| Reference Book | Ratna Sagar Integrated Social Science |
| Class | Class 7 |
| Category | Geography |
| Chapter Number | Chapter 5 |
| Chapter Name | Composition and Structure of the Atmosphere |
A) Tick (✓) the correct answers.
1) The height of the atmosphere above the mean sea level is about
a) 1600 Km
b) 2600 Km
c) 3600 Km
d) 10000 Km ✓
2) Which of the following gas present in the atmosphere is important for the growth of plants ?
a) Hydrogen
b) Oxygen
c) Carbon dioxide ✓
d) Helium
3) The amount of carbon dioxide in the atmosphere is
a) decreasing
b) increasing ✓
c) fixed
d) eliminated
4) Water vapour is confined only to the
a) troposphere ✓
b) Stratosphere
c) ionosphere
d) thermosphere
5) The lower part of the Stratosphere has concentration of
a) ozone ✓
b) argon
c) hydrogen
d) nitrogen
6) Which gas protects us from the ultraviolet rays of the Sun?
a) Helium
b) Ozone ✓
c) Nitrogen
d) Oxygen
7) Which of the following layers of the atmosphere form the uppermost layer?
a) Mesosphere
b) Ionosphere
c) Exosphere ✓
d) Thermosphere
B) Fill in the blanks
1) The density of air is highest near the – Earth’s surface
2) The air in the atmosphere is a mixture of many gases.
3) The greenhouse effect is due to the increase of Carbon dioxide in the atmosphere.
4) Jet planes normally fly in the lower part of the Stratosphere.
C) Write True or false.
1) Nitrogen is directly absorbed by plants. False
2) There is horizontal movements of air in the stratosphere. True
3) The mesosphere protects the earth from meteors. True
4) The lower part of the exosphere is called ionosphere. False
D) Match the columns.
| Column A | Column B |
| 1) Troposphere | Weather phenomena |
| 2) Mesosphere | Coldest layer |
| 3) Global Warming | Carbon dioxide |
| 4) Ionosphere | Ionised particles |
E) Write short notes on the following.
1) Nitrogen in the atmosphere – Nitrogen is the most prevalent gas in the atmosphere. It is required by plants for growth. But plants are not able to absorb nitrogen directly from the air. Bacteria in the soil and
Bacteria in the soil and roots absorb and change its form, so that plants can use it. Atmospheric nitrogen is also used to manufacture chemical fertilizers.
2) Global Warming – Carbon dioxide present in the atmosphere absorbs the earth’s heat and prevents the earth from becoming too cold at night. This is known as the greenhouse effect. Burning of coal, smoke from vehicles, factories, etc., increase the amount of carbon dioxide in the atmosphere. The increased carbon dioxide absorbs more heat and increases the earth’s temperature. This increase in earth’s temperature is known as global warming.
3) Ozonosphere – The lower part of the stratosphere (between 15 km and 35 km) has concentration of ozone and is thus called ozonosphere.Ozone absorbs the ultraviolet radiation of the sun. It is essential for protecting life on earth.
4) Ionosphere – The lower part of the thermosphere is called ionosphere as it has ionized molecules. This layer reflects low frequency radio waves back to earth. Thus, it helps in long distance communication.
F) Distinguish between the following :
1) Air and atmosphere
| Air | Atmosphere |
| It is a mixture of gases, water vapours, dust particles, smoke etc. | The atmosphere is a mixture of gases that surrounds the Earth. |
| The gases present in air are oxygen, nitrogen, carbon dioxide etc | The atmosphere is divided into 5 layers namely Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere. |
2) Troposphere and exosphere
| Troposphere | Exosphere |
| 1) It is the lowest layer and is closest to the Earth’s surface. | It is the uppermost layer. |
| 2) The temperature decreases with increasing altitude. | The temperature increases with height. |
| 3) Water vapours and dust particles are found only in this layer. | Very light gases, like hydrogen and helium are found in their layer. |
3) Stratosphere and Mesosphere
| Stratosphere | Mesosphere |
| 1) It lies above the Troposphere. | It lies above the stratosphere. |
| 2) Its estimated height is about 50 Km above the mean sea level. | It extends from about 50 Km to about 80 Km above the mean sea level. |
| 3) The temperature remains constant upto a height of about 20Km, and then increases with increasing altitude. | The temperature decreases with height and reaches about -100°C at the height of 80 Km. |
4) Thermosphere and exosphere
| Thermosphere | Exosphere |
| 1) It lies above the mesosphere. | It is the uppermost layer. |
| 2) It extends from about 80 Km to about 400 Km above the mean sea level. | The exosphere merges gradually into outer space. |
| 3) In tis layer temperature increases rapidly with the increase in height.It reaches more than 1500 ° C. | The temperature increases with height and may reach 4500 ° C. |
| 4) One does not feel warm in this layer, as thin air holds little heat. | Very light gases like hydrogen and helium, are found in this layer. |
G. Give short answers for the following questions.
Que 1 What is the importance of carbon dioxide in the atmosphere?
