School Science Health & Physical Education 6

iv. Shampoo

Shampoo is another cleaning agent used by people for washing
hair. It cleans hair better than a soap does. Thus, a shampoo is
a viscous liquid chemical substance that is used for washing
hair.

v. Disinfectants shampoo

Mostly bacteria, viruses, fungi and Protozoans are disease-
causing agents. These can be controlled by using chemical substances called
disinfectants. Thus, disinfectants are the chemicals used for killing germs.

vi. Phenol phenol

Phenol is a type of alcohol that is used as a disinfectant. It is
widely used as a bactericide. It is highly toxic. The eyes should
be protected from exposure to phenol. Phenol also burns the
skin. While using phenol, therefore, gloves should be worn on
the hands. Phenol dissolves easily in water. It is also used for
disinfecting the floor. It also repels insects like ant, house flies,
cockroach, etc.

vii. Dettol

Dettol is a multi-purpose chemical made from phenol. It can
be used for cleaning and disinfection of skin during surgical
procedures. Dettol is used for first aid in wounds, washing
children's clothes, bathing children, etc.

dettol

Chemical pollution

Chemicals are used in our everyday life. They offer us both benefits and harms.
They should be used properly and carefully to avoid their negative impacts. We
should know the proper methods of their use, quantity and time. Random use of
these chemicals causes many health and environmental problems. Let’s discuss
some of the chemicals and their impacts.

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1. Phenols are non-biodegradable and toxic to many organisms. They remain in
soil for a long duration of time. Phenol mixed in drinking water is harmful to
human health. Phenolic discharge causes chemical pollution in an aquatic
environment. It also causes soil pollution.

2. Soap and detergent are common cleaning agents. Chemicals used in soap are
biodegradable. Whereas some detergents are made with non-biodegradable
components. These chemicals affect soil microbes and plants. They also affect
the aquatic organisms when they reach water resources through rain water.

3. Use of chemical fertilisers also contributes to soil pollution and water
pollution. So, chemicals cause land and water pollution and affect life on land
and water.

Summary

1. Baking soda (sodium bicarbonate), common salt (sodium chloride), sugar
(sucrose), ghee, and vegetable oil are some of the chemicals that are used in
our kitchen.

2. Disinfectants are the chemicals used for killing germs. Phenol and Dettol are
commonly used disinfectants.

3. Phenol is a type of alcohol that is used as a disinfectant.
4. Dettol is a multi-purpose chemical made from phenol.
5. Soap is a cleaning agent made from oil, or fat.
6. Detergent is a synthetic powder or liquid chemical that is used as a cleansing

agent.
7. Stain remover is a chemical used to clean stains.
8. Shampoo is a viscous liquid chemical that is used for cleaning hair.

Exercise

1. Fill in the blanks with appropriate words.
(sodium chloride, Phenol, Dettol, baking soda, ghee)

a. Sodium bicarbonate is called…………..
b. Chemical name of common salt is …………..
c. ………………. is a source of fat.

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d. ……….. is widely used as a bactericide.

e. …………... is a multipurpose liquid disinfectant.

2. Write True for correct and False for incorrect statements.
a. The chemical name of baking soda is sodium carbonate.

b. The use of diluted Dettol prevents bacterial growth at cuts.

c. We need the knowledge of chemicals before their use in our daily life.

d. Detergents have a stronger cleaning action than a soap.

e. Overuse of chemical fertilizers is good for soil.

4. Select the best answer from the given alternatives.

a. Which cleaning agent is suitable to remove marks of paints on shirt.

i. detergent ii. washing bar

iii. stain remover iv. shampoo

b. A chemical substance that is obtained from the underground
deposits is:

i. common salt ii. sugar

iii. baking soda iv. rock salt

c. Chemicals that can cause negative impact on environment are:

i. phenol, soap, detergent ii. oil, Dettol, stain remover

iii. phenol, Dettol, detergent iv. soap, detergent, stain remover

d. The correct statement for phenol is:

i. It is widely used as a sanitizer. ii. It is not a toxic chemical.

iii. It attracts insects. iv. It does not dissolve in water.

5. Answer the following questions.
a. Write the names of four chemicals that are used in cooking.
b. Why is baking soda used in making bread, cookies, biscuits, etc.?
c. What are disinfectants? Write two examples.
d. Write two uses of phenol.
e. Make a list of chemicals that are used as cleansing agents.

f. What do you prefer to use for cleaning, detergent or soap, and why?

g. Write down the uses of each: table salt, sugar, vegetable oil, ghee.

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10.2 Metals and Non-metals

Key terms of the unit

Metals: The elements which are good conductor of heat and electricity,
malleable, ductile, lustre, etc. are called metals.

Non-metals: The pure substances which are soft, non-lustrous, non-sonorous,
non-malleable, non-ductile, insulators of electricity and bad conductor of heat
are called non-metals.

Metallic lustre: A shiny appearance on the surface of a metal is called metallic
lustre.
Sonority:  Sonority is the property of metals to produce a tinkling sound when
struck.

Malleability: The property of metals by which they can be beaten into thin
sheets is called malleability.

Ductility: Ductility is the property of metals by which they can be stretched
into wire without breaking.

Scientists have discovered about 118 elements so far. These elements show
different properties. Some elements are soft and some are hard; some are solid,
some are liquid and some are gas. On hammering, some of them can be converted
into a thin sheet whereas others break down into pieces. Some elements conduct
electricity and others do not. Based on these properties, elements are classified
into three categories: metals, non-metals and metalloids.

Metals

Most of the elements are metals. Metals are good conductors of heat and electricity.
They are malleable, ductile, lustrous and hard. They have high boiling and
melting points. Thus, the elements which are good conductors of heat and
electricity, malleable, ductile, lustre, etc. are called metals. Some examples of
metals are silver, iron, gold, platinum, copper, sodium, potassium and calcium.
Some common features of metals are discussed below:

1. Metals are good conductors of heat and electricity

Most metals conduct heat and electricity. Among them some metals are very

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good conductors of heat and electricity such as copper, aluminum, etc. These
metals are used to make electric wires.

