Being simultaneously basic and acidic, water undergoes autoprotolysis, also called self-ionization:

The equilibrium constant is

H3O+ in these equations is general simply written H+. In pure water and in solutions that do not give any additional source of H+ or OH- both ions have molar concentrations exact to 10-7. Addition of an acid decreases [OH-] and hence increases [H3O+]; addition of a base has the reverse effect.

The pH scale is defined by

Neutral water has a pH of 7, acidic solutions have lower values of ph (typically 0-7), and basic solutions or alkaline higher values (7-14). In alkaline or basic solutions [OH-] is thus greater than [H+].

Strong and weak behavior

The equilibrium constant of the protolysis reaction

is known as the acid dissociation constant (Ka) of HA or the acidity constant:

It is often expressed as a pKa value, described as