuels

10. (i) True (ii) True

Chapter 7: Conservation of Plants and Animal

Topic: Conservation of Plants

1. Deforestation can occur because of natural calamities such as forest fires, droughts, severe cyclones etc. However, the major cause of deforestation is the cutting of trees by humans for the purpose of cultivation, grazing land, making furniture, obtaining fuels, building houses and factories etc. Deforestation can lead to the following consequences: (i) A decrease in groundwater levels. (ii) An increase in pollution. (iii) An increase in the amount of carbon dioxide in the atmosphere, thereby leading to global warming. (iv) Disturbances in the water cycle and a reduction in rainfall. (v) Soil erosion (vi) Desertification

2. Some of the steps that can be taken towards the conservation of forests are as follows: (i) Replanting trees at the same spots from where they were cut or uprooted. The replanted tree must be of the species whose tree had been cut in the first place. (ii) Leaving the reforested area undisturbed can help in the rehabilitation of that area. (iii) Recycling of paper and other such similar activities can help prevent deforestation.

3. Yes. A fertile land can change into a desert. One of the causes of the conversion of fertile land into a desert is desertification. The removal of vegetation leads to rapid soil erosion, in which the top-most layer of soil gets blown away by wind. This gradually decreases the fertility of soil. As a result, it is unable to support any vegetation and the land gradually starts changing into a desert.

4. Plants are necessary for the survival of all animals on earth. They take up carbon dioxide and release oxygen, which is essential for the survival of all living organisms. The continuous cutting of trees will lead to a variety of adverse effects such as (i) a decrease in groundwater levels (ii) an increase in pollution (iii) an increase in the amount of carbon dioxide in the atmosphere, thereby leading to global warming as fewer trees will be available for utilizing the increased levels of carbon dioxide (iv) a disturbance in the water cycle because of an increase in global temperatures, which will lead to a reduction in rainfall (v) the loss of soil fertility and texture because trees hold the fertile top soil

5. Deforestation can lead to disturbances in water cycle, which can result in droughts. Also, when the ground is devoid of vegetation, it becomes difficult to hold rainwater. As a

result, the infiltration rate of water (rate at which water seeps into the ground) is reduced. This causes flooding of grounds.

Topic: Conservation of Animals

6. Wildlife Sanctuary (i) It is an area where a particular species of animals is protected. (ii)Traditional activities such as collecting firewood and grazing are allowed, e.g., Gir Sanctuary. National Park (i) It is a wide area of land where animals are allowed to freely live in their habitat. (ii)Traditional activities are not allowed, e.g., Kanha National Park.

7. Endangered species are those species whose numbers are rapidly diminishing to the level of extinction. A record of all plants and animals, which are listed in the endangered list, is maintained in the Red Data Book. Since this book keeps a record of all the endangered animals and plants, it becomes easier to identify the endangered species and take appropriate actions towards the conservation of these species.

8. Tiger is an endangered species. The main objective of project tiger is to ensure the survival of tigers and to maintain their population. Satpura national park in Madhya Pradesh was the first forest reserve in India. It consists of finest Indian teak and also has a tiger reserve. Satpura national park comes under the project tiger.

9. (i) Biodiversity refers to the variety of living organisms in a specific area. (ii) Migration is defined as the movement of animals from one habitat to another for breeding, during a particular period of time every year.

10. Birds migrate in response to changes in food availability, habitat or weather conditions. Sometimes weather conditions or competition for food and resources is so severe for birds that if they do not migrate, their survival would be at risk. Therefore, every year at a particular time, birds undertake their migratory journeys.

Chapter 8: Cell - Structure and Function

Topic: Cell - Structure and Functions

1. In prokaryotic cells (i) The nuclear material is found in the cytoplasm. (ii) It lacks the well defined nuclear membrane around it. (iii) The cell organelles are absent in prokaryotic cells. Example: A bacterial cell The eukaryotic organisms (i) It have a well defined nucleus containing nuclear material. (ii) It lacks the well defined nuclear membrane around it. (iii) The cell organelles are present in a eukaryotic cell. Examples: Cells of plants, animals etc.

2. White blood cells and Amoeba do not have a definite shape. Their cells have the ability to change their shapes. For example,Amoeba keeps changing its shape by producing various projections across its cell surface called pseudopodia. These projections help Amoeba in feeding and moving. White blood cells move along the bloodstream and out of it by changing their shape.

3.

4. Plants lack the mechanism of locomotion. Plants have the following advantages at cellular level to compensate for the loss: (i) Chloroplast is present only in plant cells to perform photosynthesis. This enables plants to prepare their own food since they cannot move in search of food. (ii) Due to lack of locomotion, plants cannot escape dangerous or adverse situations. The cell wall is an additional protective, rigid structure present outside the cell membrane. It protects them from heat, humidity, pressure, etc.

