24 December 2011

CBSE Science VIII-Pollution Of Air And Water

Air pollution is described as any change in the composition of  air either by physical or chemical methods so as to cause harmful effects on health.
The substances which contaminate the air are called air pollutants. 
Sources of Air Pollution are : 
1. Natural sources 2. Man-made (anthropogenic) sources.
Natural sources: Volcanic eruption, forest fire, sea salt sprays, biological decay, photochemical oxidation of terpenes oil , marshes, pollengrains, spores, etc.,
Man made sources : Industrial emissions, vehicles, aeroplanes, power stations and burning of fuels, etc.,
Mainly air pollution is caused due to burning of fuels to run vehicles, and from chimneys of factories and power stations.
The Green House Effect : Some of the infrared radiation from the earth passes through the atmosphere but most is absorbed and re-emitted in all directions by greenhouse gas molecules and clouds. This warms up the Earth’s surface and the lower atmosphere
Global Warming : CO2 is continuously being released because of human activities. On the other hand, area under forests is decreasing. Plants utilize CO2 from the atmosphere for photosynthesis, thereby decreasing the amount of CO2 in the air. Deforestation leads to an increase in the amount of CO2 in the air because the number of trees which consume CO2is reduced. Human activities, thus, contribute to the accumulation of CO2 in the atmosphere. 
CO2 traps heat and does not allow it to escape into space. As a result, the average temperature of the earth’s atmosphere is gradually increasing. This is called global warming.
Ozone layer is thinning due to the emission of pollutant into the
atmosphere. Holes caused in the ozonelayer allow the harmful UV rays to reach the earth.
Radioactivity: Radio active minerals present in the earth’s crust are the sources of radioactivity in the atmosphere.

Acid Rain : Oxides of nitrogen, sulphur, carbon produced by combustion of coal, petroleum, etc,. dissolve in atmospheric water vapour. They form their corresponding acids like nitric acid, sulphuric acid, etc., and reach the earth’s surface as acid rain.
Effects of acid rain
● It irritates eyes and skin of human beings.
● It inhibits germination and growth of seedlings.
● It changes the fertility of the soil, destroys plants and aquatic life.
● It causes corrosion of many buildings, bridges, etc.,
Control of air pollution :Air pollution can be minimized by the following methods:
1. Use of crude oil should be avoided and use of high quality fuels, unleaded petrol, bio-diesel and compressed natural gas(CNG) should be recommended.
2. Use of automobiles should be minimized.
3. Industrial smokes must be filtered before releasing into the atmosphere.
4. By planting more trees to get pure air (O2) and reduce the CO2 content of the environment.

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