hydroxonium ion

In chemistry, additional hydrogen ion or proton formed by the reaction of acids with water. It is the formation of hydrogen ions that give acids their characteristic properties. Their reaction in water is complex; acids do not just split apart, or dissociate, they actually react with the water present. Water acts as a base, which is a substance that can accept protons. The resulting additional hydrogen ion in the water (as H30+) is a hydroxonium ion.

Hydrochloric acid dissociates as follows:

HCl(aq) → H+(aq) + Cl−(aq)

Water added to the equation gives:

HCl(aq) + H2O(l) → H3O+(aq) + Cl−(aq)

The H3O+ is the hydroxonium ion.