5. Robert Hooke examined a section of a cork under the microscope. Cork is made up of dead cells. Therefore, the first cells to be observed by humans were dead cells.

Topic: Cell Organelles

6. (i) Lysosomes in the cell are called ‘suicide bags’ because lysosomes release digestive enzymes that digest the cell organelles and sometimes they destroy themselves. (ii) Golgi bodies in plants are called dictyosomes.

7. (i) The functions of vacuole are as follows: A. It maintains the turgidity of the cell. B. It stores water and other materials in the cell.

(ii) Tonoplast (iii) Plant cells have a single large vacuole whereas animal cells either do not have vacuole or have many small vacuoles.

8. (i) Nucleus is the control centre of a cell. It controls all the activities of the cell. It also contains gene-containing chromosomes. Genes are the units of inheritance in an organism. (ii) Chloroplast is a plastid. It is found only in plant cells. It contains chlorophyll, which is important for photosynthesis.

9. The types of plastids are: (i) Chromoplasts − These are the coloured plastids, which contain pigments. Chloroplast is a type of chromoplast that contains the green pigment chlorophyll. (ii) Leucoplasts − These are the colourless plastids. Chloroplasts are called kitchen of the cell because they have chlorophyll that helps in the process of photosynthesis. It is the process by which plants prepare their own food.

10. (i) Mitochondria are enclosed by double membranes. (ii) Outer membrane is porous while the inner membrane is deeply folded. These folds are called cristae. (iii) Mitochondria have their own DNA and proteins. Hence, they are semi-autonomous.

Chapter 9: Reproduction In Animals

Topic: Modes of Reproduction

1. The process called reproduction enables the continuous existence of an organism on earth.

2. The rearing of silkworms to obtain silk fibre is known as sericulture. A large number of cocoons are used for obtaining silk fibre. The cocoons are boiled or exposed to steam or kept under the sun, which leads to the separation of silk fibres.

3. In egg laying animals such as hens, after fertilisation, when the egg travels through the oviduct, it gets covered with various protective layers, including the shell. The hen lays eggs and it takes around three weeks for the embryo to develop into a chick. Hens sit on their eggs to keep the egg warm and to maintain a proper temperature for the growth of the embryo inside the egg. At the end of three weeks, the egg cracks open and the chick comes out.

4. Reptiles are oviparous because they lay eggs and do not give birth to young ones directly.

5.

External Fertilisation Internal Fertilisation

Fertilisation occurs outside the body of the female.

Fertilisation occurs inside the body of the female.

The female lays unfertilised eggs. The female either lays fertilised eggs or gives birth to young ones.

Example: frog Example: hen, humans

Topic: Sexual Reproduction In Humans

6. In humans, fertilization leads to the formation of the zygote which gives rise to the embryo. The embryo gets attached to the wall of the uterus for further development. The embryo develops and various structural changes begin to take place. The embryo is called a foetus when all the parts of the body can be distinguished.

7. In the female, the production and release of ova or egg follows a cyclic process every 28-30 days. This cycle is governed by hormones and is called the menstrual cycle. The first menstruation is called menarche. Menarche occurs during puberty.At the age of 45 − 50, the process of menstrual cycle stops and this stage is termed as menopause.

8. (i)

(ii) In humans, fertilisation takes place inside the body of the female. Fertilisation results in the formation of zygote, which then develops into an embryo. The embryo gets implanted on the walls of the uterus. It gradually develops body parts, and at this stage, it is called a foetus. When the development of the foetus gets completed, the mother gives birth to the baby.

9. Testes

10. The human male reproductive system includes a pair of testes, two sperm ducts, and a penis. The functions of these structures are given as follows: (i) Testes: They produce the male gametes called sperms. (ii) Sperm ducts: They help in conducting the sperms from the testes to the penis, from where they are discharged outside the body. (iii) Penis: The penis helps in the introduction of sperms into the female reproductive tract.

11. After fertilisation, the zygote so formed undergoes divisions to form the embryo. The embryo gets implanted in the endometrium. The outer tissue surrounding the embryo develops finger-like projections called villi. These projections are surrounded by uterine tissues and maternal blood. This provides a large surface area for the exchange of oxygen and nutrients. The waste materials formed are also removed with the help of the placenta.

The foetus develops inside the mother’s womb for about nine months.

