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SAI International School
                                               Subject - Chemistry
                                                     Class - VIII
                                       Topic –Pollution of Air and water
                                         Sub-Topic-Water Pollution and its causes

                                                  LESSON NOTES


        Water Pollution
        Most of the water which we use comes from the rivers and lakes. Everyday many unwanted
        and harmful substances are thrown into the rivers and lakes. This makes the water of rivers
        and lakes impure or contaminated. And we say that the water is polluted.

        The contamination of the water of rivers, lakes, and ponds, etc., with unwanted and harmful
        substances, is called water pollution. This polluted water becomes unfit for drinking.

        The unwanted and the harmful substances which make the water impure or contaminated
        are called water pollutants.

        The major water pollutants are sewage, fertilisers, pesticides, and industrial wastes.  All of
        these water pollutants are harmful and dangerous to human beings, animals, and plants.
        Hot water from power plants and industries is also considered a pollutant as when very hot
        water coming from power plants and industries is discharged into a river or lake, it raises
        the temperature of the river water (water bodies). This can affect adversely to plants and
        animals.

        Sources of Water Pollution

        the water of rivers, lakes, and ponds get polluted by various human activities. The major
        sources are:

        1.  Untreated sewage from homes
        2.  Fertilisers used in agriculture
        3.  Pesticides used in agriculture
        4.  Untreated industrial wastes

          Harmful effects of water pollution
        1.  Untreated sewage from homes
            The wastewater containing human excreta are carried in sewers (underground pipes) is
            called sewage. At many places, sewage from homes is directly dumped into the rivers
            which pollute it. Sewage contains several microorganisms such as bacteria, protozoa,
            fungi, parasites, etc., which causes serious diseases in humans. It can cause water-
            borne diseases such as cholera, typhoid, diarrhoea, dysentery, and jaundice.
        2.  Fertilisers used in agriculture
            The farmers use large amounts of chemical fertilisers in the fields to increase the yield
            of crops. The excess fertilisers dissolve in water and run into rivers, lakes, and ponds. It
            increases the nutrient level of water. This process is called ‘eutrophication’. The
            fertilisers cause rapid growth of tiny green, water plants called algae in the river known
            as ‘algal bloom’. They use to cover the whole surface of the water like a green sheet,
            with the help of fertilisers. When the algae die, the bacteria decompose the mass of
            algae by using the dissolved oxygen in the water of the river. This decreases the


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