NCERT Solutions for Class 11th: Ch 8 Composition and Structure of Atmosphere

NCERT Solutions for Class 11th: Ch 8 Composition and Structure of Atmosphere

Exercises

Page No: 78

1. Multiple choice questions.


(i) Which one of the following gases constitutes the major portion of the atmosphere?
(a) Oxygen
(b) Nitrogen 
(c) Argon
(d) Carbon dioxide
► (b) Nitrogen

(ii) Atmospheric layer important for human beings is:
(a) Stratosphere
(b) Mesosphere 
(c) Troposphere
(d) Ionosphere
► (c) Troposphere

(iii) Sea salt, pollen, ash, smoke soot, fine soil — these are associated with:
(a) Gases
(b) Dust particles 
(c) Water vapour
(d) Meteors
► (b) Dust particles

(iv) Oxygen gas is in negligible quantity at the height of atmosphere:
(a) 90 km
(b) 120 km 
(c) 100 km
(d) 150 km
► (b) 120 km

(v) Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?
(a) Oxygen
(b) Nitrogen 
(c) Helium
(d) Carbon dioxide
► (d) Carbon dioxide

2. Answer the following questions in about 30 words.

(i) What do you understand by atmosphere?

Answer

Atmosphere is a mixture of different gases and it envelopes the earth all round. It contains life-giving gases like oxygen for humans and animals and carbon dioxide for plants.

(ii) What are the elements of weather and climate?

Answer

The elements of weather and climate are temperature, pressure, winds, humidity, clouds and precipitation. These elements are subject to change and which influence human life on earth.

(iii) Describe the composition of atmosphere.

Answer

The atmosphere is composed of gases, water vapour and dust particles. Nitrogen constitutes 78.8%, oxygen constitutes 20.94% and argon constitutes 0.93%. Both gases together constitute 99% of the atmosphere. Other gases include are Carbon dioxide, Neon, Helium, Krypto, Xenon and Hydrogen. 

(iv) Why is troposphere the most important of all the layers of the atmosphere?

Answer

Troposphere is the most important of all the layers of the atmosphere:
→ All changes in climate and weather take place in this layer.
→  This layer contains dust particles and water vapour
→ All biological activities take place in this layer.

3. Answer the following questions in about 150 words.

(i) Describe the composition of the atmosphere.

Answer

The atmosphere is composed of gases, water vapour and dust particles. The given table specifies the constituent of atmosphere with their volume.

Constituent
Formulae
% by Volume
Nitrogen N2 78.08
Oxygen O2 20.95
Argon Ar 0.93
Carbon-dioxide CO2 0.036
Neon Ne 0.002
Helium He 0.0005
Krypton Kr 0.001
Xenon Xe 0.0009
Hydrogen H2 0.0005

Nitrogen and Oxygen gases together constitute 99% of the atmosphere. Other gases include are Carbon dioxide, Neon, Helium, Krypto, Xenon and Hydrogen. The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth. 
Carbon dioxide absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the green house effect. Ozone is another important component of the atmosphere which absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.

(ii) Draw a suitable diagram for the structure of the atmosphere and label it and describe it.

Answer

Structure of Atmosphere

The atmosphere consists of different layers with varying density and temperature. The column of atmosphere is divided into five different layers depending upon the temperature condition. They are: troposphere, stratosphere, mesosphere, thermosphere and exosphere.

• Troposphere: It is the lowermost layer of the atmosphere. Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator. This layer contains dust particles and water vapour. All changes in climate and weather take place in this layer. The temperature in this layer decreases at the rate of 1 C for every 165m of height. This is the most important layer for all biological activity.

→ Tropopause: The zone separating the tropsophere from stratosphere is known as the tropopause. The air temperature at the tropopause is about minus 80°C over the equator and about minus 45°C over the poles. The temperature here is nearly constant, and hence, it is called the tropopause.

• Stratosphere: It is found above the tropopause and extends up to a height of 50 km. It contains the ozone layer. This layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.

• Mesosphere: It lies above the stratosphere, which extends up to a height of 80 km. In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100 C at the height of 80 km. The upper limit of mesosphere is known as the mesopause.

• Thermososphere: It is the outermost layer of the atmosphere. It is located from 80 km with no definite upper limit. The air in this layer is very hot because heat coming from the sun strikes the thermosphere first.

→ Ionosphere: The lower layer of thermosphere is called ionosphere. It is located between 80 and 400 km above the mesopause. It contains electrically charged particles known as ions, and hence, it is known as ionosphere. Radio waves transmitted from the earth are reflected back to the earth by this layer. Temperature here starts increasing with height.

• Exosphere: The uppermost layer of the atmosphere above the thermosphere is known as the exosphere. This is the highest layer but very little is known about it. Whatever contents are there, these are extremely rarefied in this layer, and it gradually merges with the outer space.


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