Activity

Objective : To test whether metals are good conductors of electricity or not
Procedure: Make an electric circuit as shown in the figure. Then, put a coin
somewhere between points A and B of the circuit. What do you see? Does the
bulb glow? After that, take out the coin. Then, put a wooden piece between A
and B of the circuit and observe. Does the bulb glow or not? In the same way,
put a piece of paper, an iron nail, a piece of rubber, a piece of blade turn by turn
and make your observations. Does the bulb glow or not?
Observation: You have seen that the bulb glows when the circuit is connected
and completed by putting a coin, an iron nail or a piece of blade. But the bulb
does not glow when we put a piece of wood, paper, rubber, etc. in the electric
circuit.

Conclusion: From the above observations, it is clear that the bulb glows when
the electric circuit is completed with metals that conduct electricity. So iron,
steel and copper are good conductors of electricity. But when we complete the
electric circuit with wood, paper and rubber, the bulb does not glow because
they are not the conductors of electricity. So we can say that some metals are
good conductors of electricity.

2. Metals are good conductors of heat
Metals are good conductors of heat. This can be easily shown by placing metal
pots on fire. When we put metal pots on fire, they become hot. When we touch
them, our hands get a burning sensation. Why does this happen? Because the
heat from the fire passes into these pots. Can we heat water and cook food if heat
is not conducted?

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Activity

Objective: To show that metals are good conductors of heat.
Materials required: metal rod, candle, match box, wooden stick.
Procedure: Put a candle on a bench. Burn it with a match box. Now catch the
wooden stick with one hand and the metal rod with another. Now heat both
metal rod and wooden stick. After some time, observe what you feel.
Observation: You will feel the metal rod is hot, not the wooden stick.
Conclusion: This is a proof that a metal rod is a good conductor of heat but the
wooden stick is not.

3. Metals are usually hard
Most of the metals are strong and hard. Examples are iron, copper, silver and
aluminum. But some metals are soft such as lithium, sodium and potassium.

Activity

Objective: To test that metals are harder than non-metals
Materials required: Iron nail, brick, a steel plate
Procedure: Take the nail and scratch on the brick, wooden board and steel plate
one by one. What do you see?
Observation: You can see a deep scratch on the wooden board and brick but
you won't find as deep a scratch on a steel plate. The scratch on the steel plate is
faint.
Conclusion :From the above activity, it is clear that bricks and woods are soft
while steel plate is hard. So it is proved that metals are harder than non-metals.

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4. Metals have metallic lustre
Metals look shiny. Metallic lustre is a property that depends on the amount of
light reflected from a metal surface. A shiny appearance on the surface of a metal
is called the metallic lustre. Not all metals have the same ability to reflect light.
Some metals look very shiny in light because of their lustre.

Activity

Objective: To prove that metals have metallic lustre.
Materials required: stone, wooden board, brick, steel plate
Procedure : Put a brick, a steel plate and a wooden board in the sunlight. Try to
reflect the sunlight by using each of these.
Observation: The brick and wooden board do not reflect light. They do not
shine brightly. The steel plate reflects light and shines bright.
Conclusion: This observation shows that steel shines brightly and bricks and
wood don’t. This proves that metals shine and reflect light.

Reason to remember

Why do people wear gold and silver?
Metals such as gold and silver have an attractive lustre. So, people wear jewelry
made from gold and silver.

5. Most metals are hard and have high density
Most metals are hard. For example, aluminium, iron, copper, zinc, silver, etc.
Metals such as sodium and potassium are soft. Soft metals can be cut easily with
a knife. Almost all metals are solid except mercury. Mercury is in a liquid state in
room temperature.

6. Metals are sonorous
Sonority is the property of metals to produce a tinkling sound when struck. When
metals are struck or collide with each other, they vibrate and produce a special
ringing sound. Some metals such as copper and tin are used to make bells due to
their sonority.

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Activity

Objective: To prove that metals produce a tinkling sound.
Materials required: steel plate, stick, brick
Procedure: Take a stick and hit the brick. Listen to its sound. Take a plate and
hit it. Listen to its sound.

Observation: When we hit the brick, we do not get to listen a sharp sound. But
when we hit the steel plate, we listen to a sharp sound.

Conclusion: From this activity, we can say that metals produce sharp sounds
when they are hit. But non-metals do not produce such sounds.

7. Metals are malleable
Most metals such as zinc, iron, steel and tin can be hammered into thin sheets.
The property of metals by which they can be beaten into thin sheets is called
malleability. Due to their malleable property, metals can be hammered or pressed
into a particular shape without breaking or cracking them.

8. Metals are ductile
A ductile substance can be drawn into a wire. Most metals such as gold, silver,
aluminum and copper are ductile materials. When metals such as gold, silver
and copper are heated and stretched, they can be given a wire shape. Ductility is
the property of metals by which they can be stretched into a wire without
breaking. Due to ductility, it has become possible to make a copper wire, iron
rod, etc.

Fact file

Gold is highly ductile. One gram of gold can be drawn into a wire as long as 2
km.

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Activity

Objective: To show that metals are malleable and ductile in nature.

Materials required: Iron rod, brick, hammer, etc.

Procedure: Take a brick and hit it with a hammer and observe the change. After
that, hit the iron rod with the hammer and observe it. What differences do you
find?

Observation : You see that the brick breaks into pieces when you hit it but the
iron rod does not. The iron rod becomes hot and thin if you continuously hit it
for a long time.

Conclusion: This activity shows that metals do not break down easily into
pieces if they are hit, but non-metals do break down easily.

9. Metals conduct heat at different rates

Metals are good conductors of heat. This is a reason why cooking utensils are
made up of metals. Different metals conduct heat at different rates. For example,
copper conducts heat faster than iron.

