SCIENCE 8

(NCERT TEXTBOOK SOLUTION)

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 Chapter 1.
Crop Production and Management
1. Select the correct word from the following list and fill in the blanks.
float, water, crop, nutrients, preparation
(a) The same kind of plants grown and cultivated on a large scale at a place is called .
(b) The first step before growing crops is of the soil.
(c) Damaged seeds would on top of water.
(d) For growing a crop, sufficient sunlight and and from the soil are essential.
Ans. (a) crop (b) preparation (c) float (d) water; nutrients
2. Match items in column A with those in column B.
A B
(i) Kharif crops (a) Food for cattle
(ii) Rabi crops (b) Urea and super phosphate
(iii) Chemical fertilisers (c) Animal excreta, cow dung, urine and plant waste
(iv) Organic manure (d) Wheat, gram, pea
(e) Paddy and maize
Ans. i. (e) ii. (d), (a) iii. (b) iv. (c)
3. Give two examples of each.
(a) Kharif crop
(b) Rabi crop
Ans. (a) Paddy, maize (b) Wheat, gram
4. Write a paragraph in your own words on each of the following.
(a) Preparation of soil
(b) Sowing
(c) Weeding
(d) Threshing
Ans. (a) Seeds germinate in soil. Plants grow and are anchored in the soil. They absorb water and minerals from soil.
Therefore, soil preparation is necessary for the growth of healthy and bumper crop. Soil preparation involves
ploughing and levelling of soil. Ploughing loosens the soil which allows easy and deeper penetration of the roots.
(b) Sowing is the process of putting seeds in the soil. Seeds should be sown in moist soil at optimum time and season,
and at proper depth. There should be sufficient spacing between the seeds. The seeds sown too deep may fail to
germinate due to lack of water or nonavailability of oxygen.
There are two methods of sowing seeds: Manual method or Broadcasting and mechanical method. Manual Method
or Broadcasting involves scattering of seeds by hand. Seeds are later covered with soil by leveller to avoid wastage.
Seeds of Maize, Wheat, Berseem and Bajra are sown by this method. This results in unequal distribution and
wastage of seeds. Mechanical Method involves the use of a seed drill. The seed drill makes furrows in the soil and
seeds fall into the furrows at regular distance and at proper depth.
(c) The unwanted plants that grow naturally along with crop plants are called weeds. The removal of weeds from the
field without harming the crop plants is called weeding. Weeding is essentially needed because weeds compete
with the crop plants for nutrients, water, sunlight and space, and affect their growth badly. They may produce toxic
substances that interfere with the growth of crop plants, and thus affect the harvest. They may be poisonous to
humans and domestic animals. Also, they attract pests and spread them to the crop plants.
Weeding is done either by pulling out the weeds by hand or removing them with the help of harrow or rake, trowel
or hoe or by using chemicals called weedicides. Some natural enemies of weeds are released in the field which
feed on weeds and destroy them. For example, cochineal insect is used to eliminate prickly pear (Opuntia) from
the crop fields in Tamil Nadu.
(d) The process of separating grains from husk is called threshing. It can be done manually and by using animals or
machines. In manual threshing, harvested crop is threshed by striking against a hard surface. For threshing with
animals, the harvested crop is heaped on the ground and animals like oxen, buffaloes, camels, etc., are made to
walk over it in a circle. The cattle’s feet release the grains from chaff (bhusa). In mechanical threshing, a machine
called thresher is used for threshing. The farm machine, combine is used for both harvesting and threshing.

2 Science 8 (NCERT Textbook Solution)

 5. Explain how fertilisers are different from manure.
Ans. Fertilisers Manures
1. Fertilisers are synthetic chemical compounds. 1. Manures are organic substances.
2. They are manufactured in factories from different 2. They are obtained from the decomposition of
chemicals. animals wastes and plant residues by the action of
microorganisms.
3. They make the soil nutrient specific and are rich in 3. They provide all the nutrients, but are not very rich in
nutrients. nutrients.
4. They do not provide humus. 4. They provide humus to soil.
5. They restore fertility of soil, but spoil soil chemistry. 5. They replenish soil fertility, restore soil texture and
make soil porous.
6. They cause water and soil pollution. 6. They do not cause pollution.
7. They are readily soluble in water, hence, absorbed by 7. They are not readily soluble in water, hence, absorbed
plants quickly. by plants slowly.

6. What is irrigation? Describe two methods of irrigation which conserve water.

Ans. The process of watering crop plants in the field at different intervals is called irrigation.
The two methods of irrigation that conserve water are:
l Sprinkler Irrigation: In this system, perpendicular pipes are laid in the field at regular intervals. These pipes are

joined to the main pipeline at one end and have rotating nozzles at the top end. Water is pumped in the main pipe
under pressure. It escapes through the rotating nozzles and gets sprinkled over the crop plants.
l Drip Irrigation: This system provides water to the plants drop by drop just near the roots. The water is not wasted

at all and the plants get regular water supply.

7. If wheat is sown in the kharif season, what would happen? Discuss.
Ans. If wheat is sown in the kharif season, the seedlings will not grow well due to absence of optimum conditions of temperature
and water.
8. Explain how soil gets affected by the continuous plantation of crops in a field.
Ans. Soil supplies mineral nutrients to the plants, which are essential for the growth of plants. Continuous growing of crops
in the same field without leaving it uncultivated for some period, makes the soil poorer in certain nutrients and reduces
the fertility of soil.
9. What are weeds? How can we control them?
Ans. The unwanted plants that grow naturally with crop plants are called weeds.
Control of weeds:
l Manual method: In this method, weeds are uprooted by hand.

l Mechanical method: In this method, weeds are removed by using different agricultural implements such as harrow,

trowel, hoe, etc.

l Chemical method: In this method, weeds are removed by spraying weedicides such as 2, 4-D, MCPA, butachlor

and paraquat on the crop plants.

l Biological method: In this method, some natural enemies of weeds are released in the crop field which feed on

weeds and destroy them. For example, chochineal insect is used to eliminate prickly pear from the crop fields.
10. Arrange the following boxes in proper order to make a flow chart of sugarcane crop production.
Sending crop to
Irrigation Harvesting Sowing
sugar factory

1 2 3 4
Preparation Ploughing the
Manuring
of soil field

5 6 7
Ans. 1. Preparation of soil 2. Ploughing the field 3. Sowing 4. Manuring 5. Irrigation 6. Harvesting
7. Sending crop to sugar factory

Science 8 (NCERT Textbook Solution) 3

 11. Complete the following word puzzle with the help of clues given below.
Down
1. Providing water to the crops.
2. Keeping crop grains for a long time under proper conditions.
5. Certain plants of the same kind grown on a large scale.
Across
3. A machine used for cutting the matured crop.
4. A rabi crop that is also one of the pulses.
6. A process of separating the grain from chaff.
1 2
I S
3
H

