Which of these equations correctly expresses the self-ionization of water?

Upper H subscript 2 upper O superscript plus upper H subscript 3 upper O superscript plus double-headed arrow upper H subscript 2 upper O plus upper O upper H superscript minus.

Upper H subscript 2 upper O plus upper H subscript 2 upper O double-headed arrow 2 upper O upper H superscript minus.

Upper H subscript 2 upper o plus upper H subscript 2 upper O double headed arrow 2 upper H subscript 3 upper O superscript plus.

Upper H subscript 2 upper O plus upper H subscript 2 upper O double-headed arrow upper H subscript 3 upper O superscript plus, plus upper O upper H superscript minus.

Upper H subscript 2 upper O plus upper H subscript 2 upper O double-headed arrow upper H subscript 3 upper O superscript plus, plus upper O upper H superscript minus.

Explanation:

The self-ionization of water is also known as the autoionization of water or autodissociation of water. It is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H2O, deprotonates that is, it loses one of its hydrogens to become a hydroxide ion, OH−. The hydrogen adds to another water molecule to form the oxonium ion. Water being an amphiprotic substance, can act as an acid by donating a proton to a base to form the hydroxide ion, or as a base by accepting a proton from an acid to form the hydronium ion (H3O+) as typified by its self ionization.

The autoionization of liquid water therefore produces OH− and H3O+ ions. The equilibrium constant for this reaction is called the ion-product constant of liquid water (Kw) and is defined as Kw=[H3O+][OH−].