Use of metals

Metals are very important and useful for human beings. They are the foundations
of modern development. They are essential in our daily life. Metals are used to
make utensils, devices, pots, machines, vehicles, etc. Without metals, we cannot
imagine the present world. Big factories, bridges, engines, machines, factories,
industries, means of transport, communications, houses, buildings, schools,
hospitals, offices, etc. are made using metals like iron, steel, aluminium, etc.
Some metals such as gold, silver and copper, etc. are used to make ornaments.

Non-metals

Non-metals do not have the characteristics of metals. The pure substances which
are soft, non-lustrous, non-sonorous, non-malleable, non-ductile, insulators of
electricity and bad conductors of heat are called non-metals. For example,
hydrogen, oxygen, carbon, etc. In other words, non-metals are bad conductors of
heat and electricity.

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Properties of non-metals

Some major properties of non-metals are given below.

Physical state: Non-metals may be found in solid, liquid and gaseous states. For
example, carbon and sulphur are in solid states. Bromine is in a liquid state and
oxygen, hydrogen, nitrogen, etc. are in gaseous states.

Softness: Non-metals other than diamonds are softer than metals.

Malleability: Non-metals cannot be hammered into sheets. Solid non-metals
break easily if they are hit.

Ductility: Non-metals cannot be drawn into wires by stretching them.

Sonority: Non-metals do not produce a special tinkling sound when struck.

Lustre: Non-metals except graphite do not shine. They do not possess the metallic
lustre. But graphite does exhibit some metallic lustre.

Conduction or transmission: Non-metals are insulators of electricity and bad
conductors of heat.

Uses of non-metals

Like metals, non-metals are also used in a variety of applications. They are used
to make an insulating coat aound electric wires because they are non-conductors
of electricity. Some non-metals, such as sulfur, are used to make explosives. Non-
metals like chlorine are used to make medicines. For another example, oxygen is
very important for all living beings to breathe. Non-metals are used to make
cooking utensils and appliances.

Summary

1. Metals are substances which have characteristic properties of malleability,
ductility, sonority, transmission of electricity, lustre, etc. For example, iron,
copper, aluminum, silver, gold, etc.

2 The shiny appearance on the surface of metals is called the metallic lustre.
Metals such as gold have an attractive lustre.

3. Property of metals by which they produce a ringing (tinkling) sound is
called sonority. Metals with this property are used to make bells.

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4. The property of metals by which they can be beaten into thin sheets
without breaking is called malleability.

5. The property of metals by which they can be drawn into wires is called
ductility.

6. Non-metals are pure substances which are soft, non-lustrous, non-sonorous,
non-malleable, non-ductile, insulators of electricity and bad conductors of
heat. For example, hydrogen, oxygen, carbon, etc.

Exercise

1. Fill in the blanks with appropriate words.

(conductor, solid, mercury, insulator, lustre, medicine, oxygen, carbon dioxide )
a. ………… is a metal that exists in a liquid state in room temperature.

b. Non-metals are ………..of electricity and bad …………… of heat.

c. The shiny appearance on the surface of metals is called metallic…..

d. Chlorine is used to make ……………….

e. ……………. is used for breathing and burning of substances.

2. Write True for the correct and False for the incorrect statements.
a. Graphites possess lustre.
b. Iron is sonorous.
c. Non-metals cannot be drawn into wires.
d. Metals reflect light which make them look shiny.
e. Gold is brittle in nature.

3. Select the best answer from the given alternatives.
a. Which one is a non-metal?

i. lithium ii. copper iii. sodium iv. helium

b. Which of the following is a property of iodine?

i. conducts electricity ii. conducts heat

iii. produces tinkling sound iv. possesses lusture

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c. Which of the following is a set of metals?
i. iron, gold, carbon, mercury
ii. silver, copper, graphite, aluminum
iii. iron, copper, potassium, gold
iv. silver, gold, lead, graphite
d. Identify the false statement.
i. Most metals are hard.
ii. Metals are in solid, liquid or gas state.
iii. Non-metals are non-lustrous

iv. Non-metals are soft except diamond.

4. Match the following i. can be hammered into a sheet
a. lustre

b. ductile ii. produces a special 'tinkling' sound when struck

c. malleable iii. can be drawn into wire

d. sonority iv. shiny appearance

5. Answer the following questions

a. What are metals? Write down any four properties of metals with
examples.

b. Why is copper widely used to make electric wires?

c. Write two properties of gold that make it good for use as jewelry.

d. Discuss in short about the two properties of metal on the basis of which
cooking utensils are made from them.

e. Write down any two properties of non-metals with an application of
each.

f. Write any four differences between metals and non-metals.

g. Complete the table with the properties of metals and non-metals.

S.N. Proerties of metals S.N. Properties of non-metals

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Earth and Estimated Teaching Periods:
Space
Unit Theory Practical
8 2
11

Topics in Their Order according to CDC Curriculum

1. Soil
i. Introduction to soil
ii. Process of soil formation
iii. Soil erosion and deposition
iv. Conservation of soil
v. Soil profile
vi. Causes, effects and control measures of soil pollution

2. Internal structure of the earth
3. Four seasons and their effects on our daily life
4. Solar system

i. Introduction to the sun (shape, structure, temperature and source of
temperature)

ii. Introduction to members of solar system (planets, satellites, comets,
asteroids and meteors)

Learning outcomes

At the end of this unit, students will be able to:
1. Show their familiarity with soil, its formation and conservation.
2. Show their familiarity with preparing the soil profile.
3. State the causes of soil pollution, its effects and control measures.
4. Show their familiarity with the the internal structure of the earth.
5. Demonstrate the orbital revolution of the earth along with four seasons.
6. Show their familiarity with sun, solar system and heavenly bodies.

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The universe is home to everything including stars, planets, satellites, comets,
asteroid, dust, etc. Nothing is outside the universe. Thus, the group of stars,
planets, satellites, comets, asteroids, dust, etc. that exist in an infinite space is
called the universe. Earth is a part of the universe. We live on it. It is the third
planet closest to the sun in our solar system. It is mainly composed of air, water
and land.

