Water is a weak electrolyte that undergoes only little self ionization. “The ionic product of water is the product of the concentrations of H+ and OH ions in water at a given temperature.” The letter K is assigned for it.
H2O(l) + H2O(l) ⇄ H3O+(aq) + OH-(aq)
or
2 H2O(l) ⇄ H3O+(aq) + OH-(aq)
Kw = [H3O+][OH-]
H2O + H2O ⇌ H3O+ + OH−
pH + pOH = 14.
Water is amphiprotic, which means that it may behave as an acid by giving a proton to a base to generate the hydroxide ion, or as a base by taking a proton from an acid to form the hydronium ion (H3O+). Autoionization of water results in the formation of OH- and H3O+ ions. The equilibrium constant for this reaction is termed the liquid water ion-product constant (Kw), and it is defined as Kw = [H3O+][OH-]. Kw equals 1.01 1014 at 25°C;
so, pH + pOH = pKw = 14 .