Topic: Reproduction and Development in Animals

12. The lifecycle of a silkworm starts when a female moth lays eggs on the mulberry leaves. After some time, these eggs hatch into larvae. These larvae are known as caterpillars or silkworms. Slowly these caterpillars grow in size by feeding only on the leaves of mulberry trees. The next stage in the lifecycle of a silkworm is a pupa. When the caterpillar has grown large enough and is ready to go to the stage of pupa, it starts weaving a net around itself. It moves its head from one side to another, making a figure eight (8). During these movements, caterpillars secrete a fibre made of protein, which solidifies and hardens when exposed to air for some time. This is the silk fibre. Caterpillars continue to spin silk fibre till they completely get covered by these fibres. This silk covering in which a silkworm covers itself is known as a cocoon. Further development of the silkworm takes place inside the cocoon and it enters the pupa stage. After some time, the pupa enters into the adult stage and emerges out of the cocoon as a moth. The whole process thus continues and is known as the lifecycle of a silkworm.

13. In frogs, the hormone thyroxine (produced by the thyroid gland) initiates the process of a tadpole’s metamorphosis into an adult frog. In the absence of thyroxine, the tadpole will not transform into an adult and will remain in the tadpole stage.

14. Hen is an oviparous animal. In hen, after fertilization, when the egg travels through the oviduct, it gets covered with various protective layers, including the shell. After the hen lays eggs, it takes about three weeks for the embryo to develop into a chick.

15. Eggs form the first stage in the life cycle of frogs. Eggs of frogs hatch to produce the larval stage of the frog known as the tadpole. The tadpole has a tail and fins (similar to a fish), which help it to swim in water. The tadpole also has gills, which aid it in breathing underwater. During the growth phase, the tadpole grows into an adult frog and loses its tail and fins. The process of transformation from a tadpole to an adult frog is known as metamorphosis.

Chapter 10: Reaching The Age Of Adolescence

Topic: Puberty, Adolescence and Secondary Sexual Characteristics

1. (i) Adolescence is defined as the period of life in which the body undergoes changes leading to reproductive maturity. (ii) Puberty is the stage at which the changes during adolescence occur.

2. (i) Testosterone is the male sex hormone and oestrogen is the female sex hormone. (ii) Secondary sexual characters are changes that appear at puberty, which help to distinguish the male from the female.

3. In adolescents, the secretion of the sweat glands and sebaceous glands increase. This leads to the occurrence of acne and pimples.

4. Changes that occur in females during puberty: (i) Development of breasts (ii) Softening of voice (iii) The ovum begins to mature with the onset of puberty. (iv) Occurrence of the menstruation cycle (The first menstrual flow beginning at puberty is called menarche, and the stoppage of menstruation at the age of 45 − 50 years is called menopause.)

5. Personal health, hygiene, and nutrition can be maintained during adolescence by taking the following steps: (i) Consuming a balanced diet, containing all food nutrients in balanced proportions, is very essential during the stage of adolescence. (ii) An adolescent diet should include carbohydrates, fats, proteins, vitamins, and minerals in adequate amounts. (iii) Food items such as chips, cold drinks, and snacks should be avoided or their consumption should be restricted. (iv) At the time of puberty, the activity of the sweat glands increases considerably. Therefore, it is necessary for an adolescent to have a bath daily. (v) Girls should maintain personal hygiene at the time of menstruation to avoid various infections. (vi) Adolescents should do regular physical exercise to keep their body fit and healthy.

Topic: Chemical Coordination in Humans

6. Insulin helps in regulating sugar level in our blood. This hormone is secreted by pancreas gland.

7. Characteristics of hormones: (i) Act as chemical messengers

(ii) Secreted by endocrine glands (iii) Act upon specific cells/tissues/organs called target cells/tissues/organs (iv) Either accelerate or inhibit a reaction

8. Hormones are chemical messengers that regulate the physiological processes of an organism. They are secreted by glands. Hormones are required in minute quantities for the proper coordination of a particular life process. They have prolonged effects. The under secretion and over secretion of a particular hormone may have deleterious effects on the target organ. Example: (i) Thyroxin: The deficiency of this hormone results in hypothyroidism or goitre. (ii) Growth hormone: It is secreted by the pituitary, and is essential for the proper growth of the body. Over secretion of this hormone causes gigantism, while it’s under secretion causes dwarfism. (iii) Insulin: It is secreted by the pancreas. Under secretion of this hormone causes diabetes.