11.1 Soil

Key terms of the unit

Soil: The upper layer of the earth’s surface consisting of powdered rock and
humus is called soil.
Soil profile: Soil profile is a physical and chemical description of the layers that
make up the soil.
Topsoil: Topsoil is the uppermost layer of the soil.
Sub-soil: The light grey and compact soil that lies below the topsoil is called
sub-soil.
Bedrock: Bedrock means huge hard rocks at the bottom of the soil profile.
Weathering: The process in which rocks break down into the soil over time due
to physical, chemical or biological activities is called weathering.
Soil erosion: The removal of top fertile soil by various agents like air, water,
wind, river, glaciers, hurricane, cyclone, etc. is called soil erosion.
Soil deposition: Soil deposition is the process of accumulation of the eroded
soil away from the place of erosion.

Deforestation: The permanent removal of trees from forests is called
deforestation.

Soil conservation: The prevention of erosion of topsoil is called soil conservation.

Soil pollution: The presence of toxic chemicals and solid wastes in the soil at a
higher amount is called soil pollution.

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Soil is a mixture of organic matters, minerals, gases, liquids and organisms. It is
a life supporting material of the earth. A very important natural resource, soil has
different colours. Thus, the upper layer of the earth’s surface consisting of
powdered rock and humus is called soil.

Fig.: soil

Fact file

Earthworms mix soil and leave their waste in the soil increasing the fertility of
the soil.

Reason to remember

Why is soil a non-renewable resource?
Soil is a non-renewable resource because it cannot be formed in a short period
of time.

Importance of soil

a) Most plants get their nutrients from the soil.
b) We grow herbs, crops and vegetables on the soil.
c) A large amount of water is absorbed, purified and supplied by the soil.
d) Animals such as rabbits, mice, earthworms, snakes, microorganisms, etc. live

inside holes under the soil.

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plants growing on soil germination in soil earthworm in soil rabbit in the burrow

Soil profile

Soil is formed over time. Therefore, soil has layers. These layers are chemically
and physically different from one another. Thus, the soil profile is a physical and
chemical description of the layers that make up the soil. The crust is mainly
divided into three layers. These layers are topsoil, subsoil and bedrock.

Top soil

Sub soil

Bed rocks

Topsoil Fig.: soil profile

Topsoil is the uppermost layer of the soil. It contains air and water. It is dark
brown or black due to the presence of humus. Humus is a dark decaying organic
matter that makes the soil porous and fertile. Plants, microorganisms, fungi,
earthworms and other animals live in the topsoil.

Fig.: black humus topsoil and vegetation in it

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Activity

Collect some topsoil in a transparent container. Mix it with water, shake it and
keep it still for an hour. What do you see? Does it contain humus? Repeat the
experiment with subsoil and compare it with the humus of the topsoil.

Fact file

The topsoil greatly reduces the flood risk by storing 9200 tonnes of water per
acre.

Sub-soil
The compact soil that lies below the topsoil is called sub-soil. It is light grey
because it has a lower percentage of humus. It is less fertile than the topsoil. The
subsoil is used for making bricks.

Reason to remember

The topsoil is black but the subsoil is light grey or brown. Why?
The topsoil is black because it is rich in humus whereas the sub-soil is brown
because it has a lower amount of humus.

Bedrock
Bedrock is the huge hard rock at the bottom of the soil profile. Topsoil and subsoil
rest on the bedrock. It collects water in its cavities. So, it is an important source of
water.

Fact file

The top of the bedrock is called a rock head.

Formation of soil

Soil formation is a slow, continuous and gradual process. It occurs due to the
breakdown of rocks through weathering. Thus, the process in which rocks break
down into soil over time due to physical, chemical or biological activities is called
soil formation or weathering.

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Different types of weathering

The types of weathering of rocks are discussed below.

Physical weathering

Rocks expand during hot days and contract at cold nights. The continuous
expansion and contraction of rocks for millions of years crack the rocks. Water or
moist air trapped inside the cracks further aid the weathering. Gravity, frost,
snow, glacier, rainfall, flowing river, tide and wind also erode rocks to make soil.

Chemical weathering

Rocks are a mixture of various minerals. These minerals react with air, water and
other chemicals. As a result, rocks erode to make soil.

Biological weathering

Burrowing animals help water and air to enter into the cracks of rocks. The roots
of plants grow inside the cracks in the rock and split them. Finally, rocks erode
and make soil.

Fact file

It takes more than 500 years to produce just an inch of topsoil.

The collection of eroded rocks, minerals, humus, water and air over millions of
years makes soil.

Erosion and deposition

The process in which the surface of an object is destroyed and carried away
continuously is called erosion. Similarly, the sedimentation of eroded particles at
a certain place is called deposition.

Soil erosion

The displacement of top fertile soil by various agents such as air, water, wind,
river, glacier, hurricane, cyclone, etc. is called soil erosion. It is caused by natural
activities such as wind, glacier, rainfall, storm and snow. Animals may also
contribute to erosion. Human activities such as agriculture, construction and
deforestation accelerate soil erosion. The eroded soil is deposited at the bottom of
a pond, river bed and ocean. It changes into a sedimentary rock over time. Soil
deposition is the process of accumulation of the eroded soil away from the place
of erosion.

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Fact file

26.4 billion tons of soil is eroded every year.

Reason to remember

Why is grass planted in waterways in the hilly regions?
Grassed waterways help prevent soil erosion because the grass reduces the
speed of the water passing through it.

Activity

Take two flower vases. Grow a plant in the first vase and water it daily. Let no
plant be there in the second one. Observe the physical differences in the soil of
the two vases. Give reasons why the two soil surfaces are different.

