A pH scale is used to measure the amount of acid in a liquid-like water. Because acids release hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed as "pH." This scale is used to measure the acidity of rain samples.
0 = maximum acidity
7 = neutral point in the middle of the scale
14 = maximum alkalinity (the opposite of acidity)
The smaller the number on the pH scale, the more acidic the substance is. Rain measuring between 0 and 5 on the pH scale is acidic and therefore called "acid rain." Small number changes on the pH scale actually mean large changes in acidity.
For example, a change in just one unit from pH 6.0 to pH 5.0 would indicate a tenfold increase in acidity. Clean rain usually has a pH of 5.6. It is slightly acidic because of carbon dioxide which is naturally present in the atmosphere. Vinegar, by comparison, is very acidic and has a pH of 3.