As we know that normal rain water will be slightly acidic. This is because of the fact that carbon-di-oxide present in the atmosphere gets dissolved in it. Because of the presence of sulphur di oxide and nitrogen di oxide 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 rain means the presence of excessive acids in rain water. The thermal power plants, industries and the vehicles release nitrous oxide and sulphur di oxide into atmosphere due to burning of coal and oil. When these gases react with the water vapour in the atmosphere, they form acids and descend on the earth’s ‘ACID RAIN” through rain water. The air pollution of one nation could cause acid rain for another nation.