Normal rain water is always slightly acidic because of the fact that CO2 present in the atmosphere gets dissolved in it. Because of the presence of SO2 and NO2 gases as pollutants in the atmosphere, the pH of the rain water is further lowered. This type of precipitation of water is called acid rain (or) acid deposition.
Acid rains means the presence of excessive acids in rain water. The thermal power plants, industries and vehicles release nitrous oxide and Sulphur dioxide into atmosphere due to burning of coal and oil. When these gases react with water vapor in the atmosphere, they form acids and descend on the earth's "acid rain" through rain water.
SOx + H2O -------------> H2SO4
NOx + H2O --------------> HNO3
Due to the drifting of these gases in the atmosphere by the wind, their presence are felt as far as 2,000 kilometers. The air pollution of one nation could cause acid rain for another nation.
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