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

Question: Explain how the atmosphere sustain life on Earth.

Answer: The atmosphere is vital for life on the Earth. It helps sustain life in various ways:

1. The atmosphere acts as a blanket by trapping terrestrial radiation and keeping the warm. It ensures that there is the correct amount of heat required for life to exist.
2. The atmosphere blocks harmful ultra-violet radiations to reach Earth.
3. The atmosphere contains oxygen, the gas living beings need to breathe to stay alive.
4. The moisture it contains is an essential part of the water cycle.
5. Green plants need the carbon dioxide present in the atmosphere for making food.
6. Nitrogen is required for the fertility of soil and is vital for plants.

Question: How is the balance between oxygen and carbon dioxide is maintained? Mention one action that maintains its balance?

Answer: The balance between oxygen and carbon dioxide is maintained by:

1. Green plants give out oxygen during photosynthesis. This oxygen is used up when living things (including plants) respire, and carbon dioxide is released.
2. This dual process ensures that the amount of oxygen and carbon dioxide in the atmosphere is always constant.
3. This natural balance is affected when we cut down trees indiscriminately.

Question: What is global warming? What are its causes?

Answer: Carbon dioxide absorbs terrestrial radiation and radiates it back to the earth. This trapping of energy by the carbon dioxide in the atmosphere produces what is called the green house effect, and it is therefore called green house gas. If the amount of carbon dioxide in the air increase, more heat gets trapped on the earth’s surface and also the layer of atmosphere in contact with it. This warming of the earth is called global warming. Its causes are –

1. Over population
2. Deforestation
3. Use of Chemicals.
4. Burning of fossil fuels like coal and petroleum.

Question: Where is the ozone layer found? What is its importance and how it is been destroyed?

Answer: Ozone layer is found in the lower part of the stratosphere between the altitudes of 15 and 30 km. It plays a very important role- it absorbs harmful ultraviolet rays. The ozone layer is being destroyed by the release of chlorofluorocarbons (CFCs) into the atmosphere. CFC is a valuable industrial chemical which is used in refrigerators, home insulation, Plastic foam and throwaway food containers. Jet planes that fly in the stratosphere also release CFCs. Efforts are being made globally to reduce the use of CFCs in the atmosphere.