Acidity is measured on a pH scale. A pH of 1 is the most acid and a pH of 14 is the most base (or alkaline). Pure water is considered to be neutral. It has a pH of 7. Normal rainwater has a pH of about 6. That is a little acidic because even pure rain water falls through carbon dioxide in the air. Rain with a pH of less than about 5.3 is considered acid rain. Rain in the northeastern states has a pH between 4 and 5. This is serious acid rain.
