First identified in 1872 in Sweden and studied in the U.S. beginning in the 1950s, acid rain is precipitation in the form of rain, snow, hail, dew, or fog that transports sulfur and nitrogen compounds from the high atmosphere to the ground. Sulfur dioxide (SO ₂ ) and nitrogen oxides (NO, NO ₂ ) are bi-products from burning fuels in electric utilities and from other industrial and natural sources. These chemicals react with water, oxygen, carbon dioxide, and sunlight in the atmosphere to form sulfuric and nitric acids. The acids reach the ground and change the chemistry within the environment.

The acidity of any solution is determined on the pH scale of 0 to 14. A pH level of 0 to 7 is considered acidic; 7 is neural; and a level above 7 is alkaline. As the pH number decreases, acidity increases. Unopened bottled distilled water has a pH of 7, so it is neutral. In comparison, household ammonia is an alkaline with a pH of 11.5. Milk is slightly acidic with a 6.5 pH, and soft drinks, which contain phosphoric acid, have a 3.1 pH.