NCERT SOLUTIONS FOR CLASS 8 SCIENCE

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Last modified:2019-08-25

NCERT Solutions for Class 8 Science and Read NCERT Class 8 Science Notes

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Chapter 1 - Crop Production and Management

Question 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 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 the water.

(d) For growing a crop sufficient sunlight, …………….. and …….. the soil is essential.

Answer

(a) crop

(b) preparation

(c) float

(d) nutrients, water

Question 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

Answer

A

B

(i) Kharif Crops

(e) Paddy and Maize

(ii) Rabi Crops

(d) Wheat, Gram, Pea

(iii) Chemical Fertilisers

(b) Urea and Super Phosphate

(iv) Organic Manure

(c) Animal Excreta, Cow Dung Urine and plant waste

Question 3

Give two examples of each.

(a) Kharif crop

(b) Rabi crop

Answer

(a) Kharif Crop - Paddy, Maize

(b) Rabi Crop - Wheat, Gram

Question 4

Write a paragraph in your own words on each of the following.

(a) Preparation of soil (b) Sowing

(c) Weeding (d) Threshing

Answer

(a) Preparation of Soil

The preparation of the soil is the first step before growing a crop. One of the most important tasks in agriculture is to turn the soil and loosen it. This allows the roots to penetrate deep into the soil. The loose soil allows the roots to breathe easily even when they go deep into the soil. The loosened soil helps in the growth of earthworms and microbes present in the soil. These organisms are friends of the farmer since they further turn and loosen the soil and add humus to it.

(b) Sowing

Sowing is the most important part of crop production. Before sowing, good quality seeds are selected. Good quality seeds are clean and healthy seeds of a good variety. Farmers prefer to use seeds which give a high yield. Damaged seeds

become hollow and are thus lighter. Therefore, they float on water. So, we should pick good quality seeds for sowing.

(c) Weeding

Undesirable plants which grow naturally along with the crop in a field are called weeds. The process of removal of weeds is called weeding. Weeding is necessary since weeds compete with the crop plants for water, nutrients, space, and light. Thus, they affect the growth of the crop. Some weeds interfere even in harvesting and may be poisonous for animals and human beings. Weeds can be removed in various methods include physical removal of weeds by uprooting or cutting them close to the ground, from time to time or by using certain chemicals, called weedicides.

(d) Threshing

Separation of the grains from the chaff is called threshing. This is carried out with the help of a machine called ‘combine’ which is, in fact, a combined harvester and thresher.

Question 5

Explain how fertilisers are different from manure.

Answer

Fertilisers are an inorganic salt, which is prepared in factories. It does not provide any humus to the soil. Fertilisers are very rich in plant nutrients like nitrogen, phosphorus, and potassium.

Question 6

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

Answer

The supply of water to crops at different intervals is called irrigation. The time and frequency of irrigation vary from crop to crop, soil to soil and season to season.

Two Methods of Irrigation which conserve water

 (i) Sprinkler system:

This system is used on the uneven land where less water is available. The perpendicular pipes, having rotating nozzles on top, are joined to the main pipeline at regular intervals. Water is allowed to flow through the main pipe under pressure, which escapes from the rotating nozzles. In this way, water gets sprinkled on the crop.

(ii) Drip irrigation:

This system is used to save water as it allows the water to flow drop by drop at the roots of the plants. It is the best technique for watering fruit plants, gardens and trees. Water is not wasted at all.          

Question 7

If wheat is sown in the kharif season, what would happen? Discuss.

Answer

Rabi crops are sown in the winter season from October to March. If it is sown in the Kharif season, the whole crop might get destroyed because of many factors such as lack of optimum temperature, adaptability, availability of pests, etc. Kharif season includes the rainy season, which is not favourable for the growth of wheat crop.

Question 8

Explain how soil gets affected by the continuous plantation of crops in

a field.

Answer

The continuous growing of crops makes the soil poorer in certain nutrients such as nitrogen, phosphorus, potassium, etc. Plants require nutrients for their proper growth and functioning. When a farmer continues to grow’ crops one after the other, then all nutrients available in the soil reduces and the crop yield decreases automatically.

Question 9

What are weeds? How can we control them?

Answer

Undesirable plants which grow naturally along with the crop in a field are called weeds. Weeds can be removed in various methods include physical removal of weeds by uprooting or cutting them close to the ground, from time to time or by using certain chemicals, called weedicides.

Question 10

Arrange the following boxes in a proper order to make a flow chart of

sugarcane crop production.

Add Image

Answer

(6) Ploughing the field

(5) Preparation of Soil

(4) Sowing

(7) Manuring

(2) Irrigation

(3) Harvesting

(1) Sending crop to sugar factory

Question 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 the water.

Add Image

Answer

1) Irrigation

2) Storage

3) Harvester

4) Gram

5) Crop

6) Winnowing

Chapter 2 - Microorganisms: Friend and Foe

Question 1

Fill in the blanks:

(a) Microorganisms can be seen with the help of a ____________.

(b) Blue-green algae fix __________ directly from air to enhance fertility of soil.

(c) Alcohol is produced with the help of __________.

(d) Cholera is caused by __________.

Answer

(a) Microscope

(b) nitrogen

(c) Microorganisms

(d) Bacteria

Question 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

Answer

(a) (ii) alcohol

(b) (ii) Streptomycin

(c) (i) female Anopheles mosquito

(d) (ii) housefly

(e) (iii) growth of yeast cells

(f) (iii) fermentation

Question 3

Match the organisms in Column I with their action in Column II.

Column I

Column II

(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

Answer

Column I

Column II

(i) Bacteria

(e) Causing Cholera

(ii) Rhizobium

(a) Fixing Nitrogen

(iii) Lactobacillus

(b) Setting of Curd

(iv) Yeast

(c) Baking of bread

(v) A Protozoan

(e) Causing Malaria

(vi) A virus

(f) Causing Aids

Question 4

Can microorganisms be seen with the naked eye? If not, how can they be seen?

Answer

Microorganisms are smaller in size. So we cannot be seen with the naked eye. It can be seen with the help of a microscope.

Question 5

What are the major groups of microorganisms?

Answer

Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa, and some algae.

Question 6

Name the microorganisms which can fix atmospheric nitrogen in the soil.

Answer

Bacteria like Rhizobium and Azotobacter and blue-green algae can fix atmospheric nitrogen in the soil.

Question 7

Write 10 lines on the usefulness of microorganisms in our lives.

Answer

(i) Microorganisms help us in food sector. They are used for curdling of milk, preparation of bread, cake, etc.

(ii) Microorganisms are used to produce alcohol at large scale.

(iii) They are also used to produce wine.

(iv) Yeast is used in bakeries.

(v) They are also used as preservatives for food items.

(vi) They are used to make different medicines, especially antibiotics.

(vii) Microorganisms are used to prepare vaccines for various diseases.

(viii) They are useful for agriculture sector, as they enhance the fertility of soil by fixing nitrogen.

(ix) They work as natural cleaners, as they decompose the dead bodies of plants and animals.

(x) Microorganisms prepare manures by decomposing dead bodies of plants and animals.

Question 8

Write a short paragraph on the harms caused by microorganisms.

Answer

Harmful effects of microorganisms:

i. Micro-organisms cause diseases in animals. For example, in humans, bacteria cause diseases such as tuberculosis, cholera, typhoid, etc.

ii. In cattle, the foot and mouth disease is caused by a virus.

iii. Also, several microbes cause diseases in plants. For example, the productivity of wheat, orange, apple, etc. is reduced due to microbial diseases in plants.

iv. Certain microbes, on entering into our body, produce toxic substances. This leads to food poisoning. Some micro-organisms such as fungus spoil our food. For example, bread, when left unused under moist conditions, gets spoilt by fungus, producing a white cotton-like growth on the bread.

Question 9

What are antibiotics? What precautions must be taken while taking antibiotics?

Answer

Antibiotics are medicines produced by certain microorganisms to kill other disease-causing microorganisms. These medicines are commonly obtained from bacteria and fungi. Streptomycin, tetracycline, penicillin, etc. are common antibiotics.

