Answer. Burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts SO2 into H2SO4 and NOX to HNO2 and HNO3. Normal rain water is weakly acidic because it consists of dissolved CO2 of the air. Its pH is about 5.6 to 6. But rain water on dissolving air pollutants (acids) becomes more acidic and its pH reduces to 4. Thus, acid rain is formed on dissolving acidic air pollutants such as sulphur dioxide and nitrogen dioxide by rain water. Following figure shows how oxides of sulphur and nitrogen are converted into acids. These acids dissolve in rain water and damage soil, animals, plants and aquatic life.
Acid rain formation and its effects
(i) Acid rain on soil and rocks leaches heavy metals (Al, Hg, Pb, Cr, etc) with it and discharges these metals into rivers and lakes. This water is used by human beings for drinking purpose. These metals accumulate in human body to a toxic level. On the other hand, aquatic life present it lakes also suffers because of high concentration of these metals. Especially high concentration of aluminum metal clogs the fish gills. It causes suffocation and ultimately death of fish.
(ii) Acid rain attacks the calcium carbonate present in the marble and limestone of buildings and monuments. Thus, these buildings are getting dull and eroded day by day.
(iii) Acid rain increases the acidity of the soil. Many crops and plants cannot grow properly in such soil. It also increases the toxic metals in the soil that poisons the vegetation. Even old trees are being affected due to acidity of soil. Their growth is retarded. They get dry and die.
(iv) Acid rain directly damages the leaves of trees and plants, thus limiting their growth. Depending upon the severity of the damage, plants growth can be hampered. The capability of plants to resist cold or diseases reduces and ultimately they die.