9. All endocrine glands release their hormonal secretions into blood. These hormones, on reaching a particular site in the body called the target site, bring about the necessary changes to maintain proper functioning of the body. This can be illustrated with the examples of hormones testosterone and estrogen. Testosterone and estrogen are produced by the testes and ovaries respectively. The production of these hormones is under the control of the pituitary gland. The hormones released by the pituitary stimulate testes and ovaries to produce testosterone and ovaries, respectively, into the bloodstream. From the bloodstream, testosterone and estrogen reach their respective target sites, where they stimulate various changes in the body at the onset of puberty. This process can be explained using the given flow chart.

10. Testosterone is responsible for the development of low-pitched voice in human beings.

11. Pancreas is both endocrine and exocrine in nature.

Topic: Reproductive Phase and Sex Determination in Humans

12. Chromosomes are thread-like structures found in the nucleus of all living cells. All human beings have 23 pairs of chromosomes, out of which two chromosomes are sex chromosomes.

13. At the time of puberty in boys, the voice box or the larynx begins to grow and protrude from the throat. This is called an Adam’s apple. The enlargement of the larynx makes the voice low-pitched and hoarse.

14. The instructions for determining sex are present in the chromosomes of the fertilised egg. All humans have 23 pairs of chromosomes. Of these 23 pairs, one pair of chromosomes is the sex chromosomes. They are of two types − X and Y. A female has XX sex chromosomes and a male has XY sex chromosomes. The gametes carry only one set of sex chromosomes. An unfertilised egg has an X chromosome, but a sperm can have either an X or a Y chromosome. Depending upon the type of chromosome that fuses with the egg, the sex of the baby is determined. If the egg is fertilised by a sperm containing X chromosome, then it results in a girl; however, if the egg is fertilised by a sperm containing Y chromosome, then it results in a boy.

15. Some animals rely completely on environmental factors like temperature for sex determination. For example, in some animals, when the eggs are incubated at low temperature, they turn into males and when the eggs are incubated at high temperature, they turn into females.

Chapter 11: Force and Pressure

Topic: Force

1. In order to close a door, one usually has to push the door. This is an example in which force appears as a push on an object.

2. Spring balance is a device used for measuring the force acting on an object. It consists of a coiled spring that gets stretched when a force is applied on it. The pointer moves on a graduated scale due to the stretching of the spring. The reading on the scale gives the magnitude of the force.

3. Three kinds of changes are possible when force is applied to a body. (i) Change in the shape of the body. Example: The shape of a lump of dough can be changed by applying a force on it. (ii) Change in the speed of the body Example: A biker applies brakes to decrease the speed of his bike. (iii) Change in the direction of motion of the body Example: A hockey player changes the direction of a moving ball with a flick of the stick.

4. Contact forces act when an object is in contact with the force-causing agent. Muscular force and frictional force are examples of contact forces. Non-contact forces act at a distance from the object and the force-causing agent. Magnetic force, electrostatic force, and gravitational force are examples of non-contact forces.

5. Frictional force is a result of the interlocking between two surfaces.

6. The frictional force between water and the boat is responsible for the boat coming to rest. This is a contact force because the frictional force arises because of the contact between water and the boat.

7. When force is applied in the direction opposite to the object’s direction of motion, it results in a decrease in the speed of the object

8. Force exerted by Rohan = 60 N towards left Hence, the force exerted by Nikhil = 2 × 60 = 120 N towards right

And force exerted by Naina towards left Therefore, the net force on the box towards left = (60 + 30) N = 90 N And net force on the box towards right = 120 N Hence, resultant force on the box = (120 − 90) N = 30 N towards right

Topic: Pressure

9. The round piece of cloth helps increase the area of contact of the load with the head. For the same force, pressure decreases if the area of contact is large. Hence, by making the area of contact larger, the pressure exerted on the head of a porter is decreased.

10. (i) Pressure is defined as the force acting on a unit area of a surface. It is given by the relation:

Its SI unit is N/m2 or Pascal (Pa). (ii) Three characteristics of pressure exerted by liquids are given as follows:

A. Liquids exert equal pressure on the walls of the container at equal depth. B. Pressure exerted by liquids on the bottom depends on the height of the water

column. C. Liquids not only exert pressure at the bottom, but also on the walls of the

container in which they are kept.

11.

Pressure Thus, the magnitude of the pressure exerted by the table on the box is 5 units.

12. The following example will explain how liquids exert equal pressure at the same depth. Four holes are drilled at the same depth in an empty plastic bottle. The bottle is then filled with water. It will be observed that the water coming out of all four holes falls at the same distance from the bottle. This indicates that liquids exert equal pressure at the same depth.

13. We do not get crushed under atmospheric pressure, which is as high as 1000 kg, because the pressure inside our body is equal to the atmospheric pressure.

14. A bicycle tube contains air at high pressure. When the tube bursts, this air suddenly gets released into the atmosphere. This produces a loud noise.