4
G

5
C

6
W

Ans. Down: 1. IRRIGATION 2. STORAGE 5. CROP

Across: 3. HARVESTER 4. GRAM 6. WINNOWING

Chapter 2.
Microorganisms: Friend and Foe
1. Fill in the blanks.
(a) Microorganisms can be seen with the help of a .
(b) Blue green algae fix directly from air and enhance fertility of soil.
(c) Alcohol is produced with the help of .
(d) Cholera is caused by .
Ans. (a) mircroscope (b) nitrogen (c) yeast (d) bacteria
2. Tick the correct answer.
(a) Yeast is used in the production of
(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen
(b) The following is an antibiotic
(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast
(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito (ii) cockroach
(iii) housefly (iv) butterfly
(d) The most common carrier of communicable diseases is
(i) ant (ii) housefly (iii) dragonfly (iv) spider
(e) The bread or idli dough rises because of
(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading
(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection
Ans. (a) (ii) (b) (ii) (c) (i) (d) (ii) (e) (iii) (f) (iii)

4 Science 8 (NCERT Textbook Solution)

 3. Match the organisms in Column A with their action in Column B.
A B
(i) Bacteria (a) Fixing nitrogen
(ii) Rhizobium (b) Setting of curd
(iii) Lactobacillus (c) Baking of bread
(iv) Yeast (d) Causing malaria
(v) A protozoan (e) Causing cholera
(vi) A virus (f) Causing AIDS
(g) Producing antibodies
Ans. i. (e), (g) ii. (a) iii. (b) iv. (c) v. (d) vi. (f)
4. Can microorganisms be seen with the naked eye? If not, how can they be seen?
Ans. (a) No. They can be seen with the help of a microscope.
5. What are the major groups of microorganisms?
Ans. There are 5 major groups of microorganisms: Bacteria, Fungi, Algae, Protozoa, Viruses.
6. Name the microorganisms which can fix atmospheric nitrogen in the soil.
Ans. Some microorganisms which can fix atmospheric nitrogen are as follows:
(a) Symbiotic bacterium – Rhizobium
(b) Free-living soil bacteria – Azotobacter and Clostridium
(c) Some cyanobacteria or blue-green algae – Anabaena and Nostoc
7. Write 10 lines on the usefulness of microorganisms in our lives.
Ans. Usefulness of microorganisms:
l Bacteria help in the formation of curd, pickles, etc.

l They help in preparation of wine and vinegar.

l They help in making cheese.

l Bacteria help in the ripening of tea leaves that gives them aroma and flavour.

l Yeast is used in baking industry for making bread, pastries, cakes, etc.

l They are used to prepare medicines like antibiotics, etc.

l Microbes are used to prepare vaccines.

l Bacteria are used in the leather industry.

l Microorganisms are used to increase soil fertility.

l Some bacteria are used in sewage treatment plants to decompose waste.

8. Write a short paragraph on the harmful effects of microorganisms.

Ans. Many microorganisms are harmful to us. They cause diseases, spoil our food and damage our goods made of
clothes, paper, leather, etc. They cause diseases in humans, animals and plants. The diseases, which spread through
microorganisms from person-to-person, are called communicable diseases. These diseases may spread through air we
breathe, water we drink or food we eat. Communicable diseases are also known as infectious diseases. Any disease
caused due to consumption of contaminated food or drink is called food poisoning. Certain microorganisms like bacteria,
viruses or parasites grow on food and produce toxic substances that spoil the food.
9. What are antibiotics? What precautions must be taken while taking antibiotics?
Ans. Antibiotics are the substances that are obtained from some microorganisms and they kill or stop the growth of disease-
causing microorganisms.
Antibiotics should be taken under the supervision of a qualified doctor. They should not be taken in overdose because
they may cause harmful effects or may become less effective if needed in future. Also they kill the useful bacteria of
the alimentary canal.

Chapter 3.
Coal and Petroleum
1. What are the advantages of using CNG and LPG as fuels?
Ans. The advantages of using CNG and LPG as fuels are that they are less polluting and cleaner fuels. They do not produce
smoke but give more heat as compared to other fuels.
2. Name the petroleum product used for surfacing of roads.
Ans. Bitumen is used for surfacing the roads.
3. Describe how coal is formed from dead vegetation. What is this process called?
Ans. About three hundred million years ago, the earth was covered with dense forests and swamps. The trees and plants of
dense forests died and fell down on the wet and swampy floor of the forest, and began to sink into the soil. More and

Science 8 (NCERT Textbook Solution) 5

 more dead plants, gravel and soil deposited over them. The heat and pressure from the top layers gradually turned the
remains of plants into coal.
The slow conversion of dead trees and plants into coal is called carbonisation.
4. Fill in the blanks.
(a) Fossil fuels are , and .
(b) Process of separation of different constituents from petroleum is called .
(c) Least polluting fuel for vehicle is .
Ans. (a) Coal; petroleum; natural gas (b) refining (c) CNG
5. Tick True/False against the following statements.
(a) Fossil fuels can be made in the laboratory.
(b) CNG is more polluting fuel than petrol.
(c) Coke is almost pure form of carbon.
(d) Coal tar is a mixture of various substances.
(e) Kerosene is not a fossil fuel.
Ans. (a) F (b) F (c) T (d) T (e) F
6. Explain why fossil fuels are exhaustible natural resources.
Ans. Fossil fuels such as coal, petroleum and natural gas are exhaustible natural resources because they are found in limited
amount in nature, cannot be continually replenished and are likely to be exhausted by various human activities.
7. Describe characteristics and uses of coke.
Ans. The characteristics of coke are:
(a) It is a tough, porous and black substance.
(b) It is an almost pure form of carbon.
Uses of coke are:
(a) It is used in the manufacture of steel.
(b) It is used in the extraction of many metals.
8. Explain the process of formation of petroleum.
Ans. Petroleum was formed from the remains of very tiny animals and plants that lived in the sea and died millions of years
ago. After they died, their bodies sank and got buried at the bottom of the sea. Over the time, they were covered by
layers of sand, silt and clay. Over millions of years, the layers of sand, silt and clay became very thick and the remains
buried deeper and deeper. In the absence of air, enormous heat and pressure from these layers slowly changed the
dead organisms into petroleum and natural gas.
9. The following Table shows the total power shortage in India from 1991–1997. Show the data in the form of a
graph. Plot shortage percentage for the years on the Y-axis and the year on the X-axis.
S. No. Year Shortage (%)
1. 1991 7.9
2. 1992 7.8
3. 1993 8.3
4. 1994 7.4
5. 1995 7.1
6. 1996 9.2
7. 1997 11.5
Ans.