Fig.: a vase with plant Fig.: a vase with only soil

Causes of soil erosion

a) Wind, rainfall and surface water

Wind and storm erode soil over time. The raindrops and the surface water such
as rivers, glaciers, floods, tides and tsunami carry away the soil. Soil erosion is
more prominent during monsoon.

b) Deforestation

Soil erosion occurs less often on a land covered by vegetation. Forest fire, landslide
and construction of roads or cities destroy forests. The permanent removal of
trees from forests is called deforestation. When the denuded land is exposed to
rainfall, frost and flood, it is prone to erosion.

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deforestation landslide forest fire

Overgrazing

Sheep, goats, cows, etc. graze on hills, mountains and meadows. Hoofs of these
animals loosen the soil. When numerous animals consume vegetation beyond
the regeneration capacity of the land, the land becomes barren and erodes.

hundreds of sheep yaks grazing

Agriculture

Agriculture involves digging and irrigation. In
the hilly areas, agriculture and traditional
farming methods are still in practice. It makes
the soil loose making it prone to be washed
away by rain.

Fig.: unscientific farming in hills

Construction Fig.: dozer moving mass of soil

Construction of roads, buildings and bridges
involves mass movements of soil, which
increases the rate of soil erosion.

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Fact file

One tablespoon of soil has a greater number of organisms than the entire world
population of human beings.

Effects of soil erosion and soil deposition

The major effects of soil erosion and deposition are listed below:
a) Erosion makes land less fertile or even infertile.
b) Soil erosion and deposition primarily change the land structure.
c) Erosion is usually followed by a landslide.
d) The deposition makes ponds shallower, forms deltas under the ocean and

makes sedimentary rocks.

Fig.: dried cracked lands Fig.: eroded road by monsoon rain

Soil conservation

Soil is one of the major components of the environment. It helps support life on
the earth. Soil erosion causes degradation of soil. So, the soil must be conserved.
The prevention of topsoil from erosion is called soil conservation. There are many
scientific processes that help in the conservation of soil. They are listed below:

Reason to remember

The most effective process to prevent soil erosion is afforestation. Why?

The most effective process to prevent soil erosion is afforestation because the
vegetation cover on land reduces the speed of water, rain and wind there.
Moreover, the roots of the trees hold soil and decaying leaves make the soil
porous to absorb water.

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a) The most effective measure to prevent soil erosion is afforestation.

b) Overgrazing, deforestation and forest fire need to be reduced to conserve
soil.

c) Terrace farming, contour ploughing, crop rotation, grassed waterways and
various other soil conservation technologies should also be practised.

terrace farming contour ploughing

d) Bamboo, babio, khar, amriso, narkat and tilka should be planted in slopes,
gullies and riverbanks to prevent soil erosion.

e) The embankments in rivers, retention walls in slopes and spurs in the river or
beach also help prevent soil erosion.

embankment in river banks retention wall in slopes spurs in the river

Roads, buildings and bridges should be built with erosion control technologies in
place.

Soil pollution

Soil is polluted by plastics, metal scraps, glasses and decaying organic matters.
The presence of toxic chemicals and solid wastes in the soil at a high amount is
called soil pollution. Emergence of unplanned cities, deforestation and
construction are happening at a greater pace than ever these days. As a result, the
land is coming under assault. It is being rendered infertile. Agricultural production

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is decreasing. Decaying organic materials are also the source of a foul smell,
germs and epidemics.

soil pollution dumped waste in road

Causes of soil pollution

Some of the main causes of soil pollution are given below:
a) Excessive use of chemical fertilisers and pesticides in agricultural farms

causes soil pollution.
b) Plastic bags, bottles, syringes and other wastes from different sources are

responsible for soil pollution.
c) Metal scraps, plastic pieces, glass pieces, clothes, etc. are the sources of soil

pollution.
d) Lack of dumping sites and illegal dumping are also responsible for soil

pollution.

Fact file

Growing plants around the house and watering them frequently helps reduce
the effects of soil pollution.

Reason to remember

Planned urbanisation helps reduce soil pollution. How?
Planned urbanisation helps reduce soil pollution because the solid waste is
managed properly with such measures as a proper drainage system in place.

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Methods to prevent soil pollution

To avoid health hazards and to maintain environmental balance, soil pollution
must be reduced. Some of the major methods that prevent soil pollution are
listed below:
a) Proper solid waste management and drainage systems should be adopted.
b) Proper dumping sites need to be developed.
c) Waste collection containers, such as dustbins, have to be kept in houses,

schools, bus parks, footpaths and public places.
d) Use of chemical fertilizers, pesticides and detergents should be minimised.
e) Practising the culture of 3Rs (reduce, reuse and recycle) helps control soil

pollution.
f) Degradable wastes should be collected for composting.

Fig.: waste in different dustbins for degradable and non-degradable waste

Activity

Organise a program on the environment day where you, with your friends,
take part in cleaning your surroundings.

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11.2 Internal Structure of the Earth

Key terms of the unit

Crust: The crust is the thinnest and outermost layer of the earth.
Mantle: The mantle is the second layer of the earth which lies between crust
and core.
Core: The innermost layer of the earth is called core.
Axis: Axis of the earth is an imaginary straight line perpendicular to its
equatorial plane and passing through the centre.
Orbit: The orbit of the earth is an elliptical path on which it revolves around
the sun.
Rotation: Rotation refers to the anti-clockwise spinning of the earth on its axis.
It is also called diurnal motion.
Revolution: The orbiting of the earth around the sun is called revolution. It is
also called annual motion of the earth.
Day: The time duration during which the earth faces the sun while rotating on
its axis is called day.
Night: Time duration during which the earth faces away from the sun is called
night.
The internal structure of the earth was studied with the help of the earthquakes
and volcanoes in the past. But nowadays, scientists use infrasound to study the
layers of the earth. On the basis of various studies, we are now more familiar
with the three layers of the inner earth. They are crust, mantle and core.

Fig.: internal layers of earth

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Fact file

Earth has a powerful magnetic field.

