When atmospheric pollutants such as sulphur dioxide and nitrogen oxides mix with water vapour in the air, they are converted to sulphuric and nitric acids. These acids make the rain acidic, hence the term 'acid rain'. Acid ran is defined as any rainfall that has an acidity level beyond what is expected in non-polluted rainfall. Acidity is measured using a pH scale, with the number 7 being neutral. Consequently, a substance with a pH value of less than 7 is acidic, while one of a value greater than 7 is basic. Generally, the pH of 5.6 has been used as the baseline in identifying acid rain. Thus any precipitation that has a pH value of less than 5.6 is considered to be acid precipitation. |
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