6 Science 8 (NCERT Textbook Solution)

 Chapter 4.
Combustion and Flame
1. List conditions under which combustion can take place.
Ans. The conditions necessary for combustion to occur are:
(a) Presence of air or oxygen – supporter of combustion.
(b) Presence of a combustible substance – fuel.
(c) Attainment of ignition temperature.
2. Fill in the blanks.
(a) Burning of wood and coal causes of air.
(b) A liquid fuel, used in homes is .
(c) Fuel must be heated to its before it starts burning.
(d) Fire produced by oil cannot be controlled by .
Ans. (a) pollution (b) LPG (c) ignition temperature (d) water
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans. On burning, CNG produces the harmful products in very small amounts. It is a cleaner fuel as compared to petrol and
diesel. That is why, we can say that the use of CNG in automobiles has reduced pollution in our cities.
4. Compare LPG and wood as fuels.
Ans. LPG is a cleaner fuel. It burns completely and does not produce any harmful gas whereas wood does not burn completely.
As a result of incomplete combustion, wood gives poisonous carbon monoxide gas.
5. Give reasons.
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Ans. (a) Water is a good conductor of electricity. If it is used to put out fire caused by electricity, it will pass electric current
giving electric shock to the person trying to put out fire. So water is not used to put out fires involving electrical
equipments.
(b) LPG is a better domestic fuel than wood because it burns completely and does not give any poisonous gas.
(c) Ignition temperature of paper by itself is lower than a piece of paper wrapped around an aluminium pipe as aluminium
is a good conductor of heat. As a result, heat gained by paper is conducted quickly by the aluminium pipe and the
ignition temperature of a piece of paper around an aluminium pipe is not reached. Therefore, it does not burn.
6. Make a labelled diagram of a candle flame.
Ans.
Nonluminous zone

Luminous zone

Dark zone

Candle flame
7. Name the unit in which the calorific value of a fuel is expressed.
Ans. The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).
8. Explain how CO2 is able to control fires.
Ans. Carbon dioxide is a nonsupporter of burning. It displaces or takes away oxygen from the surrounding area by preventing
the supply of oxygen to the combustible substance. This extinguishes the fire.
9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans. The ignition temperature of dry leaves is lower than a heap of green leaves because green leaves contain a lot of water
in them. As a result, ignition temperature of a heap of green leaves is not reached and it is difficult to burn.
10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. A goldsmith uses the outermost zone of a flame for melting gold and silver because in this zone, a nonluminous blue
flame with very high temperature is obtained due to presence of oxygen in plenty.

Science 8 (NCERT Textbook Solution) 7

 11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ.
Calculate the calorific value of the fuel.
180, 000
Ans. Calorific value of the fuel = = 40,000 kJ/kg
4 .5
12. Can the process of rusting be called combustion? Discuss.
Ans. The process of rusting cannot be called combustion because though it occurs in the presence of oxygen gas but it does
not produce heat and light.
13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the
beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of
the flame. Whose water will get heated in a shorter time?
Ans. The beaker kept by Ramesh in outermost part of the flame will get heated in a shorter time due to its highest temperature
1400°C.

Chapter 5.
Conservation of Plants and Animals
1. Fill in the blanks.
(a) A place where animals are protected in their natural habitat is called .
(b) Species found only in a particular area is known as .
(c) Migratory birds fly to far away places because of changes.
Ans. (a) protected (b) endemic species (c) climatic
2. Differentiate between the following.
(a) Wildlife sanctuary and biosphere reserve
(b) Zoo and wildlife sanctuary
(c) Endangered and extinct species
(d) Flora and fauna
Ans. (a)
Wildlife Sanctuary Biosphere Reserve
1. It is a smaller area. 1. It is very large area.
2. Private ownership rights to collect minor forest 2. Multiple land use is allowed.
products are provided.
3. It protects and conserves the plants and animals. 3. It is meant to conserve and maintain biodiversity as
well as culture of the area.

(b) Zoo: In a zoo, wild animals are kept in cages, in captivity.

Wildlife sanctuary: A wildlife sanctuary is a protected land area reserved for the protection of wild animals, birds
and plants. Hunting is strictly prohibited there. However, private ownership rights for collecting minor forest products,
harvesting of timber and cultivation are granted so long as they do not interfere with the life of wild animals.
(c) Endangered species: The species that are not likely to survive and will soon become extinct, if the causative
factors continue, are called endangered species.
Extinct species: Species of plants and animals which are lost forever are called extinct species.
(d) The plants found typically in a particular area form the flora of that area. The term fauna represents the wild animals
found in a particular place or geographical region.
3. Discuss the effects of deforestation on the following.
(a) Wild animals
(b) Environment
(c) Villages (Rural areas)
(d) Cities (Urban areas)
(e) Earth
(f) The next generation
Ans. (a) Deforestation leads to loss of natural habitats of wild animals and depletion of their food sources. This condition
leads to loss of wildlife.
(b) Deforestation increases temperature, reduces rainfall and increases wind velocity. These changes lead to climatic
changes.
(c) Deforestation leads to flood, desertification of fertile land and causes soil erosion. This affects the agriculture and
grazing areas of cattle. Also, fodder, firewood, fibres, etc. become short to rural people.

8 Science 8 (NCERT Textbook Solution)

 (d) In the absence of forests, the climate of urban areas changes drastically. The air becomes polluted. Many useful
products obtained from forests are not available. For example, medicines, fibres, fruits, and wood or timber for
making houses, furniture, buildings, etc. will be scarce.
(e) Deforestation increases the level of carbon dioxide in the atmosphere. This causes the greenhouse effect which
leads to global warming.
(f) The next generation will be devoid of all the benefits of forests and will suffer a healthy and harmonious life.
4. What will happen if
(a) we go on cutting trees.
(b) the habitat of an animal is disturbed.
(c) the top layer of soil is exposed.
Ans. (a) It will lead to desertification, soil erosion and serious environmental problems such as global warming, rise in the
sea level, etc. This will ultimately affect all the living beings.
(b) This will disturb the life, growth and reproductive potential of that animal.
(c) It will be blown away by the wind or washed away by the running water making the soil infertile.
5. Answer in brief.
(a) Why should we conserve biodiversity?
(b) Protected forests are also not completely safe for wild animals. Why?
(c) Some tribals depend on the jungle. How?
(d) What are the causes and consequences of deforestation?
(e) What is Red Data Book?
(f) What do you understand by the term migration?
Ans. (a) Wildlife is a valuable biological resource. The conservation of biodiversity is essential because:
l Biodiversity maintains a balance in nature or in the ecosystem through food chains and food webs.

l Biodiversity regulates climate, rainfall and wind speed.

l Wild animals and plants provide a variety of commodities.

l Wildlife is needed for breeding programmes in agriculture, horticulture, sericulture, apiculture, etc.

l It helps in cycling of nutrients and preservation of soil fertility.

