Water can be considered an acid because of its ability to ionize to a proton and a negatively charged hydroxide ion:

H-O-H H ⁺ + OH ^-

Hydrogen-ion concentrations of aqueous solutions range from greater than 1 M to less than 10 ^{- 14} M. For convenience, H ⁺ concentrations are expressed on a logarithmic scale as pH. The pH of a solution is the negative log of its H ⁺ concentration, or:

Therefore, a H ⁺  concentration of 10 ^{- 7} M has a pH of 7.0. In pure water, there are always equal numbers of H ⁺ and OH ^- , and the pH is defined to be neutral.(Note that neutral pH is not always exactly 7.00. If you raise the temperature, more H₂O dissociates and the pH will be lower). Acids contribute another source of H ⁺ causing the pH to decrease from neutrality. Conversely, bases absorb H ⁺ from solution and thus make the pH higher than neutrality.