15. When a sucker is pressed against a smooth wooden surface, the air present inside comes out of the sucker cap. As a result of that, the air pressure inside the sucker becomes much less than the outside atmospheric pressure. Therefore, the atmosphere applies great pressure on the sucker from outside. Hence, the sucker gets stuck to the surface and it becomes difficult to pull it.

Chapter 12: Friction

Topic: Friction

1. The force of friction always acts in a direction opposite to the direction of motion, or in a direction opposite to the direction of the applied force.

2. As the name suggests, there is no friction on a frictionless surface. Hence, if a ball is thrown on a frictionless surface, then it will never come to stop.

3. Friction can sometimes be useful too. At other times, it can be disadvantageous. A useful application of friction is the fixing of a nail in a wall, where friction helps in holding the nail. The wearing out of soles of shoes is an example of how friction can be disadvantageous too.

4. Friction can be reduced by (i) greasing machines, hinges of doors, etc. (ii) employing air cushions between moving parts (iii) spreading a fine powder on the surface

5. When a resting body is about to move, a force of friction comes into play, which opposes the motion of the body. This force of friction is called static friction.

6. Vehicles moving through air and water have specially designed bodies to reduce frictional force offered by air and water. Bodies moving through air and water experience frictional force that opposes their motion. This frictional force offered by air and water is known as drag. The frictional force depends on the shape of the body moving through the fluid. By giving the body a special shape, drag can be minimised. This special shape is known as streamline shape.

7. Frictional force comes into play because of the irregularities on the two surfaces in contact. These irregularities interlock each other. As compared to smooth surfaces, there a large number of irregularities on rough surfaces. Hence, the force of friction is greater on rough surfaces as compared to smooth surfaces.

8. Advantages of friction: (i) Walking is made possible by friction. (ii) The burning of matchstick is possible because of friction. (iii) Writing (on board, on paper) is made possible by friction Disadvantages of friction: (i) The wear and tear of machine parts is a result of friction. (ii) The wear and tear of shoe soles and car tyres is a result of friction. (iii) To overcome excess friction in air, a lot of fuel is wasted in cars and aeroplanes.

9. This given statement can be explained by the following examples: (i) The rubbing together of palms in winter produces heat and warmth. (ii) When struck against a rough surface, a matchstick catches fire. The heat produced by friction helps light the matchstick.

10. Static friction: The force required for overcoming friction at the instant when an object starts moving from rest. Sliding friction: The force required to keep an object moving with the same speed is called sliding friction. Sliding friction is smaller than static friction. When a box, initially at rest, before sliding, static friction comes into play. On the other hand, sliding friction will come into play when the same box is in motion. Static friction is always greater than sliding friction for the same object moving or intending to move over the same surface. Therefore, it is always easy to move a box already in motion than a box at rest.

Chapter 13: Sound

Topic: Production of Sound and Propagation

1. Human voice is produced by the vibration of our vocal cords present in the larynx.

2. Sound enters through the outer part of the ear. It travels through the ear canal and reaches the other end, called the eardrum. The eardrum is like a stretched membrane. It vibrates in response to sound waves. The eardrum sends these vibrations to the brain through the middle ear and the inner ear respectively. The brain recognizes the vibrations as sound, which we are able to hear.

3. The difference in the length of the vocal cord is responsible for the difference in the voices of males and females. In males, the vocal cord is about 20 mm long, while in females, the vocal cord is about 5 mm shorter than that in males. This difference in the length of the vocal cord determines the voice of an individual.

4. Thunder and lightning are produced at the same time. However, light travels at a speed much faster than that of sound. Hence, lightning is usually seen a little before thunder is heard.

5. Sound waves require a medium for their propagation. Since vacuum is devoid of any medium, sound cannot travel through a vacuum.

Topic: Characteristic of Sound

6. A normal human ear can hear the sound of frequencies that lie in the range of 20 Hz − 20 kHz. This is the hearing range of a normal human ear.

7. For humans, the range of audible frequencies is from 20 Hz to 20 kHz. Hence, the thief will not be able to listen to the sound produced by the whistle.

8. The shrillness or the pitch of a sound is determined by frequency. If the frequency of vibration is low, the sound is less shrill and has a lower pitch e.g. a drum vibrates with a low frequency, thereby producing a low-pitched sound. If the frequency of vibration is high, the sound will be shrill and will have a higher pitch e.g. a whistle vibrates with a high frequency, thereby producing a high-pitched sound.

