Acid (see Acids And Bases) rain is any form of precipitation that has greater than normal acidity.
The acidity of precipitation is measured on a pH scale, ranging form 0 to 14,
that measures acidity and alkalinity. A chemically neutral solution measures
7, the pH of distilled (see Distillation) Water. Increasing acidity is represented by numbers less than 7, while rising
alkalinity is represented by numbers greater than 7. All rainfall is acidic due to
natural causes. Normal rainfall has a pH of between 4.8 and 5.4.
THE CAUSE OF ACID RAIN
Acid rain has a variety of causes, but the biggest non-made contributors are
sulfur dioxide (see Oxidation and Reduction) and Nitrogen oxides (see Coal - Major Combustion Byproducts). Once in the atmosphere, these Gases react with other chemicals to form sulfuric acid and nitric acid. The
acids can return to the earth as gases or particles, or in rain, snow, or fog.
In industrialized areas, the vast majority of sulfur and nitrogen emissions
are from human activities primarily the combustion of Coal and Oil.