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.