Two water molecules can react to form one hydronium ion and one hydroxide ion. This reaction is expressed through the chemical formula 2H2O(l) -> H3O+(aq) + OH?(aq), H3O+ is the hydronium ion and OH- is the hydroxide ion.
This reaction is known as a dissociation reaction, which for two water molecules is called the self-ionization of water and occurs due to an electrical attraction called hydrogen bonding. During the reaction, electrons are exchanged to form one positively and one negatively charged electron. In water, this reaction is constantly occurring in small numbers in both directions, as the hydronium ion and the hydroxide ion can react to form two water molecules.