Hydrogen ions in aqueous solution generally attach to water molecules to form the hydronium ion . The hydrogen ion necessary to form a hydronium ion may be donated by the water itself. When water donates a hydrogen ion, it is acting as a Bronsted acid. When it accepts the hydrogen ion to form a hydronium ion, it is acting as a Bronsted base. Substances that act as both a Bronsted acid and a Bronsted base (donating and accepting protons) are amphiprotic (or amphoteric). Water can ionize without any outside catalyst. This autoionization reaction proceeds according to
This reaction (and others where the forward and reverse reactions occur simultaneously at equilibrium) has an associated equilibrium constant. For water, a special equilibrium constant is used called . It is related to as follows.