Precautions to be taken while using antibiotics:

(i) Antibiotics should be taken under the supervision of a well-qualified doctor.

(ii) Course (intake) of antibiotics should be completed as per the prescription is given by the doctor.

(iii) Antibiotics should be taken in the right amount and at the right time. A wrong dose of antibiotic makes the drug ineffective. Also, excessive consumption of drugs may kill the useful bacteria present in our body.

Chapter 3 - Synthetic Fibres and Plastics

Question 1

Explain why some fibres are called synthetic.

Answer

Fibres which are prepared by man using chemicals are called synthetic fibers. These are made by joining small units called monomers into long chains. Examples of synthetic fibres are rayon, nylon, polyester, acrylic, etc.

Question 2

Mark ( ) the correct answer:

Rayon is different from synthetic fibres because

(a) it has a silk like appearance

(b) it is obtained from wood pulp

(c) its fibres can also be woven like those of natural fibres

Answer

(b) it is obtained from wood pulp

Question 3

Fill in the blanks with appropriate words:

(a) Synthetic fibres are also called ______ or _______ fibres.

(b) Synthetic fibres are synthesised from raw material called ________.

(c) Like synthetic fibres, plastic is also a __________ .

Answer

(a) Artificial and Man-made

(b) Petrochemicals

(c) Polymer

Question 4

Give examples which indicate that nylon fibres are very strong.

Answer

Nylon fibres are very strong. They are used for making ropes which are used for climbing rocks and for making parachutes. Their usage shows that nylon fibres have high tensile strength.

Question 5

Explain why plastic containers are favoured for storing food.

Answer

The characteristics that make plastics favourable for storing food items are:

(i) Lightweight

(ii) Lower price

(iii) Good strength

(iv) Easy handling

Question 6

Explain the difference between the thermoplastic and thermosetting

plastics.

Answer

Thermoplastic

Thermosetting Plastic

Thermosetting plastic cannot be bent easily. It may break when forced to bend.

Thermoplastic can be bent easily.

Thermosetting plastic cannot be softened by heating. Thus, it cannot be reshaped once molded.

Thermoplastic can be softened easily by heating. Thus, it can be reshaped.

Question 7

Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plugboards

Answer

(a) Saucepan handles are made of thermosetting plastics because these plastics do not get softened on heating. Also, thermosetting plastics such as bakelite are poor conductors of heat.

(b) Thermosetting plastics such as bakelite are poor conductors of heat and electricity. Therefore, they are used for making electric plugs, switches, plugboards, etc.

Question 8

Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’:

Telephone instruments, plastic toys, cooker handles, carry bags, ballpoint pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.

Answer

Can be Recycled

Cannot be Recycled

Telephone instruments

Plastic Toys

Cooker Handles

Plastic Chairs

Electrical Switches

Carry Bags

Question 9

Rana wants to buy shirts for summer. Should he buy cotton shirts or

shirts made from synthetic material? Advise Rana, giving your reason.

Answer

Rana should buy shirts made from cotton. This is because cotton is a good absorber of water. It absorbs the sweat coming out of our body and exposes it to the environment thereby cooling our body.

Question 10

Give examples to show that plastics are non corrosive in nature.

Answer

Plastics are not corroded even if they come in contact with strong chemicals. This is because of their non-reactive nature with most materials.

For example:

1. Plastics don't react with acids or bases which are kept in them. The cleansing chemicals that we use at home are stored in plastic bottles, instead of metal containers.

2. Plastics don't react with air and water. For example chairs, buckets all are made from plastics.

Question 11

Should the handle and bristles of a toothbrush be made of the same

material? Explain your answer.

Answer

No, the handle and bristles of a toothbrush should be made of different materials. The handle of a toothbrush should be hard and strong so, it should be made from plastic. While the bristles should be soft and flexible so, they should be made from nylon.

Question 12

‘Avoid plastics as far as possible’. Comment on this advice.

Answer

We should avoid plastics because:

Plastics are non-biodegradable. Once introduced into the environment, they take several years to decompose. Plastics cause environmental pollution.

They cannot be burnt as when burnt, they release poisonous gases. Plastic bags thrown in the garbage dump are swallowed by animals like cows. These bags choke their respiratory system and can even prove fatal. Therefore, we should avoid plastics as far as possible.

Question 13

Match the terms of column I correctly with the phrases given in column II.

A

B

(i) Polyester

(a) Prepared by using wood pulp

(ii) Teflon

(b) Used for making parachutes and stockings

(iii) Rayon

(c) Used to make non-stick cookware

(iv) Nylon

(d) Fabrics do not wrinkle easily

Answer

A

B

(i) Polyester

(d) Fabrics do not wrinkle easily

(ii) Teflon

(c) Used to make non-stick cookware

(iii) Rayon

(a) Prepared by using wood pulp

(iv) Nylon

(b) Used for making parachutes and stockings

Question 14

Manufacturing synthetic fibers are actually helping conservation of forests'. Comment.

Answer

Raw materials for natural fibres are mainly derived from plants and this means cutting a lot of trees. This leads to deforestation. But raw materials of synthetic materials are mainly petrochemicals. Hence, manufacturing synthetic fibres helps in the conservation of forests.

Question 15

Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answer

We will design a circuit to see that thermoplastics are poor conductors of electricity.

Materials needed for activity: bulb, some wires, a battery, a piece of metal, and a plastic pipe.

Procedure: Set up the circuit first with the metal and then with the plastic pipe (as shown in the figure).

After you switch on the current, you will observe that the bulb glows in the former case. In the latter case, the bulb does not glow.

Hence, a plastic pipe (which is a thermoplastic) is shown to be a poor conductor of electricity.

Chapter 4 - Materials: Metals and Non-Metals

Question 1

Which of the following can be beaten into thin sheets?

(a) Zinc (b) Phosphorus (c) Sulphur (d) Oxygen

Answer

(a) Zinc

 

Question 2

Which of the following statements is correct?

(a) All metals are ductile.

(b) All non-metals are ductile.

(c) Generally, metals are ductile.

(d) Some non-metals are ductile.

Answer

(c) Generally, metals are ductile

Question 3

Fill in the blanks :

(a) Phosphorus is very _____ non-metal.

(b) Metals are _____ conductors of heat and_____.

(c) Iron is _____ reactive than copper.

(d) Metals react with acids to produce _____ gas.

Answer

(a) reactive 

(b) good, electricity           

(c) more             

(d) hydrogen

Question 4

Mark ‘T’ if the statement is true and ‘F’ if it is false.

(a) Generally, non-metals react with acids. ( )

(b) Sodium is a very reactive metal. ( )

(c) Copper displaces zinc from zinc sulphate solution. ( )

(d) Coal can be drawn into wires. ( )

Answer

(a) False        

(b) True 

(c) False 

(d) False

Question 5

Some properties are listed in the following Table. Distinguish between metals and non-metals on the basis of these properties.

Properties

Metals

Non-metals

1. Appearance

Electro Positive

Electro Negative

2. Hardness

They react with water to form basic oxides 

They react with oxygen to form acidic 

3. Malleability

Malleable

Non- Malleable

4. Ductility

Ductile

Non-Ductile

5. Heat Conduction

Good Conduction

Poor Conduction

6. Conduction of Electricity

Good Conductor

Bad Conductor

Answer

Properties

Metals

Non-metals

1. Appearance

lustrous

non-lustrous

2. Hardness

Hard except sodium and potassium

Generally soft except diamond

3. Malleability

Generally malleable

non-malleable

4. Ductility

Generally ductile

non-ductile

5. Heat Conduction

Good conductors

Poor conductors

6. Conduction of Electricity

Good conductors

Poor conductors

Question 6

Give reasons for the following.   

(a) Aluminium foils are used to wrap food items.          

(b) Immersion rods for heating liquids are made up of metallic substances.

(c) Copper cannot displace zinc from its salt solution.           

(d) Sodium and potassium are stored in kerosene.

Answer

(a) Aluminium is malleable, soft and does not react with food items, so it is used to wrap food items.            

(b) Metals are a good conductor of heat and electricity, so immersion rods are made up of metallic substances.            