9. Number of oscillations in 40 seconds = 400

Number of oscillations in 1 second Frequency is defined as the number of oscillations in one second. Therefore, Frequency = 10 Hz

ime periodT = 1F requency

sime period 0.1T = 110 =

10. Noise pollution occurs when there is an excess of unwanted sound in the environment. Two sources of noise pollution: (i) Loud speakers and crackers (ii) Horns of buses, cars and trucks

Chapter 14: Chemical Effects of Electric Current

Topic: Conductivity of Liquids

1. The bulb does not glow when beaker X is connected to the circuit. This indicates that the liquid present in beaker X does not allow current to pass through it, i.e. the liquid is not a conductor. Pure water does not conduct electricity. Hence, beaker X contains pure water. Vinegar is a weak conductor. When beaker Y is connected to the circuit, the bulb glows dimly. This means that a weak current is generated and the filament of the bulb is not sufficiently heated. Therefore, beaker Y contains vinegar. Acid solutions are highly conducting in nature. When beaker Z is connected to the circuit, the bulb glows brightly. This means that a strong current is generated and the filament of the bulb is sufficiently heated. Therefore, beaker Z contains nitric acid solution.

2. The necessary requirements for constructing the tester are as follows: (i) An electric bulb (ii) A battery (iii) Connecting wires

3. Good conductors are substances that allow electric current to pass through them. Examples: Copper, aluminum and iron. Poor conductors are substances that do not allow electric current to pass through them. Examples: Rubber, plastic and wood.

Topic: Chemical Effect of Current

4. When electric current is passed through a solution, a chemical reaction occurs in it. Due to this reaction, the following changes may occur in the solution. (i) Bubble formation (ii) Deposition of metal on the electrode

5. The chemical effect of current is exhibited by the given activity.

6. Oxygen gas would be obtained at the positive electrode. Hydrogen gas would be obtained at the negative electrode.

Topic: Electroplating

7. Electroplating is the process of depositing a layer of a certain metal over another metal by means of electricity. It protects the electroplated metal from corrosion and rusting, thereby giving it a shiny appearance.

8. (i) Electroplating is the process used in industries for coating metallic objects with a thin layer of a different metal. When electric current is passed through silver nitrate solution, silver gets displaced from anode (positive terminal) to cathode (negative terminal). As a result, a coating of silver gets deposited at the cathode.

(ii) Tins cans are preferred over iron cans for the purpose of storing food because tin is less reactive than iron. Hence, food items are protected from being spoilt.

9. If the electrodes are interchanged, then the metal from anode (which was originally cathode) will be deposited on the plate connected to cathode (which was originally anode). This means that the same process will be carried out but in the opposite direction.

10. The two properties of chromium that make it a good electroplating element are as follows:

(i) It does not corrode and resists scratches. (ii) It has a shiny appearance.

Chapter 15: Some Natural Phenomena

Topic: Electrical Charge

1. If a charged body is brought close to an uncharged body, then the uncharged body gains a charge opposite to that of the charged body. This process is called induction.

2. It will be observed that the balloons repel each other. When the two balloons are rubbed against a woollen cloth, they acquire a similar nature of charges on their surfaces. As like charges repel each other, the balloons will move away from each other.

Topic: Interaction between Charges

3. (i) On touching the negatively charged rod, strip Q acquires a negative charge. On the other hand, strip P acquires no net charge at all. (ii) On acquiring negative charge, strip Q will attract the uncharged strip P. Therefore, an attractive force will act between the strips.

4. When a ball pen refill is rubbed vigorously against polythene, plastic refill gets electrically charged. This charge attracts the small pieces of paper.

Topic: Construction and Working of an Electroscope

5. An electroscope is an instrument that is used for detecting whether an object is carrying charge or not. When a charged body is brought in contact with the electroscope, this charge travels to the aluminium strips. The aluminium strips repel each other when they acquire this charge. This repulsion shows that some charge has been transferred to the aluminium strips.

Topic: Charging by Conduction

6. Buildings are earthed to minimise the effects of electrical shocks due to the leakage of electrical current.

Topic: Lightening

7. During a thunderstorm, air currents move upward while water droplets move downward. This movement causes separation of charges. The positive charges accumulate on the upper clouds, while the negative charges accumulate on the lower clouds. When the accumulation of charges becomes very large, the air (a poor conductor of electricity) is unable to resist the flow of negative charges from the lower clouds to the ground, which is considered positively charged. When the positive and the negative charges meet, a streak of light is produced. These streaks are called lightning

8. The safest position for the body to be in when lightning strikes is, squatting low on the ground and putting the hands on the knees with the head between the hands. This minimises the area of the body that can be struck by lightning.

Topic: Earthquakes

9.

The layer on which earthquakes occur is called the crust.

