 Self-ionization of water

Two water molecules can exchange a proton, producing hydronium and hydroxide ions: $2{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{OH}}^{-}$. Even pure water therefore supplies ions; extreme dilution eventually invalidates the strong acid assumption that it supplies all the hydronium.

Self-ionization of water, also known as autoionization, is a process in which water molecules dissociate into ions. In this process, two water (H₂O) molecules interact, resulting in the formation of a hydroxide ion (OH⁻) and a hydronium ion (H₃O⁺).