„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.[1] Acid deposition as an environmental issue (discussed later in the article) would include additional acids other than H2CO3.