The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or an aqueous solution, in which a water molecule, H₂O, 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 hydronium, H₃O. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.