The pH scale is a measure of how acidic or basic a liquid substance is. The pH is calculated as a negative logarithmic function of the concentration of hydrogen ions, which are acidic by nature. A high concentration of hydrogen ions will result in a low pH.
The pH scale runs from 1.0 to 14.0. A substance can have a pH level of any finite number on that scale. Water, a neutral substance, has a pH of 7.0. Any level below 7 is acidic and any level above 7 is basic. The formula for pH is pH=-log[H+], where [H+] is the concentration of hydrogen ions in moles per liter. As a result of being a logarithmic relationship, the acidity increases by a factor of 10 from one level to the next.