Water molecules can behave as both acids and bases. One water molecule can donate a H⁺ ion (acid) to another water molecule, which accepts the H⁺ ion (base), forming an OH^- ion and an H₃O⁺ ion respectively.

However, the OH^- ion is a very strong base and the H₃O⁺ ion is a very strong acid, therefore they will react together almost immediately to produce water again. At any instant, there is a very small amount of H₃O⁺ and OH^- ions present. An equilibrium is set up:

\(2H_2O_{(l)} ⇌ H_3O^+_{(aq)} + OH^-_{(aq)} \)

This is usually written in its simplified form:

\(H_2O_{(l)} ⇌ H^+_{(aq)} + OH^-_{(aq)} \)

The equilibrium constant for this slight dissociation of water is known as the ionic product of water, K_w.

\(K_w = [H^+][OH^-] \)

Like any other equilibrium constant, the value of K_w varies with temperature. At room temperature, K_w is assumed to be 1.00 x 10^-14 mol² dm^-6.

The relationship between K_w and pK_w is the same as that between K_a and pK_a, or [H+] and pH.

\(pK_w = - \log_{10}{K_w} \)

At room temperature, pK_w is assumed to be 14.

The variation of K_w with temperature

The dissociation of water to form H⁺ and OH^- ions is an endothermic process:

\(H_2O_{(l)} ⇌ H^+_{(aq)} + OH^-_{(aq)} \)

According to Le Chatelier's Principle, if you increase the temperature, the equilibrium will move in the direction that counters the change, i.e. the endothermic direction. So, increasing the temperature will favour the forwards reaction, and produce more H⁺ and OH^- ions.

Therefore, as temperature increases, the value of K_w also increases.