Water undergoes autoprotolysis, dissociating to form a positive hydrogen ion and a negative hydroxyl ion. Autoprotolysis occurs in equilibrium balance, with the ion product of water, a short-hand equilibrium constant, kw, which describes the degree to which water will dissociate.
A Brønsted acid or base introduced to the aqueous system establishes a coupled equilibrium with the autoprotolysis reaction. For example, a Brønsted acid added to water increases the concentration of hydrogen ions. In response, autoprotolysis runs in reverse to recapture the equilibrium described by kw, but with each hydrogen ion it takes up, it also takes up a hydroxide, reforming water, until the ion product of water is recovered. However, now the hydroxide ion concentration is lower than the hydrogen ion concentration. The pH is acidic.