(c) Copper is less reactive than zinc, so it can not displace zinc from its salt solution.            

(d) Sodium and Potassium are very reactive; they react with air and water, so they are stored in kerosene.

Question 7

Can you store lemon pickle in an aluminium utensil? Explain.

Answer

No, we cannot store the lemon pickle in aluminium utensil because aluminium is a metal and lemon is acidic. The acids react with metals to give hydrogen which would spoil the food and makes it unfit to use.

 

Question 8

Match the substances given in Column A with their uses given in Column B.

Column I

Column II

(i) Gold

(a) Thermometers

(ii) Iron

(b) Electric wire

(iii) Aluminium

(c) Wrapping Food

(iv) Carbon

(d) Jewellery

(v) Copper

(e) Machinary

(vi) Mercury

(f) Fuel

Answer

Column I

Column II

(i) Gold

(d) Jewellery

(ii) Iron

(e) Machinary 

(iii) Aluminium

(c) Wrapping Food

(iv) Carbon

(f) Fuel

(v) Copper

(b) Electric wire

(vi) Mercury

(a) Thermometers

Question 9

What happens when

(a) Dilute sulphuric acid is poured on a copper plate?

(b) Iron nails are placed in copper sulphate solution?

Write word equations of the reactions involved.

Answer

(a) When dilute sulphuric acid is poured on a copper plate, copper reacts with an acid to give copper sulfate and hydrogen.                 

Sulphuric acid + Copper → Copper sulphate + Hydrogen            

(b) When iron nails are placed in copper sulphate solution, displacement reaction takes place in which iron displaces copper.             

Copper sulphate + Iron → Iron sulphate + Copper                  

The blue colour turns into green.

Question 10

Saloni took a piece of burning charcoal and collected the gas evolved in a test tube.

(a) How will she find the nature of the gas?

(b) Write down word equations of all the reactions taking place in this Process.

Answer

(a) When charcoal burnt then carbon dioxide gas is formed. This gas turns lime water into milky substance. It can also be tested by red and blue litmus. The solution of gas turns blue litmus into red so it is acidic.       

(b) Carbon + Oxygen → Carbon dioxide         

Carbon dioxide + Lime water → Milky

Question 11

One day Reeta went to a jeweller’s shop with her mother. Her mother gave old gold jewellery to the goldsmith to polish. Next day when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a reason for the loss in weight?

Answer

Gold is a metal which is washed in an acidic solution. Some gold dissolves in acid to form an oxide. This causes the loss of gold in the form of gold oxide. In this process, a certain amount of gold is lost to the acidic solution.

Chapter 5 - Coal and Petroleum

Question 1

What are the advantages of using CNG and LPG as fuels?

Answer

The advantages of using compressed natural gas (CNG) and liquified petroleum gas (LPG) as fuels are:

  • They can be burnt directly
  • They can be transported easily through pipelines
  • They are clean fuels and do not give smoke when burnt
  • They give a lot of heat energy when burnt

Question 2

Name the petroleum product used for surfacing of roads.

Answer

Bitumen, a petroleum product, is used for surfacing roads.

Question 3

Describe how coal is formed from dead vegetation. What is this process called?

Answer

Million years ago the earth had dense forests in low lying wetland areas. Due to natural processes, like flooding, these forests got buried under the soil. As more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. Under high pressure and high temperature, dead plants got slowly converted to coal. As coal contains mainly carbon, the slow

process of conversion of dead vegetation into coal is called carbonisation.

Question 4

Fill in the blanks :

(a) Fossil fuels are _____, _____ and _____.

(b) The process of separation of different constituents from petroleum is

called _____.

(c) Least polluting fuel for vehicle is _____.

Answer

(a) Coal, Petroleum, Natural gas

(b) Refining

(c) CNG

Question 5

Tick True/False against the following statements :

(a) Fossil fuels can be made in the laboratory. (T/F)

(b) CNG is more polluting fuel than petrol. (T/F)

(c) Coke is almost pure form of carbon. (T/F)

(d) Coal tar is a mixture of various substances. (T/F)

(e) Kerosene is not a fossil fuel. (T/F)

Answer

(a) False

(b) False

(c) True

(d) True

(e) False

Question 6

Explain why fossil fuels are exhaustible natural resources.

Answer

Fossil fuels require millions of years to form from the dead vegetation and animals that get buried deep inside the earth. They require high temperature and pressure for their formation, which cannot be provided in the laboratory. Thus, fossils are limited. Therefore, the use of fossil fuels at this rate will lead to their exhaustion.

Question 7

Describe characteristics and uses of coke.

Answer

Characteristics of coke are:

  • Tough
  • Porous
  • Black in colour

Uses of Coke:

  • In the manufacture of steel
  • In the extraction of metals (as a reducing agent)

Question 8

Explain the process of the formation of petroleum.

Answer

Petroleum was formed from dead organisms that got buried in the sea millions of years ago. These dead bodies got covered with layers of sand and clay. Lack of air, high temperature, and high pressure transformed these dead organisms into petroleum and natural gas.

Question 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.

Add Image

Answer

Add Image

Chapter 6 - Combustion and Flame

Question 1

List conditions under which combustion can take place.

Answer

Conditions necessary for combustion are:          

(i) Presence of a combustible substance.          

(ii) Attainment of ignition temperature.         

(iii) Proper supply of air to provide oxygen.

Question 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 _____.

Answer

(a) pollution                                   

(b) kerosene

(c) ignition temperature         

(d) water

Question 3

Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer

(i) It produces less carbon monoxide gas.          

(ii) It produces less carbon dioxide gas.          

(iii) It produces less amount of sulphur dioxide and nitrogen dioxide which cause acid rain.         

(iv) No residue remains after combustion.

Question 4

Compare LPG and wood as fuels.

Answer

LPG

Wood

It is a gaseous fuel

It is a solid fuel

It does not produce smoke

It produces smoke

Its calorific value is more (55000 kJfkg)

Its calorific value is less (17000 kJfkg)

It is easily stored in cylinders

It requires more space to store

It does not cause any pollution

It causes more pollution

Question 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.

Answer

(a) Water is a good conductor of electricity. It conducts electricity and may result in electric shock.          

(b) LPG has more calorific value and produces no pollution. So it is better domestic fuel than wood.          

(c) The ignition temperature of paper is less, so it catches fire easily. It does not catch fire when wrapped around aluminium pipe because aluminium absorbs the heat, so paper does not attain its ignition temperature.

Question 6

Make a labelled diagram of a candle flame.

Answer

Add image

Question 7

Name the unit in which the calorific value of a fuel is expressed.

Answer

Kilojoules per kg (kJ/kg)

Question 8

Explain how CO2 is able to control fires.

Answer

(i) CO2 forms a blanket around fire due to which supply of air is stopped.          

(ii) CO2 also brings down the temperature of the fuel.

Question 9

It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer

The green leaves contain some water due to which the ignition temperature of leaves increases and they do not catch fire easily while dry leaves have no water, so they catch fire easily.

Question 10

Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer

A goldsmith uses the outer zone (non-luminous zone) of a candle flame to melt gold and silver because it is the hottest zone and has more temperature.

Question 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.

Answer

Total mass of fuel = 4.5 kg        

Total heat produced = 180,000 kJ        

Heat produced by burning 1 kg of fuel = 180,000 kJ/4.5 kg = 40,000 kJ/kg.       

So, calorific value of fuel = 40,000 kJ/kg.

Question 12

Can the process of rusting be called combustion? Discuss.

Answer

The process of rusting cannot be called combustion because in this process no heat and light is produced. Due to this reason iron is not considered as combustible substance.

Question 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?

Answer

The water heated by Ramesh will get heated in a shorter time because he kept his beaker near the hottest zone of the flame.

Chapter 7 - Conservation of Plants and Animals

Question 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.

Answer

(a) Wildlife sanctuary

(b) Endemic species

(c) Climate

Question 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.

