CBSE Class 8 Science, Pollution of Air and Water

•   By volume, air consists of about 78% nitrogen and about 21% oxygen. Carbon dioxide, argon, methane, ozone and water vapour are present in very small quantities.

•   When air is contaminated by unwanted substances which have a harmful effect on both the living and non-living, it is known as air pollution.

•   The substances which contaminate the air are known as air pollutants.

•   The sources of air pollutants are factories, power plants, automobile exhausts and burning of firewood and dung cakes.

•   Vehicles produce pollutants such as carbon monoxide, carbon dioxide, nitrogen oxides, oxides of sulphur and smoke.

•   Carbon monoxide is a poisonous gas. It reduces the oxygen-carrying capacity of the blood.

•   Smog: This is a thick fog-like layer in the atmosphere, especially during winters, made up of fog and smoke.

•   Sulphur dioxide is mainly produced by burning fuels such as coal. It causes respiratory problems, including permanent lung damage.

•   CFCs (chlorofluorocarbons) used in refrigerators, air conditioners and aerosol sprays cause depletion of ozone layer. Ozone layer protects us from ultraviolet rays of the sun.

•   Acid Rain: Pollutants like sulphur dioxide and nitrogen dioxide react with water vapour present in air to form sulphuric acid and nitric acid. The acid drops come down with rain which is known as acid rain.

•   Marble Cancer: Acid rain corrodes the marble, especially marble monuments. This phenomenon is called marble cancer.

•   Greenhouse Effect: A part of radiation of the sun that falls on the earth is absorbed by it and a part is reflected back into space. A part of the reflected radiation is trapped by the atmosphere. This trapping of radiations of the earth's atmosphere causes warming of the earth, which is known as greenhouse effect.

        Note: Green houses are made of glass, especially in cold region, in which potted plants are placed. During day, the heat waves of the sunlight enter into the green house through glass walls and roofs but all the heat radiation effected are not allowed by the glass walls to go back into the atmosphere. So, the temperature of green house remains much higher than the temperature of the atmosphere outside the green house. So, this phenomenon is called green house effect.

        Like CO₂, methane, nitrogen oxide and water vapours are also called green house gases.

•   Global Warming: Due to human activities there is accumulation of CO₂ in the atmosphere. Carbon dioxide traps heat and does not allow it to escape into space. As a result, the average temperature of atmosphere of the earth gradually increases. This gradual increase in temperature is known as global warming.

•   The substances such as sewage, toxic chemicals, slit, agricultural chemical etc. that pollute water are called water pollutants.

•   Ganga Action Plan was launched in 1985 to reduce the pollution levels in the river.

•   Agricultural chemicals such a fertilizers, pesticides and weedicides dissolve in water in the crop fields. From there they are washed to water bodies such as pond, lake etc. These substances act as nutrients for algae and growth of algae enhances.

        After sometimes algae start dying due to over growth. Dead mass of algae is decomposed by decomposers like bacteria.

        In this process a lot of oxygen dissolved in the water gets used up. This results in a decrease in the oxygen level that may cause the death of aquatic organisms.

•   Water contaminated with sewage may contain viruses, bacteria, fungi, eggs of worms and parasites which cause diseases like cholera, typhoid and jaundice.

•   Potable Water: Water which is purified and suitable for drinking is known as potable water.

•   Chlorination: By adding chlorine tablets to water for purification is a common chemical process. We should not use more chlorine tablets than specified.

•   Water is a precious natural resource. We should conserve it and should follow the mantra—reduce, reuse and recycle.