We, the human beings, are a part of nature and all the components of nature are interdependent. Any damage
to biodiversity will threaten the human existence. Therefore, biodiversity needs to be conserved.
(b) Encroachment by people living in the neighbourhood, indiscriminate killing and poaching of wild animals for skin,
fur, horn and tusk has caused reduction and elimination of many wild animals living in protected forests.
(c) Forests support tribal people living there by providing them with a variety of essential commodities such as food,
fodder, firewood, fruits, fibres, medicines, essential oils, etc.
(d) Causes of deforestation: Human activities such as felling trees for timber wood, cleaning land for cultivation of
crops, vegetables and fruits, building houses, factories, roads and for mining, etc. There are some natural causes
such as forest fires, severe droughts, floods, earthquakes, landslides, pests, and viral and fungal diseases of plants
that lead to deforestation.
Consequences of deforestation: Deforestation leads to:
l Global warming due to increased level of carbon dioxide in nature.

l Climate change due to decrease in rainfall and increase in temperature and wind speed.

l Desertification due to reduction in water-holding capacity of the soil.

l Droughts due to disturbed water cycle, reduced rains and lowered water table.

l Soil erosion and floods due to decreased water-holding capacity of soil.

l Loss of wildlife due to loss of natural habitats of wild animals and plants.

l Depletion of resources displaces tribal people who depend on forests for their livelihood.

(e) The Red Data Book is a record of all those species of plants and animals which are under the threat of extinction or
are rare and vulnerable for extinction. It is maintained by WCU, Switzerland. This book provides information about
the distribution and status of threatened species, and guides their conservation programmes.
(f) Migration is the seasonal movement of an animal from its habitat to some safer area, due to climatic changes,
where it breeds and returns when climatic conditions become favourable.
6. In order to meet the ever-increasing demand in factories and for shelter, trees are being continually cut. Is it
justified to cut trees for such projects? Discuss and prepare a brief report.
Ans. Because of great increase in human population, more and more trees are cut every day. This has led to the rapid
decline in biodiversity. This is also causing changes in the temperature and rainfall patterns. These are responsible for
global warming and melting of glaciers. Such changes have adverse effects on biodiversity.
Thus, alternative sources for forest products should be explored. To restore forest cover and forest wealth, tree saplings
should be planted regularly. This will ensure regular supply of forest products and will not disturb the ecological balance
in nature.

Science 8 (NCERT Textbook Solution) 9

 7. How can you contribute to the maintenance of green wealth of your locality? Make a list of actions to be taken
by you.
Ans. (a) Creating awareness in mass regarding the importance of plants through different media.
(b) Establishing the Ecoclubs in housing societies.
(c) Organising debates, strect shows, poster making competitions, etc.
(d) Plantings the saplings of trees on bare land.
8. Explain how deforestation leads to reduced rainfall.
Ans. Deforestation leads to increased amount of carbon dioxide in air which results in global warming, i.e., increase in the
earth’s temperature. This disturbs water cycle which reduces rainfall.
9. Find out about national parks in your state. Identify and show their location on the outline map of India.
Ans. Do yourself.
10. Why should paper be saved? Prepare a list of ways by which you can save paper.
Ans. Paper is made from wood. It takes 17 fully grown trees to make one tonne of paper. By saving paper, we can save
many trees. Also, we save energy and water required for manufacturing paper.
Ways to save paper:
(a) Not wasting paper and minimising the use of paper.
(b) Taking maximum use of paper by writing on both sides of it.
(c) Using e-card instead of paper greeting cards or using old greeting cards and wedding cards for making new cards
or invitation cards, etc.
(d) Reusing gift wrappers.
(e) Sending e-invitation wherever possible instead of paper invitations.
11. Complete the word puzzle.
1
C

G
3
D

4 5
E

Down
1. Species on the verge of extinction.
2. A book carrying information about endangered species.
5. Consequence of deforestation.
Across
1. Species which have vanished.
3. Species found only in a particular habitat.
4. Variety of plants, animals and microorganisms found in an area.
Ans. Down: 1. ENDANGERED 2. RED DATA BOOK 5. DESERTS
Across: 1. EXTINCT 3. ENDEMIC 4. BIODIVERSITY

10 Science 8 (NCERT Textbook Solution)

 Chapter 6.
Reproduction in Animals
1. Explain the importance of reproduction in organisms.
Ans. Reproduction is important in the life of organisms because it helps in the continuity of life from one generation to the
next generation.
2. Describe the process of fertilisation in human beings.
Ans. The union of sperm and ovum is called fertilisation. In human beings, fertilisation occurs in the anterior part of oviduct.
3. Choose the most appropriate answer.
(a) Internal fertilisation occurs
(i) in female body. (ii) outside female body.
(iii) in male body. (iv) outside male body.
(b) A tadpole develops into an adult frog by the process of
(i) fertilisation (ii) metamorphosis (iii) embedding (iv) budding
(c) The number of nuclei present in a zygote is
(i) none (ii) one (iii) two (iv) four
Ans. (a) (i) (b) (ii) (c) (ii)
4. Indicate whether the following statements are True (T) or False (F).
(a) Oviparous animals give birth to young ones.
(b) Each sperm is a single cell.
(c) External fertilisation takes place in frog.
(d) A new human individual develops from a cell called gamete.
(e) Egg laid after fertilisation is made up of a single cell.
(f) Amoeba reproduces by budding.
(g) Fertilisation is necessary even in asexual reproduction.
(h) Binary fission is a method of asexual reproduction.
(i) A zygote is formed as a result of fertilisation.
(j) An embryo is made up of a single cell.
Ans. (a) (F) (b) (T) (c) (T) (d) (F) (e) (F) (f) (F) (g) (F) (h) (T) (i) (T) (j) (F)
5. Give two differences between a zygote and a foetus.
Ans. Zygote is a single-celled structure while foetus is a multicellular structure. Zygote does not resemble with adult form
whereas a foetus resembles the human form.
6. Define asexual reproduction. Describe two methods of asexual reproduction in animals.
Ans. In asexual reproduction, a single organism (parent) produces genetically identical individuals of its own kind. The second
parent is not needed.
The offspring produced by asexual reproduction are identical to each other and to the parent. They are called clones.
Asexual reproduction is carried out in many ways:
l Binary Fission: It occurs in unicellular organisms like bacteria, Ameoba and Paramecium. First the nucleus of parent

cell divides into two daughter nuclei. This is followed by the division of parent cell. Each half receives one daughter
nucleus and forms a daughter organism. Thus, two daughter organisms are formed from one organism.
l Budding: In budding, a new organism is formed as an outgrowth from the parent body. In Hydra, a small bulge

appears from the lower part of the body. This grows into a bud, detaches from parent body and develops into a
young Hydra.
7. In which female reproductive organ does the embryo get embedded?
Ans. Uterus
8. What is metamorphosis? Give examples.
Ans. The change in form and shape from larva to adult is called metamorphosis.
In frogs, the development of the embryo takes place outside the female body. The embryo develops inside the egg.
The embryo hatches out of the egg as larva, called tadpole. The tadpole undergoes several changes in its form and
structure and changes into an adult frog.
9. Differentiate between internal fertilisation and external fertilisation.
Ans. Internal Fertilisation: In internal fertilisation, fusion of male and female gametes takes place inside the body of the
female. Internal fertilisation occurs in reptiles, insects, birds and mammals including human beings.
External Fertilisation: When the fusion of male and female gametes takes place outside the body of female, it is called
external fertilisation. This type of fertilisation takes place in water. It is found in frog and aquatic animals such as fishes
and starfishes.