Answer

(a) Wildlife sanctuary is the place where wild animals are protected in their natural habitat. Here poaching or capturing the animals is prohibited.            Biosphere reserves are the area used to conserve biodiversity like plants, animals and microorganisms. It contains many wildlife sanctuaries and national parks.            (b) Zoos are the places where some animals are protected in artificial habitats for public view. Wildlife sanctuaries are used to protect and conserve wildlife in their natural habitats.           

(c) The species which are at the verge of extinction and required protection and conservation are called endangered species. The species which exist no more on the earth are called extinct species.           

(d) Flora is used for wide varieties of plants we fauna is used to refer to all the varieties of animals.

Question 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.

Answer

(a) Wild animals: Deforestation destroys the habitats of many wild animals. They are left to die without their natural shelter.           

(b) Environment: Deforestation affect the environment adversely due to disturbance of ecological balance. The amount of carbon dioxide is increased in the atmosphere and causes global warming.           

(c) Villages (Rural areas): The villagers depend on forests. They are closely related to forests. Due to deforestation various wild animals run to nearby villages and cause danger to them. Villagers will not get fruits, fuel, wood due to deforestation.           

(d) Cities (Urban areas): Deforestation does not affect the life of cities directly badly but changes in climate affect cities. Calamities like flood and droughts do not spare cities.          

(e) Earth: Deforestation affects the earth. The fertile land of earth is converted into deserts and its temperature rises. It changes its climate and environment.            (f) The next generation: Many beautiful fauna and flora are destroyed due to deforestation. So, the next generation will not be able to see them. They will not get clean and cool environment. They will not have a clean and healthy environment.

Question 4

What will happen if:           

(a) we go on cutting trees.           

(b) the habitat of animal is disturbed.           

(c) the top layer of soil is exposed.

Answer

(a) If we go on cutting trees then:                

(i) Ecological balance is disturbed.                

(ii) Earth will loose top fertile layer and will be converted into a desert.                 (iii) Floods and droughts will become more frequent.                

(iv) Many animals lost their shelters.                 

(v) There will be a scarcity of things like fruits, a paper which we get from forests.           

(b) Animals become unprotected and unconserved when the habitat of animal is disturbed. It becomes endangered.           

(c) Top layers of soil are fertile. If they are exposed, they lose their fertility and humus. It causes desertification.

Question 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?

Answer

(a) Biodiversity is conserved, to save them from extinction.           

(b) The protected forests are not also safe completely for wildlife because the poaching takes place at large scale in these areas. The poachers kill wild animals in those protected areas.           

(c) Some tribals depend on the jungle for their food, clothes, shelter and other requirements.           

(d) Deforestation is caused due to urbanization and industrialization. Deforestation is responsible for desertification and natural calamities. Many species of plants and animals are destroyed due to deforestation.          

(e) Red Data Book is the record book and source book of all the endangered animals and plants.           

(f) Periodical movement of a species of birds from their native place to other places due to changes in climate is called migration. The purpose of migration is for breeding and to enjoy long summer days.

Question 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 purposes? Discuss and prepare a brief report.

Answer

The continuously cutting of trees is not justified for any project. We not only cut the trees, but we also snatch away the shelters of many animals, birds and other organisms. The ecological balance is also disturbed. It also causes many natural calamities.

Question 7

How can you contribute to the maintenance of green wealth of your locality? Make a list of actions to be taken by you.

Answer

(i) We can grow more and more plants in our locality.           

(ii) We can protect them and provide water to them.           

(iii) All the residents should be taught about the benefits of trees.           

(iv) They should also be taught about the harms of cutting trees.

Question 8

Explain how deforestation leads to reduce rainfall.

Answer

Plants are the main agent to maintain the water cycle in the environment. So cutting of trees reduces the rainfall of that area. If plants will not absorb water from the soil they will not evaporate in the environment to form clouds. If clouds will not be formed then no rainfall takes place. In this way deforestation reduces rainfall.

Question 9

Why should paper be saved? Prepare a list of ways by which you can save paper.

Answer

The wood pulps and the bark of trees are used in the manufacturing of paper. If the paper is not saved it causes cutting more and more trees. So we should save paper to protect our trees.           

Ways to Save Paper:         

(i) A paper should be recycled.           

(ii) We should not throw paper here and there.           

(iii) The waste paper should be collected and sent for recycling.

Question 10

Complete the word puzzle           

Down           

1. Species on the verge of extinction.           

2. A book carrying information about endangered species.           

5. A 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.

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Answer

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Chapter 8 - Cell Structure and Functions

Question 1

Indicate whether the following statements are True (T) or False (F).

(a) Unicellular organisms have one-celled body. (T/F)

(b) Muscle cells are branched. (T/F)

(c) The basic living unit of an organism is an organ. (T/F)

(d) Amoeba has irregular shape. (T/F)

Answer

(a)  True     

(b)  True     

(c)  False     

(d)  True

Question 2

Make a sketch of the human nerve cell. What function do nerve cells

Perform?.

Answer

Functions of a nerve cell are

(a) it helps in the transfer of messages from various body parts to the brain and from the brain to various body parts of the body.     

(b) Nerve cells help in the coordination of the functions of the organs of the body.

Question 3

Write short notes on the following:

(a) Cytoplasm

(b) Nucleus of a cell

Answer

(a) Cytoplasm- it is a jelly-like substance which is present between the nucleus and the cell membrane. There are various other organelles present in the cytoplasm. It is made up of carbohydrates, proteins, and water.     

(b) A nucleus of a cell- Nucleus is a dense, spherical structure situated in the centre of a cell. A nuclear membrane separates the nucleus from the cytoplasm. It contains genetic materials like RNA and DNA.

Question 4

Which part of the cell contains organelles?

Answer

Cytoplasm contains organelles.

Question 5

Make sketches of animal and plant cells. State three differences between them.

Answer

Animal cell

Plant cell

(a) A chloroplast is not there

(a) Chloroplast is there

(b) A cell wall is absent

(b) A cell wall is present

(c) Centrosome is present

(c) It doesn’t have a centrosome

(d) Vacuoles are much smaller in size

(d) Vacuoles are large in size

Question 6

State the difference between eukaryotes and prokaryotes.

Answer

Eukaryotes have a well-designed nucleus and organelles covered with membranes but prokaryotes do not have a well designed nuclear membrane.

Question 7

Where are chromosomes found in a cell? State their function.

Answer

Chromosomes are present in the nucleus of the cell and they carry characteristics of parent cells to daughter cells.

Question 8

‘Cells are the basic structural units of living organisms’. Explain.

Answer

Numerous cell units combine to form tissues. These tissues combine to form organs which form a complete body. Hence cell is the structural unit of an organism that performs all basic functions of life.

Question 9

Explain why chloroplasts are found only in plant cells?

Answer

Photosynthesis is the main process in plants which takes places in the presence of chlorophyll which is found in chloroplasts. That’s why chloroplast is found only in plant cells.

Question 10

Complete the crossword with the help of clues given below:

Across

1. This is necessary for photosynthesis.

3. Term for component present in the cytoplasm.

6. The living substance in the cell.

8. Units of inheritance present on the chromosomes.

Down

1. Green plastids.

2. Formed by a collection of tissues.

4. It separates the contents of the cell from the surrounding

medium.

5. Empty structure in the cytoplasm.

7. A group of cells.

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Answer

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Chapter 9 - Reproduction in Animals

 

Question 1

Explain the importance of reproduction in organisms.

Answer

Reproduction is a very important process for living organisms. It facilitates the continuation of a species.

Question 2

Describe the the process of fertilization in human beings.

Answer

Sexual reproduction takes place in human beings. Male partner releases sperm inside a female’s body. Sperms and ovum fuse together in the fallopian tube to form a zygote. During this process, the nucleus of sperm fuses with the nucleus of ovum to form a single nucleus. This entire process is called fertilization.

Question 3

Choose the most appropriate answer:

(a) Internal fertilization 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) fertilization. (ii) metamorphosis. (iii) embedding. (iv) budding.

(c) The number of nuclei present in a zygote is

(i) none. (ii) one. (iii) two. (iv) four.

