The reaction in which two water molecules react to give ions with proton transfer is called auto-protolysis of water or self ionization of water. A proton from one $\mathrm{H_2O}$ molecule is transferred to another water molecule leaving behind OH– ion and forming a $\mathrm{H_3O^+}$ ion. $\mathrm{H_2O}_{(l)} + \mathrm{H_2O}_{(l)}$ ⇌ $\mathrm{H_3O^+}_{(aq)} + \mathrm{OH^-}_{(aq)}$ Significance : Auto-protolysis proves that water is amphoteric in nature.