Acid rain is measured using a scale called "pH", a scale that runs from zero, the most acidic, to 14, the most alkaline. A change of one unit on the pH scale represents a 10-fold change in acidity. Organisms generally thrive near pH 7, the neutral point, and function less successfully toward either end of the scale.
The following diagram shows the pH scale and the pH of some common items:
Pure water has a pH of 7.0. Normal rain is slightly acidic because carbon dioxide dissolves into it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the US has a pH of about 4.3
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