Answer

(a) in female body    

(b) metamorphosis     

(c) one

Question 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 fertilization takes place in frog. ( )

(d) A new human individual develops from a cell called gamete. ( )

(e) Egg laid after fertilization is made up of a single cell. ( )

(f) Amoeba reproduces by budding. ( )

(g) Fertilization is necessary even in asexual reproduction. ( )

(h) Binary fission is a method of asexual reproduction. ( )

(i) A zygote is formed as a result of fertilization. ( )

(j) An embryo is made up of a single cell. ( )

Answer

(a) False     

(b) True     

(c) True     

(d) False     

(e) True     

(f) False     

(g) False     

(h) True     

(i) True     

(j) False

Question 5

Give two difference between a zygote and a foetus.

Answer

Zygote

Foetus

It contains a single cell.

 

It contains many cells.

It is formed by the fusion of ovum and sperm

It is formed by repeated divisions of the zygote

Question 6

Define asexual reproduction. Describe two methods of asexual reproduction in animals.

Answer

Asexual reproduction is a mode of reproduction in which only a single parent is involved. It takes place in different forms:

(i) Buddings - This process takes place in hydra and bacteria. A part of these organisms starts growing further and bulging out and gradually separates from the organisms when fully developed.

(ii) This type of reproduction takes place in amoeba. The nucleus of amoeba gets divided into two followed by division of their bodies. Each part gets a separate nucleus and develops into a separate amoeba. This type of reproduction is common in unicellular organisms.

Question 7

In which female reproductive organ does the embryo get embedded?

Answer

Uterus

 

Question 8

What is metamorphosis? Give examples.

Answer

The transformation of a larva into an adult through drastic changes is called metamorphosis.

Question 9

Differentiate between internal fertilization and external fertilization.

Answer

Internal fertilization

External fertilization

(i) Takes place inside the body of a female

the female’s body

(i) Takes place outside the body of a female

(ii) Sperms are released by the male inside

(ii) Sperms are released in open

 

Question 10

Complete the crossword puzzle using the hints given below

Across

1. The process of the fusion of the gametes.

6. The type of fertilization 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 the fertilized egg.

4. These animals lay eggs.

5. A type of fission in amoeba.

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Answer

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Chapter 10 - Reaching the age of Adolescence

Question 1

What is the term used for secretions of endocrine glands responsible for changes taking place in the body?

Answer

Hormones

Question 2

Define adolescence.

Answer

The period of life, when the body undergoes changes, leading to reproductive maturity is called adolescence.

Question 3

What is menstruation? Explain.

Answer

When the egg produced by ovary does not get fertilized, the released egg and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding in women which is called menstruation. Menstruation occurs once in about 28 to 30 days.

Question 4

List the changes in the by that take place at puberty.

Answer

The changes in the body of puberty are:       

(i) A sudden increase in height.       

(ii) The bones of arms and legs elongate and make a person tall.       

(iii) Growing of hair at different parts of the body.       

(iv) The voice of a boy gets hoarse.       

(v) Development of sex organs.       

(vi)Growth of mustaches and beards in the boys.       

(vii) The breast in girls develops and hips become wider whereas in boys shoulders generally broaden.

Question 5

Prepare a table having two columns depicting names of endocrine glands and hormones secreted by them.

Answer

Endocrine glands

Hormones

1. Pituitary gland

1. Growth Hormones

2. Ovaries

2. Estrogen

3. Testes

3. Testosterone

4. Thyroid

4. Thyroxine

5. Pancreas

5. Insulin

6. Adrenal glands

6. Adrenalin

Question 6

What are sex hormones? Why are they named so? State their functions.

Answer  

The hormones which help and control the formation of secondary sexual characters are called sex hormones. They are called sex hormones because they control the sexual activities and help us to distinguish boys from girls. The sex hormones help to develop secondary sexual characters. Such as, in boys, they control the formation of sperms by testes and growth of facial hair. In girls they help to develop breasts, milk secreting glands or mammary glands.

Question 7

Choose the correct option.       

(a) Adolescence 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) tastes buds are well developed in teenagers.       

(b) Reproductive age in women starts when their:             

(i) menstruation starts.             

(ii) breasts start developing.             

(iii) body weight increasing.             

(iv) height increases.       

(c) The right meal for adolescents consists of:             

(i) chips, noodles, cake.             

(ii) chapati, dal, vegetable.              

(iii) rice, noodles and burger.             

(iv) vegetable cutlets, chips and lemon drink.

Answer

(a) (ii) proper diet is needed for the rapid growth taking place in their body.       

(b) (i) menstruation starts.       

(c) (ii) chapati, dal, vegetable.

Question 8

Write notes on:       

(a) Adam's apple

(b) Secondary Sexual characters       

(c) Sex determination in the unborn baby.

Answer 

(a) Adam's apple: The protruding part in the throat is called Adam's apple. It is the enlarged voice box or larynx. It makes the voice of boys coarse.       

(b) Secondary sexual characters: The characters which help to distinguish the male from female are called secondary sexual characters. The characters like the hair on the chest under arms, development of breast, beard, and mustaches are called secondary sexual characters.       

(c) Sex determination in unborn baby: The sex of the unborn baby is determined by the sex chromosomes. An unfertilized egg always has X chromosomes. Sperms are of two types. One contains X chromosomes and the other contains Y chromosomes. If sperm contributes a Y chromosome to the egg at fertilisation, the zygote would develop into male child. If a sperm containing X chromosome fertilizes the egg, the zygote would develop into a female.

Question 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 the brain      

8. Secretion of endocrine glands       

9. Pancreatic hormone       

10. Female hormone

Down

1. Male hormone

2. Secretes thyroxine

3. Another term for teenage

5. Hormone reaches here through bloodstream

6. Voice box

7. Term for changes at adolescence

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Answer 

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Question 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?

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Answer 

We conclude that initially, girls grow faster than boys but by about 18 years of age both reach about their maximum height. The rate of growth in height varies in different individuals.

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Chapter 11 - Force and Pressure

Question 1

Give two examples each of the situations in which you push or pull to change the state of motion of objects.

Answer 

We push a bicycle to move it, we pull the table to change its position.

Question 2

Give two examples of situations in which applied force causes a change in the shape of an object.

Answer 

  • When we press the foam, its shape is changed.
  • When we stretch the rubber band, its shape is changed.

Question 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______ and 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.

Answer 

(a) To draw water from a well we have to pull at the rope.

(b) A charged body attracts an uncharged body towards it.

(c) To move a loaded trolley we have to pull it.

(d) The north pole of a magnet repels the north pole of another magnet.

Question 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 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.

Answer 

(a) shape

(b) muscular

(c) contact

(d) gravity, friction.

 

Question 5

In the following statements identify the agent exerting the force and object on which its 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.

Answer

      (a) Fingers

            Lemon

Lemon juice is extracted

      (b) Fingers

            Toothpaste tube

Toothpaste come out

      (c) A load

            Spring

      The spring expands

      (d) An athlete

            On the legs

      Cleans the height

 

 

Question 6

A blacksmith hammers a hot piece of iron while making tool. How does the force due to hammering affect the piece of iron?

Answer

The force is hammering causes the change in the shape of iron and iron can be molded in the shape of the required of iron.

Question 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?

Answer

Electrostatic force.

Question 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.

Answer

The forces that act on the bucket are as follows:-

(i) The pressure of water contained in it exerted on its wall and on the base of the bucket.

(ii) Force of gravity by the earth.

(iii) The water is contained in a bucket and the pressure of force is exerted on the wall of the bucket, which is sufficient to take it as a force. As the bucket is held in my hand, so I overcome the force of gravity of the earth. That is why these forces do not bring a change in the state of motion.

Question 9

A rocket has been fired upwards to launch a satellite in its orbits. Name the two forces acting on the rocket immediately after leaving the launching pad.

Answer

i) Gravitational force        

ii) Force of friction

Question 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.

(b) Gravity of the earth.

(c) shape of a rubber bulb.

(d) Atmospheric pressure.

Answer

Atmospheric pressure.

Chapter 12 - Friction

Question 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.

Answer

(a) relative motion          

(b) nature          

(c) heat        

(d) reduces                         

(e) less

Question 2

Four children were asked to arrange forces due to rolling, static and sliding frictions in 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

Answer

(c) Static, sliding, rolling.