Crust

The crust is the thinnest and outermost layer of the earth. Its thickness varies
from 5 km to 50 km. In the crust, the temperature ranges from 25°C to 600°C. The
important elements found on the earth's crust are oxygen, sodium, silicon,
calcium, iron, etc. The crust consists of forests, crop fields, deserts, hills, mountains,
plains, oceans, etc. Various life forms exist in this layer.

Mantle

The mantle is the second layer of the earth which lies between the crust and core.
It is the thickest layer. It is approximately 3000 km thick. It is a semi-solid layer.
In the mantle, the temperature ranges from 200°C to 4000°C. This layer contains
oxygen, silicon and magnesium.

Differences between Crust and Mantle

S.N. Crust S.N. Mantle

The crust is a thin outer layer of The layer of the earth below the
1. 1.
the earth where we live. crust is called the mantle.

The crust varies from around 5 The mantle is much thicker than

2. km thick (in the ocean floor) to 2. the crust. Its thickness is about

around 50 km thick (on land). 3000km.

Fact file

The layer that separates crust from the mantle is called moho.

Reason to remember

Why is magma there at the mantle of the earth?

There is magma at the mantle of the earth because the intense pressure from the
crust causes heating and melting of rocks into magma.

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Core

The innermost layer of the earth is called its core. It is the hottest layer. It has two
sections: the inner core and outer core.
(a) The outer core: It is the second thickest layer of the earth which is present

between the inner core and mantle. Its thickness is about 2400 km. In the
outer core, the temperature ranges from 2730°C to 7730°C. The materials
found in this layer are at a liquid state. The important elements present in this
layer are iron, cobalt and nickel.

(b) The inner core: It is the hottest layer of the earth. It has the highest pressure.
So, different elements present in this layer are at a solid state. Its thickness is
about 1300 km. Its temperature is about 5430°C. It has different elements such
as iron, cobalt and nickel.

Reason to remember

Despite a very high temperature in the inner core, it is in a solid state. Why?

Despite a very high temperature, the inner core is solid because the pressure at
the core is extremely high due to which molecules are compressed in the lowest
possible volume.

Activity

Take an egg and boil it for some time. Cut the boiled egg vertically with a knife.
Compare the different parts of the egg with the internal structure of the earth.
The eggshell represents the crust.
The white part represents the mantle.

And the yellow nucleus represents the core.

Fig.: internal structure of the earth Fig.: cut off the boiled egg

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Axis and orbit of the earth

The axis of the earth is an imaginary straight line, perpendicular to its equatorial
plane, that passes through the centre. The earth’s orbit around the sun is not
completely circular, but it is oval or elliptical. Thus, the orbit of the earth is an
elliptical path on which it revolves around the sun.

Rotation and revolution

The earth shows two types of motion. They are rotation and revolution. The earth
rotates about its axis and revolves around its orbit. Rotation refers to the anti-
clockwise spinning of the earth on its axis. It is also called a diurnal motion. One
rotation takes about twenty-four hours and is called a mean solar day. The
orbiting of the earth around the sun is called its revolution. It is also called an
annual motion of the earth. For earth to make one revolution, it takes 365.25 days.

Day and night

The sun is a major source of light on the earth. As the earth spins continuously on
its axis, a half of the surface of the earth faces the sun and another half of its
surface turns away from the sun. The time duration during which the earth faces
the sun while rotating on its axis is called a day and the time duration during
which the earth faces away from the sun is called a night. As the earth takes 24
hours to complete a rotation, a complete day is of nearly 12 hours. Similarly, a
night is of 12 hours, too. But, the days and nights are not equal due to the tilting
(slanting) of the earth. Thus, the continuous rotation of the earth on its axis is the
cause of day and night on the earth.

Fig.: day and night on the earth

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Activity

Take a globe and a burning candle. Draw an elliptical circle around the candle
and make the room dark. Rotate the globe through the elliptical path around
the candle. Assuming the globe to be the earth and the candlelight as the
sunlight, study the following things:

1. During the rotation of the earth, which side is the day and which side is the
night?

2. During the revolution of the earth, which side is cold and which side is hot?

Reason to remember

Why is there a variation in the day and night times on earth throughout the
year?

The earth’s axis is not perpendicular to the orbital plane of the earth. It makes
an angle of 66.5° with the orbital plane. This tilting causes variations in the
duration of the light received by the earth. So, variations occur in the day time
and night time throughout the year.

Change in seasons Fig.: change in season

The earth revolves around the sun on
an elliptical orbit. While revolving, the
earth reaches different distances from
the sun. The point on the earth’s orbit
nearest to the sun is called perihelion.
At this point, the earth is about 147.3
million km away from the sun. It
occurs on January 3. The point farthest
from the sun is called aphelion. At this
point, the earth is about 152.1 million
km away from the sun. It occurs on
July 4.

The elliptical orbit causes the variation of the earth’s distances from the sun. Yet,
this phenomenon is not responsible for the seasons on the earth. The tilting of the
earth’s axis is the major cause of change in season. There are four main seasons.
They are:

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1. Spring season

In the northern hemisphere, the spring season comes after the winter season.
In Nepal, this season lasts from Falgun to Baishakh. As the spring starts, the
days become longer and the nights become shorter. Around Chaitra 7
(March 21), the day and the night become equal. Spring is the season of
flowers and greenery.

2. Autumn season

In the northern hemisphere, the autumn season comes after the summer
season. In Nepal, this season lasts from Bhadra to Kartik. During this period,
the solar radiation falls equally on northern and southern hemispheres. Due
to this reason, the days and nights are nearly equal in autumn. Around
Ashoj 7 (September 23), the day and night become equal.

3. Winter season

In the northern hemisphere, the winter season comes after the autumn
season. In Nepal, this season lasts from Mangsir to Magh. In the winter, the
solar radiation declines in the northern hemisphere. Therefore, the days are
shorter and the nights are longer. Poush 7 (December 22) tends to be the
shortest day and the longest night.

4. Summer season

In the northern hemisphere, the summer season comes after the spring
season. In Nepal, the summer season lasts from Jesth to Shrawan. Ashad 7
(June 21) is the longest day and the shortest night.

