The self-ionization of water (also autoionization of water, and autodissociation of water, or simply dissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, HO, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH. The hydrogen nucleus, H, immediately protonates another water molecule to form a hydronium cation, HO. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.