Topic: Richter Scale

10. The scale on which the power or intensity of an earthquake is measured is called the Richter scale. An earthquake measuring 7 or more on the Richter scale can cause severe damage to life and property. Hence, Kashmir is at higher risk of damage.

11. This statement means that an earthquake of magnitude 4 is not twice as destructive as an earthquake of magnitude 2. An increase of 2 in the magnitude means that the earthquake is 1000 times more destructive. Therefore, an earthquake of magnitude 4 is 1000 times more destructive than an earthquake of magnitude 2.

Topic: Seismograph

12.

13. (i) The outermost layer of the earth consists of some moveable pieces called plates. The boundaries of these pieces form weak zones, called seismic zones, in the earth’s crust. (ii) An earthquake is said to be destructive if the reading on the Richter scale is more than 7.

Topic: Precautions During Earthquakes

14. During an earthquake, if you are indoors, then you (i) Should protect yourself by taking shelter under a bed, table, chair, etc. (ii) Should not stand below heavy objects such as wall clocks, wall hangings, etc. On the other hand, if you are outdoors during an earthquake, then you should try to remain in an open area, away from buildings, trees and electric wires If you are in a vehicle during an earthquake, then you should remain in the vehicle.

15. Raju should not get up from the bed. Instead, he should cover his head with a pillow and stay put.

Chapter 16: Light

Topic: Laws of Reflection

1. There are two laws of reflection: (i) The angle of incidence is always equal to the angle of reflection. (ii) The incident ray, the normal on the surface at the point of incidence, and the reflected ray all lie in the same plane.

2. A plane mirror forms images such that the left of an object appears on the right of the image and vice-versa. This phenomenon is known as lateral inversion.

3. The situation is shown in the given ray diagram:

Let the angle made by the reflected light ray with the mirror be x°. The angle made by the incident ray AO with the normal PO i.e. ∠AOP = 32°. From the law of reflection, the angle of incidence is always equal to the angle of reflection. ∴ ∠AOP = ∠BOP = 32° (OB is the reflected ray) But ∠BOP + ∠BON = 90° 32° + x = 90° ∴ x = 90° − 32° = 58°

4. The magnification of an image produced by a plane mirror is 1.

5. Diffused reflection: In this type of reflection, all the incident parallel rays reflected by a plane surface are not parallel to each other after reflection. The reflecting surface, in this case, is not smooth and has many irregularities.

Regular reflection: In this type of reflection, all the incident parallel rays reflected by a

plane surface are parallel to each other after reflection. The reflecting surface, in this case, is smooth.

Topic: The Human Eye

6. The given figure shows all parts of the human eye.

(i) Iris: It controls the amount of light entering the eye through the pupil. It produces colour appearance to the eye. (ii) Lens: The eye lens focuses the incoming light rays onto the retina. (iii) Retina: The retina consists of light sensitive cells, which convert the incoming light rays into electrical signals and sends them to the brain via the optic nerves.

7. Convex lens is present in our eyes.

8. The iris adjusts the size of the pupil according to the intensity of light received by our eyes. As the intensity of light is great outside a cinema hall, the pupil becomes small. This limits the amount of light entering the eye. The intensity of light inside a cinema hall is low. So, the pupil needs to expand and become big to allow more light to enter the eye. As the pupil takes some time to dilate, we cannot see the objects clearly for the first few seconds.

9. To keep the shape of the eye spherical, the eyeball is filled with transparent liquids called aqueous and vitreous humour. There are two separate chambers in our eyes—one between the cornea and the lens, and the other between the lens and the retina. The first chamber is filled with aqueous humour, while the second chamber is filled with vitreous humour.

10. Visual aids: These can magnify words Tactual aids: Braille writer, slate and stylus Auditory aids: Cassettes, tape recorders and talking books Electronic aids: Talking calculator, television

Chapter 17: Stars and The Solar System

Topic: Eclipses and Tides

1. The next full moon day will be on a Monday. The time period between two full moons is approximately 29 days. This means that the next full moon will occur on the 29th day, starting from the first full moon day. If a full moon appears on a Sunday, then the 28th day will also be a Sunday. Therefore, the next full moon will appear on a Monday, i.e. on the 29th day.

2. The moon appears at night because it reflects the light coming from the sun.

Topic: Surface of the Moon

3. (i) The phases of the moon are caused by the Sun’s rays reflecting off the moon’s surface as the moon moves around the Earth. We are able to see only the part of the moon that reflects the Sun’s rays toward us. This results in the various shapes of the bright part of the moon as seen from the Earth during an entire month. The changes in the shape of the moon are called phases of the moon. (ii) The surface of the moon is barren and dusty. It has several craters and mountains.

