Water itself dissociates to a small but significant extent

As with ammonia, the constant term, [H₂O] = 55.5 moles per liter, can be incorporated into the equilibrium constant:

in which K_w, is the "ion product" for water. The numerical value of K_w varies with temperature:

(Can you use these data and Le Chatelier's principle to predict whether the dissociation of water is endothermic or exothermic? Check your prediction against Appendix 2.) The value of K_w that conventionally is used in calculations is

Because this ion product is an equilibrium constant, hydroxide and hydrogen ion concentrations can be related in aqueous solution. If we increase the hydrogen ion concentration by adding acid to a solution, we will repress the dissociation of water. Some of the added H⁺ will combine with OH^-, and the hydroxide ion concentration will decrease until the ion product again is 10^-14 .

After equilibrium has been reestablished, the hydrogen and hydroxide ion concentrations will be different than before, but their product will be the same. Similarly, if we add hydroxide ions to a solution, they will combine with some of the H⁺ originally present, until the ion product once again is 10^-14 .

When an acid and a base are mixed, some of the H⁺ and OH^- ions combine to form H₂O. This is called neutralization. A solution in which [H⁺] and [OH^-] are equal is termed a neutral solution.

In pure water or any neutral solution, the hydroxide and hydrogen ion concentrations are