Question 3

Alida runs her toy car on a 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, 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.

Answer

(a) wet marble floor, dry marble floor, newspaper, towel.

Question 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.

Answer

The book moves downwards. The frictional force is acting opposite to the movement of book. So it acts upwards.

Question 5

You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you on the floor? Why?

Answer

The layer of soap makes floor smooth due to which the friction is reduced and the foot cannot make a proper grip on the floor. Therefore it is difficult to walk on a soapy floor and we start to slip on the floor.

Question 6

Explain why sportsmen use shoes with spikes.

Answer

Sportsmen use shoes with spike to increase the friction between shoes and the surface. The shoes with spike do not slip while they run or play.

Question 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?

Answer

The heavy object will be pressed hard against the opposite surface and produces more friction. So Seema will have to apply a larger force due to more friction.

Question 8

Explain why sliding friction is less than static friction.

Answer

The two sliding objects find less time to get interlocked against each other (objects and irregularities of the surface). So they get less friction. Therefore sliding friction is always less than static friction.

Question 9

Give examples to show that friction is both a friend and a foe.

Answer

Examples to show that friction is a friend and a foe:             

Friction is a friend:   

(i) Friction allows us to grip and catch different objects.             

(ii) It helps us to walk comfortably on the surface.             

(iii) It helps to minimise the speed or to stop the moving objects.             

(iv) It helps us to write on paper or blackboard.             

(v) The things do not move from their place due to the friction between the surfaces.             

Friction is a foe:         

(i) Friction causes wear and tear in objects.             

(ii) It causes damage to the parts of machines.             

(iii) The machines or tools require regular maintenance due to which a lot of money is wasted.             

(iv) It reduces the speed of moving objects, so more force is required.             

(v) It does not allow the free movement of objects.

Question 10

Explain why objects moving in fluids must have special shapes.

Answer

The object moving in fluids must have a special shape. This type of shape is called a streamlined shape. The streamlined shape helps to overcome the friction between objects and fluids. The objects have pointed fronts with the little broader middle portion which gets tapered at the back.

 

Chapter 13 - Sound

Question 1

Choose the correct answer :

Sound can travel through

(a) gases only (b) solids only

(c) liquids only (d) solids, liquids and gases.

Answer

(d) solids, liquids and gases

Question 2

Which of the following voices is likely to have a minimum frequency?

(a) Baby girl (b) Baby boy

(c) A man (d) A woman

Answer

(a) Baby girl

Question 2

In the following statements, tick T against those which are true, and F

against those which are false:

(a) Sound cannot travel in a vacuum. (T/F)

(b) The number of oscillations per second of a vibrating object is called

its time period. (T/F)

(c) If the amplitude of vibration is large, the sound is feeble. (T/F)

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)

(e) The lower the frequency of vibration, the higher is the pitch. (T/F)

(f) Unwanted or unpleasant sound is termed as music. (T/F)

(g) Noise pollution may cause partial hearing impairment. (T/F)

Answer

(a) True

(b) False

(c) False          

(e) False

(f) False

(g) True

Question 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) The shrillness of a sound is determined by the ................. of vibration.

Answer

(a) time period

(b) amplitude

(c) hertz

(d) noise         

(e) frequency

Question 5

A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer

Number of oscillations = 40          

Total time = 4 seconds          

Time taken to complete one oscillation = 4/40 = 1/10 = 0.1 second

So time period = 0.1 second

Frequency = 1/time period = 1/0.1 = 10Hz

Question 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?

Answer

Total vibrations = 500

Time taken = 1 second

Time taken to complete one vibration = 1/500 = 0.002 seconds.

So time period = 0.002 seconds.

Question 7

Identify the part which vibrates to produce sound in the following

instruments:

(a) Dholak (b) Sitar (c) Flute

Answer

(a) Stretched membrane

Question 8

What is the difference between noise and music? Can music become noise sometimes?

Answer

The sound which is unpleasant for our ears is called noise while music is the sound which is pleasant for our ears. Music becomes noise sometimes when it crosses the bearable range of sound for our ears.

Question 9

List sources of noise pollution in your surroundings.

Answer

Sources of noise pollution: Honking of horns, loudspeakers, loud sounds of machines in factories, loud sounds of T.V., radio, domestic appliances etc.

Question 10

Explain in what way noise pollution is harmful to humans.

Answer

Harmful effects of noise pollution:          

(i) It causes deafness.          

(ii) It causes mental illness.          

(iii) It causes headache and high blood pressure.

Question 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.

Answer

I would suggest my parents buy house three lanes away from the roadside. This house would safeguard our health and peace of mind.

Question 12

Sketch larynx and explain its function in your own words.

Answer

The larynx is also known as the voice box. It has vocal cords which have air column vibrating in them, which cause sound in humans.

Sketch Larynx by yourself

Question 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?

Answer

The speed of light is more than that of sound. Due to more speed, the light reaches us before the sound does. So lightning is seen earlier and thunder is heard later.

Chapter 14 - Chemical Effects of Electric Current

Question 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__________effect.          

(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 metallic object, by means of electricity, is culled __________.

Answer

(a) acids, bases, salts          

(b) chemical          

(c) negative          

(d) electroplating

Question 2

When the free ends of a tester are dipped into a solution the magnetic needle shows deflection. Can you plain the reason?

Answer

Yes, we can explain the reason. The magnetic needle will show deflection when the circuit is complete.          

The deflection of the magnetic needle shows that the solution is a good conductor.

Question 3

Name three liquids, which when tested in the manner shown in Fig. 14.1 may cause the magnetic needle to deflect.

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Answer

Tap water, hydrochloric acid, sodium hydroxide.

Question 4

The bulb does not glow in the set up shown in Fig 14.2. List the possible reasons. Explain your answer.

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Answer

The possible reasons may be:          

(i) The connections of the circuit may b loose.          

(ii) Bulb may b fused.          

(iii) Cells may be used up.          

(iv) The liquid may be poor conductor.

Question 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.

Answer

We conclude that option (i) ‘liquid A is a better conductor than liquid B'.

Question 6

Does pure water conduct electricity? If not what can we do to make it conducting?

Answer

No, pure water does not conduct electricity. Pure water can be made to conduct' dissolving salt in it.

Question 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.

Answer

The water used in water hoses is a good conductor of electricity. Firemen shut off the main electrical supply, because if the supply of electricity continues this may be a high risk of electrocution due to water.

Question 8

A child staying in the coastal region tests the drinking water and also the sea water with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Answer

Seawater contains more amount of salt than drinking water. So the sea water is a better conductor of electricity. This is the reason that the compass needle deflects more in case of seawater.

Question 9

Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

Answer

No, It is not safe for the electrician to carry out electrical repairs during a heavy downpour. It is because during heavy downpour there is a higher risk of electrocution.

Question 10

Paheli had heard that rain. water 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 show deflection. What could be the reasons?

Answer

Rain water is like a distilled water but some impurities are mixed in from the atmosphere. These impurities make the rainwater conducting. This could be the reason for the deflection of compass needle.

Question 11

Prepare a It of objects around you that are electroplated.

Answer

Pots of metals, bath taps, ornaments, rims of vehicles, the handlebar of cycles and motorcycles, kitchen gas burner, the bottom of cooking utensils, handles of doors, tin cans are some objects around us that are electroplated.

Question 12

The process that you saw in Activity 14.7 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 trans cued to the thin copperplate. Which electrode should be attached to the positive terminal of the battery and why?

Answer

The rod of the copper plate should be connected to the positive terminal of the battery. Because when an electric current is passed through the copper sulfate solution, it dissociates into copper and sulfate. The free copper drawn to the negative terminal of the battery gets deposited on it. On the other hand the loss of copper from the solution would be regained from the impure copper rod which is connected to the positive terminal of the battery.

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Chapter 15 - Some Natural Phenomena

Select the correct option in questions 1 and 2.

Question 1

Which of the following cannot be charged easily by friction?          

(a) A plastic scale

(b) A copper rod           

(c) A inflated balloon

(d) A woolen cloth.

