Acid rain is the collective name given to any rain containing acidic impurities. These impurities are generally the product of industrial pollution.

Normal, unpolluted rain has a pH of 5.6 due to the presence of dissolved carbon dioxide. Acid rain is regarded as rain with a pH more acidic than this value (a pH value less than 5.6). The greater acidity of acid rain is due to different oxides of nitrogen and sulfur dissolved in the rain. These oxides are generally referred to as NO_x and SO_x to indicate their variable composition. These pollutants are released into the atmosphere by the combustion of fossil fuels such as coal, oil, and gas.

Acid rain has been implicated in a number of environmental problems. In the 1960s and 1970s, fish stocks were depleted in a large number of Scandinavian lakes and this occurred at exactly the same time...