Topic: Facts about Stars

4. We do not see stars during the day because the amount of light that reaches the earth from the stars is far less than the light of the sun.

Topic: Constellations

5. In the northern hemisphere, the Pole Star always appears at the same position in the night sky throughout the year.

Topic: Sun and Planets

6. Solar system consists of the Sun and other celestial bodies that revolve around it. The celestial bodies constitute the eight planets, comets, asteroids, and meteors. The given table lists the names of the inner and outer planets.

Inner Planets Outer planets

Mercury Jupiter

Venus Saturn

Earth Uranus

Mars Neptune

7. The essential environmental conditions responsible for the existence of life on planet Earth are as follows: (i) Presence of water (ii) Presence of suitable atmosphere (iii) Presence of ozone layer (iv) Suitable temperature range No, there is no other planet in the solar system that has all these environmental conditions.

Topic: Asteroids, Comets, Meteors

8. (i) Asteroids are small celestial objects composed of rocks and metals. Along with the eight planets, asteroids are also present in the solar system. They revolve around the sun. Their orbit lies between the orbits of Mars and Jupiter. (ii) Meteors enter the earth’s atmosphere at very high speeds. Their speed is opposed by the gas molecules present in the atmosphere. This opposition to the motion of meteors (or friction) heats up the meteors and causes them to evaporate quickly.

Topic: Artificial Satellites

9. (i) The satellites that have an orbital period equal to the rotation period of the earth are called geostationary satellites. Such satellites appear to be fixed in space. (ii) Communication satellites are placed at a height of 36,000 km in an orbit around the earth. The microwaves that are directed towards the satellites are dispersed to a large area or to a specified place. Devices, such as T.V., telephone and internet, function because of these satellites.

10. With the help of electronic cameras fitted into remote sensing satellites, they take clear pictures of the part of the earth and send them to the earth. Data obtained from these pictures help in providing information regarding agriculture, water table, earth resources, meteorology, etc.

Chapter 18: Pollution of Air and Water

Topic: Sources of Air Pollution

1. Power plants pollute the air by releasing tiny particles of ash into the atmosphere.

Topic: Effects of Air Pollution on Living Organisms

2. Harmful effects of air pollution: (i) Air pollutants cause respiratory problems such as asthma, cough and cold. (ii) The presence of pollutants such as CFCs destroys the ozone layer. (iii) Pollutants such as sulphur dioxide, sulphur trioxide and nitrogen dioxide combine with the water vapours present in the atmosphere to form acid droplets. These droplets fall on earth and cause acid rain. Acid rain has a harmful effect on both the living and the non-living. (Note: Any two will do.)

Topic: Effects of Air Pollution on Non-Living Objects

3. he white marble used in the Taj Mahal is turning pale because of discolouration, which is happening because of the high levels of pollution in its immediate environment.

Topic: Initiatives to Reduce Air Pollution and Global Warming

4. CNG is considered to be a cleaner fuel because it undergoes complete combustion to produce only carbon dioxide and water vapour. Hence, it does not cause air pollution.

5. The Delhi government has made the use of compressed natural gas (CNG) in public transport vehicles mandatory. This move has significantly helped reduce air pollution in the city.

Topic: Greenhouse Effect and Global Warming

6. (i) CFC is primarily responsible for the depletion of the ozone layer. (ii) Sulphur dioxide is produced by the burning of coal.

(iii) is the molecular formula of ozone. (iv) An increase in earth’s temperature by a layer of gases is known as global warming.

7. An increase in the Earth’s global average temperature will lead to an increase in sea levels. As a result of the increase in temperature, the water trapped in glaciers and ice caps will melt, thereby causing sea levels to rise. Apart from affecting rainfall patterns and climatic conditions, it could also lead to the flooding of coastal areas.

Topic: Sources of Water Pollution: A Case Study of River Ganga

8. The Ganga Action Plan is aimed at reducing the pollution levels in the river Ganga.

Topic: Water Pollution

9. The factors that could have contributed to the increase in the pollution levels of the river and consequently a decrease in the fish population are as follows: (i) Dumping of large quantities of garbage into the river. (ii) Releasing untreated sewage water into the river. (iii) Throwing of dead bodies into the river. (iv) Washing, bathing and defecating near the shores of the river. (v) Throwing flowers and idols of gods and goddesses into the river. (vi) Dumping non-biodegradable substances such as polythene bags into the river.

Topic: Conservation and Purification of Water

10. Tap water sometimes appears milky as it contains chlorine. This water is safe for drinking as chlorine kills the germs present in water, thereby making it safe for consumption.