Ans Carbon dioxide is important because of its ability to absorb heat. It is responsible for the greenhouse effect on earth. The amount of carbon dioxide exhaled by humans and animals is almost equal to the amount absorbed by plants.The amount of carbon dioxide in the atmosphere is gradually increasing. This is due to the increased burning of fossil fuels like coal and petroleum, and deforestation. Thus, we must plant more trees to maintain the balance of carbon dioxide in the atmosphere.
Que 2 Name the different layers of the atmosphere and their extent.
Ans The different layers of the atmosphere and their extent are –
1) Troposphere – It is the lowest layer and is closest to the Earth’s surface. It extends roughly upto a height of about 8 Km near the poles and about 18 Km over the equator,
2) Stratosphere – Its estimated height is about 50 Km above the mean sea level.
3) Mesosphere – It extends from about 50 Km to about 80 Km above the mean sea level.
4) Thermosphere – It extends from about 80 Km to about 400 Km above the mean sea level.
5) Exosphere – It is the uppermost layer that merges gradually into outer space.
Que 3 Which layer of the atmosphere is most important for us and why?
Ans Troposphere is the most important layer, as all weather phenomena like the formation of clouds, fog, frost, dew, thunder, lightning, precipitation, etc. occur in this layer. This layer protects us from the excessive heat of the sun during the day and keeps the earth’s surface warm at night.
Que 4 Explain the main features of the atmosphere.
Ans The main features of the atmosphere are:
1) The presence of the atmosphere has made the earth a unique planet.It has life giving gases like oxygen for humans and animals, carbon dioxide for plants.
2) It prevents the sun’s harmful ultraviolet rays from reaching the earth’s surface.
3) Change in weather is due to presence of atmosphere.
4) It is divided into 5 different layers – Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.
5) Upper limit of the atmosphere is at a height of about 10000 Km from the mean sea level.
6) The atmosphere is held to the Earth due to it gravitational pull.
H. Give long answers for the following questions.
Que 1 Describe the main features of the composition of the atmosphere.
Ans The main features of the composition of the atmosphere are as under :
1) Air is a mixture of gases, water vapour, dust particles and smoke.
2) The two main gases present in the atmosphere are nitrogen (about 78 per cent) and oxygen (about 21 per cent).
3) The other gases present in the atmosphere are carbon dioxide, helium, ozone, argon, hydrogen, etc. (about one per cent).
4) The lower layer of the atmosphere contains water vapour and dust particles.
5) The amount of water vapour and dust particles varies from time to time and from place to place.
6) Heavier gas, like oxygen, is mostly found in the lower layer.
7) The composition of air varies from place to place and from time to time.
Que 2 Explain the importance of nitrogen and oxygen in the atmosphere.
Ans Nitrogen is the most prevalent gas in the atmosphere. It is required by plants for growth. But plants are not able to absorb nitrogen directly from the air. Bacteria in the soil and roots absorb and change its form, so that plants can use it. Atmospheric nitrogen is also used to manufacture chemical fertilizers.
Oxygen is the most important gas. All living organisms need it to survive. They breathe oxygen present in the air. During photosynthesis green plants absorb carbon dioxide and release oxygen. This process helps to maintain the level of oxygen in the air. Oxygen is also essential for combustion.
Que 3 What are the main features of the troposphere?
Ans
1) The troposphere is the lowest layer and is closest to the earth’s surface.
2) It extends roughly upto a height of about eight km near the poles and about 18 km over the equator.
3) The temperature in this layer decreases with increasing altitude. It decreases roughly at the rate of 1°C per 165 m of ascent. This is called normal lapse rate.
4) Water vapour and dust particles are found only in this layer.
5) Troposphere is the most important layer, as all weather phenomena like the formation of clouds, fog, frost, dew, thunder, lightning, precipitation, etc., occur in this layer.
6) This layer protects us from the excessive heat of the sun during the day and keeps the earth’s surface warm at night.
7) Troposphere has about 90 per cent of the air in the atmosphere.
Que 4 What is the significance of atmosphere for the earth?
Ans
1) The lower layer of the atmosphere has life giving oxygen, which is needed for breathing.
2) Nitrogen and carbon dioxide are needed by plants for their growth.
3) It prevents the sun’s harmful ultraviolet rays from reaching the earth’s surface.
4) It acts like a greenhouse and controls the extremes of temperature during day and night.
5) It softens the sun’s glare during the day.
6) The change of weather and climate is due to the presence of the atmosphere.
7) The presence of water vapour causes condensation and precipitation.
8) Sound waves travel through air.
9) It shields us from meteors.
HOTS
Que 5 Why does the density of air decrease with increase in height?
Ans The density of air decreases with increasing altitude. As we go far from the Earth, the layers of air become thinner. The layers of air closest to the Earth are very close to each other and hence are thick. The layers of air at higher levels are not so close to each other and hence are thinner.
Que 6 How is carbon dioxide responsible for global warming?
Ans The temperature of the Earth surface is increased due to the radiation received from the Sun. This heat energy is radiated back into the atmosphere by the hot surface of the Earth. Molecules of carbon dioxide gas have a tendency to absorb the heat. The carbon dioxide molecules present in the atmosphere absorb this heat energy. Hence, an increase in the amount of carbon dioxide molecules in the atmosphere also increases the heat energy, which increases the temperature of the Earth. This leads to global warming.
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