„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:
- H2O (l) + CO2 (g) ⇌ H2CO3 (aq)
Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions:
- H2O (l) + H2CO3 (aq) ⇌ HCO3− (aq) + H3O+ (aq)
However, 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 H2CO3.