"Acid rain" is a popular term referring to the deposition of a mixture from wet (rain, snow, sleet, fog,  cloudwater, and dew) and dry (acidifying particles and gases) acidic components. Distilled water, once carbon dioxide is removed, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are alkaline. "Clean" or unpolluted rain has an acidic pH, but usually no lower than 5.7, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid according to the following reaction:
Carbonic acid then can ionize in water forming low concentrations of carbonate and hydronium ions:
Unpolluted rain can also contain other chemicals which affect its pH (acidity level). A common example is nitric acid produced by electric discharge in the atmosphere such as lightning. Acid deposition as an environmental issue (discussed later in the article) would include additional acids other than .
Occasional pH readings in rain and fog water of well below 2.4 have been reported in industrialized areas.
The main sources of the SO and NO pollution that causes acid rain are burning fossil fuels to generate electricity and power internal combustion vehicles, to refine oil, and in industrial manufacturing and other processes.