The seasons in the southern hemisphere are the opposite of those in the northern
hemisphere. That is, when there is autumn in the northern hemisphere there is
spring in the southern hemisphere. Similarly, when there is summer in the
northern hemisphere, there is winter in the southern hemisphere. The change in
seasons in both the hemispheres is described below in solstice and equinox.

Reason to remember

Why do seasons change on the earth?
Changes in season take place on the earth due to its tilted axis as well as its
revolution around the sun.

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Seasons in Nepal

Generally, there are four seasons on the earth, but in the context of Nepal, there
are six seasons. They are given below with their tentative Nepali months:
1. Spring season: CHAITRA and BAISHAKH.
2. Summer season: JESTHA and ASHAD.
3. Rainy season: SHRAWAN and BHADRA.
4. Autumn season: ASHOJ and KARTIK.
5. Pre-winter season: MANGSIR and POUSH.
6. Winter season: MAGH and FALGUN.

Project Work

Bring a chart paper and write down the characteristics of seasons that are
described in the context of Nepal.

11.3 Solar System

Key terms of the unit

Solar system : The sun along with other heavenly bodies like planets, satellites,
comets, asteroids, dwarf planes etc. that revolve around the sun is called solar
system.

Sun: The sun is a medium-sized yellow coloured star. It lies at the centre of the
solar system.

Planets: The heavenly bodies that revolve around the sun on their own elliptical
orbits are called planets.

Satellite: The heavenly objects that revolve around the planet are called satellite.

Asteroids: The rocky and metallic objects which are present in between the
orbits of Mars and Jupiter that revolve around the sun are called asteroids or
minor planets.

Comet: A broom-shaped shining object with a bright head is called a comet.

Meteors: The pieces of stone or metal which enter the earth’s atmosphere are
called meteors.

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The solar system refers to the family of the sun. It includes eight planets, satellites,
comets, asteroids, dwarf planets etc. Thus, the system of sun along with other
heavenly bodies that revolve around it is called the solar system.

Fig.: solar system

Sun

The sun is a medium-sized yellow coloured star. It lies at the centre of the solar
system. It is a huge ball of burning gases. In the presence of high temperature,
high pressure and high amount of hydrogen, the sun constantly sees a stream of
a nuclear fusion of hydrogen gases givng helium. In this process, tremendous
amounts of heat and light are released. All the plants and animals including
human beings get the heat and light energy from the sun.

Fig.: Sun

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Fact file

1. Diameter of the sun: About 14,00,000 km

2. The distance between the earth and the sun: 15,00,00,000 km

3. The surface temperature of the sun is about 5500°C while its inner core has
temperatures of up to 15.6 million degrees Celsius.

Planet

The word planet has been derived from the Greek word ‘planetes’ that means
‘wanderers’. In other words, the heavenly bodies that revolve around the sun on
their own elliptical orbits are called planets. Planets do not have their own light
like that of stars.

The eight planets of the solar system in the order of their
increasing distance from the sun are given below:

Mercury

Mercury is the smallest planet in the solar system. It is Fig.: Mercury
the planet closest to the sun. It has no atmosphere. So,
there is no possibility of life in Mercury.

Venus

Venus is the second closest planet from the sun. It is the Fig.: Venus
second-brightest natural object after the moon in the night
sky. The atmosphere of Venus mainly consists of carbon
dioxide. Life cannot exist on Venus. 

Activity

Mercury can be seen just before the sunrise in September and October. Similarly,
it can be seen just after the sunset in March and April. Observe Mercury and
show it to your family members also.

Venus can be seen just before the sunrise and just after the sunset. Observe the
Venus and show it to your family. Note that Venus is the brightest heavenly
body in the night sky after the moon.

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Earth Fig.: Earth

We live on the earth. It is the third planet near the sun. This
planet has life-supporting gases, water, land and suitable
temperatures. Presence of various life forms in the earth
has earned it a title, ‘the living planet’. Its rotation on its
axis makes the day and night.

Fact file

The earth takes 365.25 days to complete one revolution around the sun. It takes
24 hours to complete one rotation about its axis.

Mars

Mars is the fourth planet from the sun. It looks like a bright
orange-red star in the night sky. Therefore, it is also called
the red planet. The atmosphere of Mars mainly consists of
carbon dioxide, nitrogen and a little amount of oxygen.

Fig.: Mars

Jupiter

Jupiter is the fifth planet from the sun and the largest planet
in the solar system. Jupiter looks like a bright star with our
naked eyes. It consists of bands of clouds and a big red spot.

Fig.: Jupiter

Fact file

Pluto used to be called a planet until 2006.

Reason to remember

Pluto is not considered a planet anymore. Why?

Pluto is not considered a planet anymore because it intersects the orbit of
Neptune.

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Saturn
Saturn is the sixth planet from the sun and the second-
largest planet in the solar system. This planet is surrounded
by rings of dust, ice and rocks.

Fig.: Saturn

Fact file

Saturn is over 95 times bigger than the earth but it can float in water.

Uranus
Uranus is the second farthest planet of the solar system.
It is a very cold planet. Uranus also has rings around it
like that of saturn.

Fig.: Uranus

Neptune
Neptune is the eighth planet in the solar system. It is
the farthest planet from the sun. It is the fourth-largest
planet by diameter. It is the coldest planet.

Fig.: Neptune

Activity

Make a colourful chart of the solar system with the help of your textbook or
your science teacher.
Name the eight planets of the solar system. Also, write the main features of
each planet in your exercise book.

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Satellites

The heavenly objects that revolve around the planet are called satellites. The
earth has one natural satellite called the moon. The moon revolves around the
earth. Similarly, there are two satellites of Mars. Mercury and Venus do not have
satellites. Jupiter, Saturn, Uranus and Neptune have 79, 82, 27 and 14 satellites
respectively discovered until 2021. Artificial satellites are used for the study of
weather and the universe.

