Autoionisation of water
Autoionisation of water

Pure water is not made only of neutral molecules. A tiny fraction of water molecules transfer protons between themselves, so the equilibrium 2H2O(l)⇌H3O+(aq)+OH−(aq)2\text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{OH}^-(aq)2H2O(l)⇌H3O+(aq)+OH−(aq) is always present.