The acidity of a solution is the measure of the concentration of hydrogen ions (H+) in the solution. Alkalinity and acidity are measured using a pH scale. On a pH scale, 7.0 is neutral and any measure lower than 7 is acidic, whereas a pH value greater than 7 is basic (alkaline). Usually, the pH of normal rain is about 5.6 which is slightly acidic. This is because carbon dioxide dissolves into it to form a weak carbonic acid. The acid rain pH, on the other hand, is between 4.2 and 4.4.