Science 8 (NCERT Textbook Solution) 11

 10. Complete the crossword puzzle using the hints given below.
1 2 3 4
F T I O

5
B

6
I
7
B
8
O

Across
1. The process of the fusion of the gametes.
6. The type of fertilisation in hen.
7. The term used for bulges observed on the sides of the body of hydra.
8. Eggs are produced here.
Down
2. Sperms are produced in these male reproductive organs.
3. Another term for in vitro fertilisation.
4. These animals lay eggs.
5. A type of fission in amoeba.
Ans. Across: 1. FERTILIZATION 6. INTERNAL 7. BUDS 8. OVARY
Down: 2. TESTES 3. IVF 4. OVIPAROUS 5. BINARY

Chapter 7.
Reaching the Age of Adolescence
1. What is the term used for chemical secretions of endocrine glands responsible for changes taking place in the
body?
Ans. Hormone.
2. Define adolescence.
Ans. The period of life when the body undergoes changes leading to reproductive maturity is called adolescence.
3. What is menstruation? Explain.
Ans. Menstruation is the periodic discharge of blood and lining of uterus in females. Between two successive ovulations, the
wall of uterus becomes thick and vascular to receive the developing embryo. In case the egg is fertilised, pregnancy
is results. In case ovum is not fertilised, the thickened lining of uterine wall and its blood capillaries break down. This
causes bleeding. This blood discharge is called menstrual flow and it lasts for 3 to 7 days. The cycle of breakdown of
wall of uterus and its blood capillaries causing menstrual flow every 28th day is called menstruation or menstrual cycle.
4. List changes in the body that take place at puberty.
Ans. The changes that take place at puberty are:
(a) Increase in height
(b) Changes in body shape
(c) Change in voice box and voice
(d) Increase in the activity of sweat glands and sebaceous glands
(e) Maturation of sex organs
(f) Growth of beard and moustaches in boys
(g) Beginning of menstruation in girls

12 Science 8 (NCERT Textbook Solution)

 5. Prepare a Table having two columns depicting names of endocrine glands and hormones secreted by them.
Ans.
Endocrine Gland Hormones Secreted
1. Pituitary Gland (Master gland) l Growth hormone (GH)
l Follicular Stimulating Hormone (FSH)
l Several other hormones
2. Adrenal (2 in number) l Adrenaline (emergency hormone)
l Other hormones
3. Thyroid l Thyroxine
4. Pancreas l Insulin
5. Testes (In males) l Testosterone
6. Ovaries (In females) l Estrogen
l Progesterone

6. What are sex hormones? Why are they named so? State their function.
Ans. Hormones secreted by testes and ovaries are called sex hormones. Testes secrete testosterone and ovaries secrete
estrogen. They are secreted by sex organs so they are called sex hormones. They stimulate changes in the body and
produce secondary sexual characters in males and females.
7. Choose the correct option.
(a) Adolescents should be careful about what they eat, because
(i) proper diet develops their brains.
(ii) proper diet is needed for the rapid growth taking place in their body.
(iii) adolescents feel hungry all the time.
(iv) taste buds are well developed in teenagers.
(b) Reproductive age in women starts when their
(i) menstruation starts.
(ii) breasts start developing.
(iii) body weight increases.
(iv) height increases.
(c) The right meal for adolescents consists of
(i) chips, noodles, coke.
(ii) chapati, dal, vegetables.
(iii) rice, noodles and burger.
(iv) vegetable cutlets, chips and lemon drink.
Ans. (a) (ii) (b) (i) (c) (ii)
8. Write notes on
(a) Adam’s apple
(b) Secondary sexual characters
(c) Sex determination in the unborn baby
Ans. (a) At puberty in boys, the larynx begins to grow and protrudes out in the throat region. This is called Adam’s apple.
(b) The external features that help to distinguish a male from a female are called secondary sexual characters. For
example, beard and moustaches, hair on chest and thighs, adam’s apple in males, and developed breasts, wide
pelvic region and shrill voice in females.
(c) The sex chromosomes are the X and Y-chromosomes that determine the gender (sex) of a human baby. Males
have X and Y chromosomes, whereas females have two X chromosomes.
The sperm carries either an X chromosome or a Y chromosome. The egg or the ovum of a female contains only
X chromosomes.
If sperm carrying X chromosome fertilises the egg, then the resulting zygote will be XX, i.e., a female child.
If the sperm carrying Y chromosome fertilises the egg, then the resulting zygote will be XY, i.e., a male child.
9. Word game : Use the clues to work out the words.
Across
3. Protruding voice box in boys
4. Glands without ducts
7. Endocrine gland attached to brain
8. Secretion of endocrine glands

Science 8 (NCERT Textbook Solution) 13

 9. Pancreatic hormone 1
10. Female hormone
Down E
1. Male hormone
2. Secretes thyroxine
3. Another term for teenage
5. Hormone reaches here through blood stream
6. Voice box 2
7. Term for changes at adolescence

3
A

4
D
5

7
P
8
M E

Y C
9 10
S T

Ans. Across: 3. ADAM’S APPLE 4. ENDOCRINE 7. PITUITARY 8. HORMONE 9. INSULIN 10. ESTROGEN
Down: 1. TESTOSTERONE 2. THYROID 3. ADOLESCENCE 5. TARGET SITE 6. LARYNX 7. PUBERTY
10. The table below shows the data on likely heights of boys and girls as they grow in age. Draw graphs showing
height and age for both boys and girls on the same graph paper. What conclusions can be drawn from these
graphs?

Ans. During childhood, the height of boys increases more than the girls. But, on reaching adolescent stage, the height of
girls increases very fast and then slows down after reaching 16 years of age.

14 Science 8 (NCERT Textbook Solution)

 Chapter 8.
Force and Pressure
1. Give two examples each of situations in which you push or pull to change the state of motion of objects.
Ans. Two examples showing push or pull in order to change the state of motion of objects are pushing a trolley and pulling
a drawer.
2. Give two examples of situations in which applied force causes a change in the shape of an object.
Ans. Two examples showing a change in the shape of an object on applying force are pressing a lump of dough with hands
and stretching a rubber band.
3. Fill in the blanks in the following statements.
(a) To draw water from a well we have to at the rope.
(b) A charged body an uncharged body towards it.
(c) To move a loaded trolley we have to it.
(d) The north pole of a magnet the north pole of another magnet.
Ans. (a) pull (b) attracts (c) push (d) repels
4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move
towards the target. Based on this information, fill up the gaps in the following statements using the following
terms.
muscular, contact, non-contact, gravity, friction, shape, attraction
(a) To stretch the bow, the archer applies a force that causes a change in its .
(b) The force applied by the archer to stretch the bow is an example of force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a
force.
(d) While the arrow moves towards its target, the forces acting on it are due to and that due
to of air.
Ans. (a) shape (b) muscular (c) contact (d) gravity, friction
5. In the following situations identify the agent exerting the force and the object on which it acts. State the effect
of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
Ans. (a) Here, fingers are agent and a piece of lemon is object on which force acts. When force acts on the lemon, its shape
changes and juice is pushed out of it.
(b) Here, thumb is agent and a toothpaste tube is an object on which force acts. When pushing a toothpaste tube,
paste comes out of it and the shape of the tube gets changed.
(c) Here, spring is an object and a load is an agent. When a load is suspended from a spring, spring gets stretched.
(d) Here, an athlete is an agent and the earth is an object on which force acts. When an athlete makes a high jump,
he pushes the ground in order to change the position.
6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect
the piece of iron?
Ans. The force due to hammering changes the shape of the piece of iron.
7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was
found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon
and the wall?
Ans. Electrostatic force is responsible for the attraction between the balloon and the wall.
8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss
why the forces acting on the bucket do not bring a change in its state of motion.
Ans. Muscular force and gravitational force. Because muscular force acts upwards and gravitational force acts downwards
in order to maintain the balance and there is no possibility of change in the state of motion.
9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket
immediately after leaving the launching pad.
Ans. Gravitational force and mechanical force act on the rocket immediately after leaving the launching pad.
10. When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the
form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water
in the dropper is due to
(a) pressure of water.