Answer

(b) A copper rod

Question 2

When a glass rod is rubbed with a piece of silk cloth the rod:           

(a) and the cloth both acquire a positive charge.           

(b) becomes positively charged while the cloth has a negative charge.            (c) and the cloth both acquire a negative charge.           

(d) becomes negatively charged while the cloth has a positive charge.

Answer

(b) becomes positively charged while the cloth has.a negative charge.

Question 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) The lightning conductor cannot protect a building from lightning.          

(d) Earthquakes can be predicted in advance.

Answer

(a) False        

(b) True        

(c) False        

(d) False

Question 4

Sometimes the crackling sound is heard while taking off a sweater during winters. Explain.

Answer

The electric discharge takes place between body and sweater. At the time of electric discharge, some energy is released. In this case, energy is released in the form of cracking sound.

Question 5

Explain why a charged body loses its charge if we touch it with our hand.

Answer

When we touch a charged body, it loses its charge, due to the process of the earth. Our body is a good conductor of electricity. It transfers the charges to the earth.

Question 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?

Answer

The scale used to measure earthquake is Richter Scale. Yes, it would be recorded by a seismograph. The earthquake with a magnitude of 3 on the Richter Scale is not likely to cause much damage.

Question 7

Suggest three measures to protect ourselves from lightning.

Answer

Three measures of protection are:           

(i) Stay under a covered area or inside the room.           

(ii) We should not use T or phone during lightning.           

(iii) We should not take bath during lightning.

Question 8

Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon.

Answer

A charged balloon is repelled by another charged balloon because both the balloons contain the same type of charges. We know that like charges repel each other. A balloon is charged while other is uncharged so they have no same charge. Therefore charged balloon attracts uncharged balloon.

Question 9

Describe with the help of a diagram an instrument which can be used to detect a charged body.

Answer

An electroscope is used to detect that a body is charged or not. It works on the principle that like charges repel while unlike charges attract each other. When the metal strips repel each other proves that the body is charged because repulsion is the sure test to detect that a body is charged or not.

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Fig. 15.5. A simple electroscope.

Question 10

List three states in India where earthquakes are more likely to strike.

Answer

(i) Kashmir       

(ii) Rajasthan       

(iii) Gujarat       

(iv) Punjab

Question 11

Suppose you are outside your home and earthquake strikes. What precautions would you take to protect yourself?

Answer

(i) We should move to open space.           

(ii) We should not take shelter under trees or buildings:            

(iii) If we are driving, we should slow down the vehicle and move slowly away from that area to a clear spot.

Question 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 to Explain.

Answer

No, we will not take an umbrella at the time of thunderstorm. The taking umbrella will increase the risk of lightning. The wide objects are more prone to a lightning strike.

Chapter 16 - Light

Question 1

Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Answer

We see only those objects from which reflected rays enter our eyes. When we are in a dark room then we do not see objects, We can see objects outside the room because out of the room the light is available and those rays of light enter our eyes after reflection.

Question 2

Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the law of reflection?

Answer

Regular Reflection

Diffused Reflection

 1. It takes place on a smooth and shiny surface.

1. It. takes place on a rough surface

2. In this case, all rays are parallel after reflection.

2. Reflected rays are in a different direction.

Question 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 ease.

(a) Polished wooden table          (b) Chalk powder            

(c) Cardboard surface        (d) Marble floor with water spread over it.             (e) Mirror     (f) Piece of paper

Answer

(a) A regular reflection takes place on a polished wooden table because its surface is smooth.            

(b) A diffused reflection takes place on a chalk powder because its surface is uneven.            

(c) Cardboard surface has minute irregularities on it so it will have an irregular reflection.            

(d) Marble floor with water spread overacts as a surface for a regular reflection as it will act as a plane surface.            

(e) Mirror has a regular reflection, Its surface is also shiny and has a smooth surface,            

(f) At a piece of a paper takes irregular or diffused reflection because it has an uneven surface.

Question 4

State the laws of reflection,

Answer

There are two laws of reflection.           

(i) The angle of incidence is always equal to the angle of reflection.   i.e. ∠i = ∠r            

(ii) The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

Question 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.

Answer

For this experiment take a plane mirror and stand it on a plain sheet of paper with a block. Now draw an incidence line AB. Now see in the mirror and mark the points on the paper, where you fill the line is traveling after getting reflected from the mirror. Remove the mirror and draw a perpendicular on the mirror line. Join the points to make the reflected ray. You will see that incident ray, reflected ray and normal will be in the same plane, i.e. on the sheet of paper.

Question 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.

Answer

(a) 2         

(b) left, left hand         

(c) large         

(d) lesser

Choose the correct option in

Question 7

The angle of incidence is equal to the angle of reflection.            

(a) Always       (b) Sometimes  (c) Under special condition (d) Never.

Answer

(a) Always

Question 8

The image formed by a plane mirror is:            

(a) virtual, belt, and the mirror and enlarged.            

(b) virtual, behind the mirror and of the same size as the object.             (c) real at the surface of the mirror and enlarged.            

(d) real, behind the mirror and of the same size as the object.

Answer

(b) Virtual, behind the mirror and of the same size as the object.

Question 9

Describe the construction of a kaleidoscope.

Answer

Kaleidoscope is made up of three rectangular mirror strips each about 15 cm long and 4 cm wide. Join them together to form a prism. Fix them in a circular cardboard tube. Make sure that the tube is slightly longer than the mirror strips. Close one end of the tube by a cardboard disc having a hole in the center. To make the disc durable, paste a piece of the transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate.

Question 10

Draw a labeled sketch of the human eye.

Answer

Question 11

Gurmit wanted to perform activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher's advise?

Answer 

Laser light is harmful to the eye and can cause a permanent defect in the eye. She can lose her eyesight also.

Question 12

Explain how you can take care of your eyes.

Answer 

Following cares are required for our eyes:            

(i) Do not read in too little or too much light.            

(ii) Wash your eyes with cold water at least three times.            

(iii) Do not look at the sun or powerful light directly.            

(iv) Always read at the normal distance for vision.             

(v) Do not rub your eyes with a dirty hand.

Question 13

What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Answer 

The angle of incidence ray will be 45°.

 

Question 14

How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Answer 

An infinite number of images will be formed.

Question 15

Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.

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Answer 

Add Image

Question 16

Boojho stands at A just on the side of a plane mirror as shown in Fig.16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

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Answer 

He cannot see his image himself. He can see only the image of P but not Q and R.

Question 17

(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).

(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?

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Answer 

(a) Add Image

(b) Yes, Paheli can see the image at B.            

(c) Yes, Boojho can see the image.            

(d) No change of image A.

Chapter 17 - Stars and The Solar System

Choose the correct answer in Questions 1-3:

Question 1

Which of the following is NOT a member of the solar system?

(a) An asteroid

(b) A satellite

(c) A constellation

(d) A comet

Answer 

(c) A constellation

Question 2

Which of the following is NOT a planet of the sun?

(a) Sirius

(b) Mercury

(c) Saturn

(d) Earth

Answer 

(a) Sirius

Question 3

Phases of the moon occur because

(a) we can see only that part of the moon which reflects light towards us.

(b) our distance from the moon keeps changing.

(c) the shadow of the Earth covers only a part of moon’s surface.

(d) the thickness of the moon’s atmosphere is not constant.

Answer 

(a) We can see only that part of the moon which reflects light towards us.

Question 4

Fill in the blanks:

(a) The planet which is farthest from the Sun is ____________ .

(b) The planet which appears reddish in colour is ____________ .

(c) A group of stars that appear to form a pattern in the sky is known as a ____________ .

(d) A celestial body that revolves around a planet is known as __________.

(e) Shooting stars are actually not ____________.

(f) Asteroids are found between the orbits of _________ and _________ .

Answer 

(a) Neptune     

(b) Mars     

(c) Constellation           

(d) Satellite     

(e) meteors     

(f) Mars, Jupiter.

