NCERT 7th Class (CBSE) Social Science: The Atmosphere

Question: Which layer of the atmosphere is important for us and why?

Answer:  Troposphere is the most important layer:

1. As all weather phenomena like the formation of clouds, fog, mist, dew, thunder, lightening, precipitation etc. occur in this layer.
2. This layer protects us from the excessive heat of the sun during the day and keeps the earth’s surface warm at night.
3. Water vapour and dust particles are found only in this layer.
4. The oxygen present in this layer is useful for breathing.

Question: Describe the main features of the composition of the atmosphere.

Answer: 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 18%) and oxygen (about 21%).
3. The other gases present in the atmosphere are carbon dioxide, helium, ozone, argon, hydrogen etc.
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. That is why mountaineers have to carry oxygen cylinders while scaling very high peaks.

Question: Explain the importance of nitrogen and oxygen in the atmosphere.

Answer: 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.

Question: What is the significance of atmosphere for the earth?

Answer: The significance of atmosphere for the earth is:

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 as a green house and controls the extremes of temperature during day and night.
5. It softens the sun’s glare during 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.