Science 8 (NCERT Textbook Solution) 15

 (b) gravity of the earth.
(c) shape of rubber bulb.
(d) atmospheric pressure.
Ans. (d)

Chapter 9.
Friction
1. Fill in the blanks.
(a) Friction opposes the between the surfaces in contact with each other.
(b) Friction depends on the of surfaces.
(c) Friction produces .
(d) Sprinkling of powder on the carrom board friction.
(e) Sliding friction is than the static friction.
Ans. (a) relative motion (b) nature (c) heat (d) reduces (e) smaller
2. Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order.
Their arrangements are given below. Choose the correct arrangement.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
Ans. (c)
3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The
force of friction acting on the car on different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper
Ans. (a)
4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional
force acting on it.
Ans. D
ir
fric ectio
tio n o
na
l fo f
rce

Bo
ok
Bo
in ok is
thi
s d mov
ire ing
cti
on

5. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you
to walk on the floor? Why?
Ans. It would make it more difficult to walk on the floor because smoothness of floor and slippery nature of soap will reduce
the friction.
6. Explain why sportsmen use shoes with spikes.
Ans. Sportsmen use shoes with spikes in order to have an extra grip on the ground while playing.
7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have
to apply a larger force and why?
Ans. Seema will apply a larger force because heavier the box, larger is the force of friction acting on it.
8. Explain why sliding friction is less than static friction.
Ans. Lesser force is required to move the object while sliding whereas larger force is required to move the object in static
condition because it is in rest position. Therefore, sliding friction is less than static friction.

16 Science 8 (NCERT Textbook Solution)

 9. Give examples to show that friction is both a friend and a foe.
Ans. Fixing a nail on the wall is not possible without friction. This example shows that friction is necessary or a friend. Rubbing
of mechanical parts produces heat and damages the moving parts. This example shows that friction is a foe or an evil.
10. Explain why objects moving in fluids must have special shapes.
Ans. The objects moving in fluids must have special shapes because these objects require a lot of energy to overcome the
drag. To reduce this problem, these objects are having a special body shape called streamlined shape.

Chapter 10.
Sound
1. Choose the correct answer.
Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases.
Ans. (d)
2. Voice of which of the following is likely to have minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Ans. (c)
3. In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false.
(a) Sound cannot travel in vacuum.
(b) The number of oscillations per second of a vibrating object is called its time period.
(c) If the amplitude of vibration is large, sound is feeble.
(d) For human ears, the audible range is 20 Hz to 20,000 Hz.
(e) The lower the frequency of vibration, the higher is the pitch.
(f) Unwanted or unpleasant sound is termed as music.
(g) Noise pollution may cause partial hearing impairment.
Ans. (a) T (b) F (c) F (d) T (e) F (f) F (g) T
4. Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called .
(b) Loudness is determined by the of vibration.
(c) The unit of frequency is
(d) Unwanted sound is called .
(e) Shrillness of a sound is determined by the of vibration.
Ans. (a) Time period (b) amplitude (c) hertz (Hz) (d) noise (e) frequency
5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Ans. Number of oscillation (n) = 40, time (t) = 4 s
t 4
∴ Time period (T ) =
= = 0.1 s
n 40
1 1
and frequency (υ) = = = 10 Hz
T 0 .1
6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per
second. What is the time period of the vibration?
Ans. Given that υ = 500, T = ?
1 1
T = = = 0.002 s
υ 500
7. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute
Ans. (a) Dholak – Stretched skin (b) Sitar – string (c) Flute – air column

Science 8 (NCERT Textbook Solution) 17

 8. What is the difference between noise and music? Can music become noise sometimes?
Ans. Irritating, unpleasant and tiring sounds are called noise. On the other hand, soothing, pleasant and refreshing sounds
are called music.
Yes, music becomes noise when we hear it in the high volume which may disturb people.
9. List sources of noise pollution in your surroundings.
Ans. Do yourself.
10. Explain in what way noise pollution is harmful to human.
Ans. Noise pollution is the unwanted and displeasing human created sound that disrupts the environment.
The noise pollution affects both health and behaviour. It can cause annoyance and aggression, hypertension, high stress
levels, hearing loss, sleep disturbance and tinnitus. Tinnitus can lead to forgetfulness, severe depression and at times
panic attacks. High noise pollution can cause cardiovascular disorders.
11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes
away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Ans. We would suggest our parents to buy the house three lanes away from roadside because traffic on road is one of the
causes of noise pollution.
12. Sketch larynx and explain its function in your own words.
Ans. The sound producing organ of our body is called larynx or voice box. It is located in the throat at the upper end of the
windpipe. Inside the voice box, there are two ligaments called vocal cords. When lungs force out air through the larynx,
the vocal cords vibrate to produce sound. The muscles attaching the vocal cords can make them loose or tight. This
is called changing the tension in the vocal cords. The tension on the vocal cords can change to produce a higher or a
lower tone.

13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is
seen earlier and thunder is heard later. Can you explain why?
Ans. The speed of sound in air (340 m/s) is very much less than the speed of light in air (3 × 108 m/s). Due to this fact, we
always see the lightning much before hearing the thunder.

Chapter 11.
Chemical Effects of Electric Current
1. Fill
in the blanks.
(a)
Most liquids that conduct electricity are solutions of , and .
(b)
The passage of an electric current through a solution causes effects.
(c)
If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the
terminal of the battery.
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called
.
Ans. (a) acids, bases, salts (b) chemical (c) negative (d) electroplating
2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain
the reason?
Ans. Yes, the taken sample of solution would be a good conductor of electricity hence, due to magnetic effect of electricity,
the magnetic middle shows deflection.