Question 5

Mark the following statements as true (T) or false (F):

(a) Pole Star is a member of the solar system. ( )

(b) Mercury is the smallest planet of the solar system. ( )

(c) Uranus is the farthest planet in the solar system. ( )

(d) INSAT is an artificial satellite. ( )

(e) There are nine planets in the solar system. ( )

(f) Constellation Orion can be seen only with a telescope. ( )

Answer 

(a) False      

(b) True       

(c) False      

(d) True           

(e) False      

(f) False

Question 6

Match items in column A with one or more items in column B:

A

B

(i) Inner Planets

(a) Saturn

(ii) Outer Planets

(b) Pole Star

(iii) Constellation

(c) Great Bear

(iv) Satellite of the Earth

(d) Moon

Answer 

A

B

(i) Inner Planets

(e) Earth (g) Mars

(ii) Outer Planets

(a) Saturn

(iii) Constellation

(c) Great Bear (f) Orion

(iv) Satellite of the Earth

(d) Moon

Question 7

In which part of the sky can you find Venus if it is visible as an evening star?

Answer 

In the west side of the sky.

Question 8

Name the largest planet of the solar system.

Answer 

The largest planet is Jupiter.

Question 9

What is a constellation? Name any two constellations.

Answer 

A group of the star which has a recognizable shape is called constellation. Two constellations are great Bear and Orion.

Question 10

Draw sketches to show the relative positions of prominent stars in

(a) Ursa Major and (b) Orion

Answer 

(a) Ursa Major appears like a big dipper. There are three bright stars in the handle and four stars in the bowl of the dipper.

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(b) Orion appears like a hunter. Three bright stars appear in the belt, while five bright stars are arranged in the form of a quadrilateral.

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Question 11

Name two objects other than planets which are members of the solar system.

Answer 

  • Comets and asteroids.

Question 12

Explain how you can locate the Pole Star with the help of Ursa Major.

Answer 

Pole star can be located with the help of the three stars at the end of Ursa Major. Imagine a straight line passing through these stars. Extend the imaginary line in the north direction. This line is about five times the distance between two stars. A star is to be seen in this direction. This is a pole star.

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Question 13

Do all the stars in the sky move? Explain.

Answer 

No, all the stars do not move in the sky. They appear to move from east to west. It is due to the rotation of the earth on which we live. The earth moves from west to east. But pole star does not appear to move.

Question 14

Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the Earth?

Answer 

Stars are far away from each other. The distance between the two stars is millions of kilometers. The distance between sun and earth is 150,000,000 km, whereas the distance of alpha century is 40,000,000,000,000 km. It is not convenient to show in km. So it is expressed in a light year. A light year is a distance covered by light in one year. Eight light year means the distance covered by light in eight years.

Question 15

The radius of Jupiter is 11 times the radius of the Earth. Calculate the

ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate?

Answer 

Planets are spherical.

Volume of sphere = 4/3 πr3

Ratio of volume of Jupiter to earth = 1331/1

So 1331 Earths can actually fit in Jupiter.

Question 16

Boojho made the following sketch (Fig. 17.29) of the solar system. Is the sketch correct? If not, correct it.

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Answer 

No, the given sketch is not correct. The following figure is correct.

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Chapter 18 - Pollution of Air and Water

Question 1

What are the different ways in which water gets contaminated?

Answer 

Water gets contaminated by the following ways:           

(i) Many industries discharge harmful chemicals to rivers and streams.           

(ii) Water gets contaminated when sewage is disposed off in rivers.           

(iii) Pesticides, weedicides and other chemicals also dissolve in water and get contaminated.           

(iv) Breeding of microorganisms makes water polluted.

Question 2

At an individual level, how can you help reduce air pollution?

Answer 

By this process we reduce air pollution:           

(i) We can plant trees to reduce the level of carbon dioxide and air pollution.         

(ii) By using CNG and unleaded petrol in our vehicle.           

(iii) By using public transport as far as possible.           

(iv) By giving general awareness about air pollution to our family and friends.

Question 3

Clear, transparent water is always fit for drinking. Comment.

Answer 

Clear, transparent water is always fit for drinking. This is not true, because many microorganisms are present in it. So we clean it by boiling.

Question 4

You are a member of the municipal body of your town.

Make a list of measures that would help your town to ensure the supply of clean water to all its residents.

Answer 

A list of measures that would help our town to ensure the supply of clean water to all its residents       

(i) Proper cleaning of water tank.       

(ii) Chlorine tablets should be made available.       

(iii) To get clean water in every household, it is necessary that water reaching the houses should be treated.

Question 5

Explain the differences between pure air and polluted air.

Answer 

Pure air is free from germs and harmful gases. It contains 78% nitrogen, 21% oxygen and small amount of other gases and dust particles.  Polluted air contains harmful gases and unwanted substances.

Question 6

Explain circumstances leading to acid rain. How does acid rain affect us?

Answer 

Some pollutants like sulphur dioxide and nitrogen dioxide react with the water vapour present in the atmosphere to form sulphuric acid and nitric acid. The acid drops down with rain, making the rain acidic. This is called acid rain. Acid rain affects us in many ways.       

(i) Acid rain corrodes the marble of the monument.       

(ii) Acid rain damages building and sculptural materials.       

(iii) Acid rain damages fresh water also.       

(iv) Acid rain also damages our crops and also makes it poisonous.       

(v) Acid rain damages the leaves of plants.

Question 7

Which of the following is not a greenhouse gas?

(a) Carbon dioxide

(b) Sulphur dioxide

(c) Methane

(d) Nitrogen

Answer

Nitrogen

Question 8

Describe the ‘Greenhouse Effect’ in your own words.

Answer

Greenhouse Effect means warming the Environment. When the sun rays reach on earth, a part of the radiation that falls on the earth is absorbed by it and a part is reflected back into space. A part of the reflected radiation is trapped by the atmosphere. This trapped radiation further warms the earth. This is called Carbon House effect. CO2 is one of the gases responsible for this effect.

Question 9

Prepare a brief speech on global warming that you have to make in your class.

Answer

Continuous increase in temperature of the earth is called global warming. There are many reasons for it. One of them is increase in population. Increase in the number of vehicles is also one of the causes. Pollution of air has also increased the global warming. Cutting trees, grazing the plants by animals increase the global warming.

Question 10

Describe the threat to the beauty of the Taj Mahal.

Answer

Over the past 2 decades, India's most famous tourist attraction Taj Mahal located at Agra, has become a matter of concern. Experts have warned that air pollution is discolouring its white marble. So it is not only living organisms that get affected by polluted air but are also the non-living things like buildings, monuments and statues. The industries located in and around Agra like rubber processing, automobiles, chemicals and especially, the Mathura Oil Refinery have been responsible for producing pollutants like sulphur dioxide and nitrogen dioxide. These gases react with the water vapour present in the atmosphere to form sulphuric acid and nitric acid. These come down with the rain, making the rain acidic. This is called acid rain. Acid rain has resulted in corrosion of the marble of the monument. The phenomenon is also called Marble cancer. Suspended particulate matter, such as the soot particles emitted by the Mathura Oil Refinery, have contributed to the yellowing of the marble. The Supreme Court has taken steps to save the Taj. It has ordered the industries to switch to cleaner fuels like CNG (Compressed Natural Gas) and LPG (Liquefied Petroleum Gas). Moreover, the automobiles should switch over to unleaded petrol in the Taj zone.

Question 11

Why does the increased level of nutrients in the water affect the survival of aquatic organisms?

Answer

Excessive quantities of chemicals get washed away from the fields. These act as nutrients for algae to flourish. Once these algae die, they serve as food for decomposers like bacteria. A lot of oxygen in water body gets used up. This results in the decrease in the oxygen level which may kill aquatic organisms.

Read CBSE Class 8 Science Notes for all Chapters

In this page, candidates can find the NCERT Solutions for Class 8 Science for all chapters. By checking the NCERT Solutions for class 8 Science notes given here helps you to find answers for textbook questions so you complete your homework. Moreover, the NCERT Solutions for Class 8 Science given here is useful for you to prepare for your monthly, quarterly, half-yearly, and Annual Exams. Find the complete NCERT Solutions for Class 8 Science on this page.


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