Before considering the consequences of acid rain, it is important to remember that rain is naturally acidic, having a pH of about 5.6. The pH scale ranges from 0 to 14 with the midpoint, 7, taken as neutral; pH <7 represents an excess of hydrogen ions and, hence, acidity, while pH >7 represents alkalinity. The pH scale is logarithmic, i.e. pH = -logl, (Hi). Hence, a change of 1 on the pH scale corresponds to a change in a factor of 10 in the hydrogen ion concentration (H+).