18 Science 8 (NCERT Textbook Solution)

 3. Name three liquids, which when tested in the manner shown in Fig. 11.9, may cause the magnetic needle to
deflect.

Fig. 11.9
Ans. Tap water, salt solution, lemon juice, etc.
4. The bulb does not glow in the setup shown in Fig. 11.10. List the possible reasons. Explain your answer.

Fig. 11.10
Ans. The possible reasons of not glowing the bulb joined in tester may be:
(i) The sample liquid may be a bad conductor of electricity.
(ii) The bulb may be fused.
(iii) The battery may be discharged.
If the liquid is a bad conductor of electricity or the bulb is fused, then circuit will not be completed, and hence, bulb will
not glow. If the battery is discharged, there will be no current in the circuit and the bulb will not glow.
5. A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the
bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that
(i) liquid A is a better conductor than liquid B.
(ii) liquid B is a better conductor than liquid A.
(iii) both liquids are equally conducting.
(iv) conducting properties of liquid cannot be compared in this manner.
Ans. (i)
6. Does pure water conduct electricity? If not, what can we do to make it conducting?
Ans. No, we can mix some table salt, lemon juice or soda in pure water to make it conduct electricity.
7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area.
Explain why they do this.
Ans. In case of a fire, the firemen involved at the site use impure water to extinguish the fire. The impure water is a good
conductor of electricity and it causes electric shocks so they shut off the main electrical supply for the area.
8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that
the compass needle deflects more in the case of seawater. Can you explain the reason?
Ans. Seawater contains more salts as compared to drinking water because drinking water gets purified to remove excess
salt. Hence, seawater is a better conductor than the drinking water so the child gets more deflection in compass needle
in case of seawater.
9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Ans. No, it is not safe because rainwater can get slightly acidified as well as it wets the equipments that can cause electric
shock.

Science 8 (NCERT Textbook Solution) 19

 10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass
tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection.
What could be the reasons?
Ans. In atmosphere, a plenty of smoke particles is present. They contain many nonmetallic oxides like CO2, NO2 SO2, etc.
which mix with rainwater and form acids like HNO3 (nitric acid), H2CO3 (carbonic acid), H2SO3 (sulphurous acid) or
H2SO4 (sulphuric acid). Thus, mixing of acids can cause acid rain, therefore, rainwater gets impure and becomes good
conductor of electricity.
11. Prepare a list of objects around you that are electroplated.
Ans. Car parts, taps, kitchenware, gas stove, cutlery, etc., are electroplated.
12. The process that you saw in Activity 11.7 is used for purification of copper. A thin plate of pure copper and a
thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the
thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?
Ans. Impure thick copper rod should be attached to the positive terminal of the battery because when copper sulphate
(CuSO4) gets decomposed, the copper (copper ion) gets attracted towards the negative electrode and converting into
copper gets deposited here. Further, sulphate part goes to positive electrode and again converts its copper sulphate
by decreasing the thickness of impure copper plate.

Chapter 12.
Some Natural Phenomena
Select the correct option in questions 1 and 2.
1. Which of the following cannot be charged easily by friction?
(a) A plastic scale (b) A copper rod (c) An inflated balloon (d) A woollen cloth.
Ans. (b)
2. When a glass rod is rubbed with a piece of silk cloth the rod
(a) and the cloth both acquire positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire negative charge.
(d) becomes negatively charged while the cloth has a positive charge.
Ans. (b)
3. Write T against true and F against false in the following statements.
(a) Like charges attract each other.
(b) A charged glass rod attracts a charged plastic straw.
(c) Lightning conductor cannot protect a building from lightning.
(d) Earthquakes can be predicted in advance.
Ans. (a) F (b) T (c) F (d) F
4. Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.
Ans. A crackling sound is heard while taking a sweater off because sweater gets charged electrically on rubbing with our
skin.
5. Explain why a charged body loses its charge if we touch it with our hand.
Ans. When we touch a charged body with our hand, the charge flows from that body to our hand because human body
behaves as a good conductor of electricity. Thus, the charged body loses its charges.
6. Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on
this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?
Ans. Richter scale. Yes, an earthquake of measure 3 be recorded by a seismograph. No, it will cause no damage.
7. Suggest three measures to protect ourselves from lightning.
Ans. Some important safety measures to protect from lightning are as follows:
When inside
(a) Stay away from doors and windows.
(b) If the thunderstorm is present, do not plug or unplug TVs, telephones, stereos or other electrical appliances.
(c) Stay away from plumbing, avoid running water and do not take a shower or bath.
When outside
(a) If caught outdoors, seek cover indoors as quickly as possible.
(b) Do not stand under a tree for cover because taller and moist objects are more prone to lightning strike.
(c) Do not use an umbrella, lawn mower, bicycle or similar objects.
(d) If caught in an open field, crouch low, with your head bent in between your arms and legs close together.

20 Science 8 (NCERT Textbook Solution)

 8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted
by another charged balloon?
Ans. As the two charged balloon have the similar charges, hence they repel each other. But when an uncharged balloon is
brought near a charged balloon, it is induced an apposite charge to itself and hence gets attracted.
9. Describe with the help of a diagram an instrument which can be used to detect a charged body.
Ans. An electroscope is used to detect a charged body. When we touch the metal wire of
electroscope with the charged straw, the charge flows through the metal wire to the
aluminium strip, as the straw has high level of charge and the metal wire has low (no)
charge on it. The two halves of aluminium strip acquire same type of charge from
the straw and they separate apart due to repulsion caused by similar charges. The
aluminium strip is now charged.
When we touch the metal wire with our hand, charge from the foil strip (high level)
flows to the zero level charge on our hand, and therefore, the repulsion experienced
by the two halves of strip is vanished.
10. List three states in India where earthquakes are more likely to strike.
Ans. Jammu and Kashmir, Rajasthan and Gujarat
11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect
yourself?
Ans. We should take the following precautions to protect ourselves:
If we are Driving
(a) Pull over to the side of the road, stop and set the parking brake.
(b) Avoid bridges, power lines, poles, signboards, buildings and trees, as much as possible.
(c) Stay inside the vehicle until shaking stops.
(d) If a power line falls on the vehicle, stay in till a trained person removes it.
If we are Outdoors
(a) Look for a place away from trees, poles, power lines, buildings and hoardings.
(b) Drop to the ground, till the tremors stop.
12. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you
have to go out on that day. Would you carry an umbrella? Explain.
Ans. If it is predicted that a thunderstorm is likely to occur on a certain day and we have to go out on that day, then carrying
an umbrella is not a good idea because an umbrella has metallic wires and rods that can cause electric shock during
thunderstorm. Also, we have to face a lot of difficulties due to storms.