Fact file

Ganymede is the largest satellite of solar system that revolves around Jupiter.

Reason to remember

Moon is called a natural satellite. Why?
Moon is called a natural satellite because it is a naturally formed heavenly object
that revolves around the planet earth.

Asteroid

The space between Mars and Jupiter contains millions of rocks that revolve
around the sun. These objects are called asteroids. Thus, the rocky and metallic

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objects which are present between the orbits of Mars and Jupiter that revolve
around the sun are called asteroids or minor planets. Ceres is the largest asteroid.

Fig.: Asteroid

Comet

Comets are tailed flying bodies that can be observed in the sky. They are small
bodies in the solar system which are made up of ice, dust, rocks and trapped
gases. Thus, a broom-shaped shining object with a bright head is called a comet.
There are about 3,296 known comets.

Fig.: Comet

Meteor

The space around the earth contains millions of rocks. If these rocks are pulled by
the earth’s gravity, they enter into the earth’s atmosphere. They are called
meteors. Thus, the pieces of stone or metal which enter the earth’s atmosphere
are called meteors. Moving meteors drag themselves in the atmosphere to

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produce a bright light. So, they are also known as ‘shooting stars’, falling stars’,
or ‘fireball’. The meteor that does not burn completely in the atmosphere and
reaches the earth surface is called a meteorite.

Fig.: Meteor

Summary

1. The upper layer of the earth’s surface consisting of weathered rock and
humus is called soil.

2. Soil is said to be a non-renewable resource because it cannot be formed in a
short period of time.

3. A soil profile is a physical and chemical description of the layers that make
up the soil.

4. The process in which rocks break down into the soil over time due to physical,
chemical or biological activities is called weathering.

5. The removal of top fertile soil by various agents such as air, water, wind,
river, glacier, hurricane, cyclone, etc. is called soil erosion.

6. Soil deposition is the process of accumulation of the eroded soil away from
the place of erosion.

7. The prevention of erosion of topsoil is called soil conservation.
8. The presence of toxic chemicals and solid wastes in the soil at a higher amount

is called soil pollution.
9. The crust is the thinnest and outermost layer of the earth.

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10. The mantle is the second layer of the earth which lies between crust and core.
11. The innermost layer of the earth is called core.
12. Axis of the earth is an imaginary straight line perpendicular to its equatorial

plane and passing through the centre.
13. The orbit of the earth is an elliptical path on which it revolves around the sun.
14. Rotation refers to the anti-clockwise spinning of the earth about its axis. It is

also called a diurnal motion.
15. The orbiting of the earth around the sun is called revolution. It is also called

an annual motion of the earth.
16. The time duration in which the earth faces the sun while rotating on its axis

is called the day and the time duration in which the earth faces away from the
sun is called the night.
17. The sun and heavenly bodies such as planets, satellites, comets, asteroids,
dwarf planets etc. that revolve around the sun make up the solar system.
18. The heavenly bodies that revolve around the sun on their own elliptical orbits
are called planets.
19. The heavenly objects that revolve around the planet are called satellites.
20. The rocky and metallic objects which are present between the orbits of Mars
and Jupiter that revolve around the sun are called asteroids or minor planets.
21. A broom-shaped shining object with a bright head is called a comet.
22. The pieces of stone or metal which enter the earth’s atmosphere from space
are called meteors.

Exercise

1. Fill in the blanks with appropriate words.
a. The physical and chemical description of the layers of the soil is
called………..
b. The removal of top fertile soil is called…….

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c. …………… is a serious problem in the agricultural land.

d. The internal structure of the earth is divided into …….layers.

e. ……..lies between crust and core.

2. Write True for the correct and False for the incorrect statements.

a. The continuous expansion and contraction for millions of years cracks
rocks.

b. The collection of eroded rocks, minerals, humus, water and air over
millions of years makes soil.

c. Human activities such as agriculture, construction and deforestation
also accelerate soil erosion.

d. Erosion makes land fertile in the hilly areas.

e. Soil erosion causes degradation of soil.
3. Select the best answer from the given alternatives.

a. Which of the following does not cause soil pollution?

i. agricultural waste ii. afforestation

ii. deforestation iv. chemical fertilizer

b. Which one of the following is not the effect of soil pollution?

i. infertile land ii. decrease in agricultural production

iii. unpleasant scenery iv. pure water

c. This layer is the innermost layer of the earth.

i. crust ii. mantle iii. outer core iv. inner core

d. Where do we live?

i. crust ii. mantle iii. outer core iv. inner core

e. Which of the following is not a part of the solar system?

i. sun ii. earth iii. galaxy iv. satellite

4. Match the following:

Sun i. natural satellite of earth
ii. mars
Earth

Largest planet iii. planet

Red planet iv. jupiter

Moon v. star

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5. Write your answer to the following questions in a word each.
a. It is called a living planet.
b. These two planets do not have any satellite.
c. The small rocky bodies revolving around the sun.
d. It is called a tailed star.
e. It is seen as a streak of light in the night sky.

6. Write any two differences between:
a. Crust and mantle
b. Sun and planet

7. Give reasons:
a. Terrace farming is done in the hilly region.
b. Jupiter is called a planet.
c. Earth is called a living planet.

8. Study the given diagram and answer the following questions.

a. What is shown in the given diagram?
b. Write the name of layers 1, 2 and 4.
c. Which one of the given layers has humus?
9. Answer the following questions.
a. What is soil? Describe its formation process.
b. List the importance of soil.
c. What is a soil profile?
d. What is soil erosion? Write any three causes and three effects of soil

erosion.

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e. What is soil conservation? List ways to ensure soil conservation.
f. Define soil pollution. List causes and preventive measures of soil

pollution.
g. Describe internal structure of the earth with the help of a diagram.
h. What is season? What is responsible to change the season? Describe

different types of season.
i. Define the solar system. List the names of eight planets with their brief

description.
10. Project Work
Make a clay model of the solar system in an A4-size cardboard paper.

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