Those gases which combine with water to form acids are called acidic gases. Examples of acidic gases are sulfur dioxide ( SO₂) and nitrogen dioxide (NO2). The sulfur dioxide gas present in polluted air forms sulfuric acid (H2SO4) whereas nitrogen dioxide gas present in polluted air forms nitric acid (HNO₃).
When the ‘acids‘ formed from sulfur dioxide and nitrogen dioxide present in air dissolve in rainwater and fall on the earth, we call it acid rain. This is because due to the presence of acids in it, the rainwater becomes slightly acidic.
Rainwater is turned acidic when its pH falls below 5.6. In fact, clean or natural rainwater has a pH of 5.6 at 20ºC because of the formation of carbonic acid due to the dissolution of CO2 in water.
The term acid rain was coined by Robert Angus (1872).
Effects of Acid Rain:
Aquatic life especially fish are badly affected by lake acidification.
Aquatic animals suffer from the toxicity of metals such as aluminum, mercury, manganese, zinc, and lead which leak from the surrounding rocks due to acid rain. Dissolved aluminum is known to clog gills of fish. Birds dependent on aquatic animals are also killed. Such lakes come to have only some algae and fungi.
It damages foliage and weaken trees.
It makes trees more susceptible to stresses like cold temperature, drought, etc Many insects and fungi are more tolerant of acidic conditions and hence they can attack the susceptible trees and cause diseases.
Spoilage of Human Assets: Marble structures, limestone structures, metallic articles, jewellery, textiles, and paintings are spoiled by acid rain. This happens due to the corroding effect of acids on metals through the formation of sulfates, nitrates, and chlorides. Paintings are decolorized. Jewellery is similarly affected. Marble and limestone are made of CaCO3. It is insoluble in water. Acid rain changes CaCO3 into CaSO4 and CaNO3. The chemicals are soluble and are washed away or peeled off. This results in pitting and corrosion of these structures.