Chapter 13.
Light
1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room.
Explain.
Ans. No, we cannot see the objects kept in a dark room when we are also present in the room. But, we can see the objects
outside the room because the light comes from them to our eyes.
2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of
reflection?
Ans.
Regular Reflection Irregular Reflection
1. When a beam of light falls on a smooth and highly 1. When a beam of light falls on a rough and uneven
polished surface, almost entire light gets reflected in surface, the light gets reflected in different directions,
the same medium in a definite direction. This kind of i.e., light rays do not follow uniformity of direction.
reflection is called regular reflection. This kind of reflection is known as irregular or diffused
reflection.
2. We can see our image formed by a mirror due to the 2. In case of diffused reflection from rough surfaces, either
phenomenon of regular reflection. there is no image formed or a blurred (hazy) image is
formed.
3. Regular reflection creates glare and we cannot see the 3. We are able to see things comfortably because of
things clearly and comfortably. irregular reflection.
No, diffused reflection does not mean the failure of laws of reflection.

Science 8 (NCERT Textbook Solution) 21

 3. Mention against each of the following whether regular or diffused reflection will take place when a beam of
light strikes. Justify your answer in each case.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper
Ans. Regular reflection will take place in case a beam of light strikes on (a) polished wooden table, (d) marble floor with
water spread over it and (e) mirror. In these cases, surfaces are too smooth.
Diffused reflection will take place in case of light striking on (b) chalk powder, (c) cardboard surface and (f) piece of
paper. In these cases, surfaces are so rough that regular reflection cannot take place.
4. State the laws of reflection.
Ans. When light falls on a plane smooth surface, it follows the laws of reflection. These laws are:
(a) When a ray of light (incident ray) falls on a plane smooth surface, it is reflected in the same medium in such a way
that the angle of incidence is equal to the angle of reflection.
(b) The incident ray, the reflected ray and the normal always lie in the same plane.
5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie
in the same plane.
Ans. Place a white sheet of paper on a drawing board. Take a plane mirror, put it vertically and draw a line MN along the
margin of the mirror. Fix two pins A and B at a gap of a few cm apart. They should appear in one line in the mirror.
Now, by looking at the reflection of these two pins, fix two pins C and D on the other side in such a way that images
of all four pins appear in a straight line. Remove the pins and draw a straight line along the path of AB and CD. The
point where these two lines meet, mark it as O. Draw a perpendicular OR. This is called normal. The angle formed by
the incident ray (OP) and the normal is called angle of incidence (i) and angle formed by the normal and reflected ray
(OQ) is called angle of reflection (r).

From the figure, it is clear that the incident ray, the reflected ray and the normal at the point of incidence lie in the same
plane.
6. Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be m away from his image.
(b) If you touch your ear with right hand in front of a plane mirror it will be seen in the mirror
that your right ear is touched with .
(c) The size of the pupil becomes when you see in dim light.
(d) Night birds have cones than rods in their eyes.
Ans. (a) 2 (b) left, left (c) bigger (d) less
Choose the correct option in questions 7-8.
7. Angle of incidence is equal to the angle of reflection.
(a) Always (b) Sometimes (c) Under special conditions (d) Never
Ans. (a)
8. Image formed by a plane mirror is
(a) virtual, behind the mirror and enlarged. (b) virtual, behind the mirror and of the same size as the object.

22 Science 8 (NCERT Textbook Solution)

 (c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.
Ans. (b)
9. Describe the construction of a kaleidoscope.
Ans. Take three strips of plane mirrors of equal sizes, say 15 cm long and 4 cm wide. Join the long edges of the mirror strips,
with an adhesive tape on their outer side, so that their reflecting surfaces face each other and they form a triangular tube.
Close one end of the triangular tube with a small triangular sheet of glass. Paste a piece of butter paper on the outer
side of the triangular glass sheet. Put some small coloured objects from its open end, into the tube. These objects may
be coloured beads, stars, broken pieces of bangles or plastic toys. Be sure that these coloured objects have enough
space to move inside the tube. Finally, close the open end of the tube with a piece of cardboard, having a 2 mm wide
hole in its centre. The kaleidoscope is ready. To make it more durable and decorative, you can wrap the sides (only)
of your kaleidoscope with a brightly coloured and designed paper. Look through the hole of kaleidoscope with one eye.
Rotate the kaleidoscope to see different and new patterns made by coloured objects, due to multiple reflections.
10. Draw a labelled sketch of the human eye.
Ans.

11. Gurmit wanted to perform Activity 13.8 using a laser torch. Her teacher advised her not to do so. Can you
explain the basis of the teacher’s advise?
Ans. Laser beam is not preferred to human eyes because it can damage the living cells (rods and cones specially). Hence,
the teacher advised Gurmit not to perform the activity related to varying the pupil’s size in bright and dim light, using a
laser torch.
12. Explain how you can take care of your eyes.
Ans. Ways to take care of the eyes:
(a) The eyes should be washed every day with fresh and clean water at normal temperature.
(b) Never rub the eyes.
(c) In case of dust particles get into the eyes, splash a lot of clean and cold water into the eyes so as to wash away
the dust.
(d) If a foreign particle like a splinter or a metal particle got into the eyes, wash with clean and cold water immediately.
If it does not come out with water, rush to the doctor.
(e) Never look at very bright sources of light like the sun or a welding spark directly.
(f) Do not read or write in dim or very bright light and also in a moving vehicle.
(g) While reading or writing, keep your books or notebooks at a distance of distinct vision (25 cm) from the eyes.
(h) Do not spend much time looking at computer screen or a television screen.
13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90°
to the incident ray?
Ans. According to question, angle between incident ray and reflected ray is 90°. Let us i r
sketch a ray diagram to understand it clearly.
Here, ∠AOB = 90° (from question)
 ∠AON = ∠BON (Law of reflection)
i.e., ∠i = ∠r
So, ∠i + ∠r = 90°
or ∠i + ∠i = 90°
or 2∠i = 90°
90°
∴ ∠i = = 45°
2
Science 8 (NCERT Textbook Solution) 23
 14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by
40 cm?
Ans. Infinite images, because the number of images formed by two plane mirrors does not depend upon distance between
them. It depends upon the angle between them.
As the angle between two parallel plane mirrors is 0°.
So number of images,
360° 360°
= –1= –1=∞–1=∞
θ 0
So, infinite number of images will be formed.
15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 13.19.
Draw the reflected ray from the second mirror.

Fig. 13.19

Ans.

16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 13.20. Can he see himself in the mirror?
Also can he see the image of objects situated at P, Q and R?

Fig. 13.20

Ans. From the ray diagram shown below, we observe that

l Boojho standing at A cannot see his image himself because reflected rays (as drawn O A and O A from two ends)
1 1 2 2
go away from him.
l Position of A lies between reflected rays O P and O P as well as O Q and O Q , hence he can see the images
1 1 2 2 1 1 2 2
of P and Q only.

24 Science 8 (NCERT Textbook Solution)

 l Similar to that of A, reflected rays O R and O R move away from A so he cannot see R also.
1 1 2 2
17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 13.21).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?

Fig. 13.21

Ans. (a) From the sketched ray diagram, it is clear that A′ is the image of A formed by the given plane mirror.

(b) Yes, B can see this image.
(c) Yes, C can see this image.
(d) Position of A does not change so position of image will also not change when Paheli moves from B to C.

Science 8 (NCERT Textbook Solution) 25

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First published 2023
Second edition 2025