The acid-base chemical properties of water
contribute to
the dissolution of solutes that also have acid-base properties. In pure water, about two molecules in every billion react to form a hydroxide ion, OH–(aq), and a hydronium ion, H3O+(aq):
In this reaction, a hydrogen ion is released from a bonding pair of electrons in one molecule and transferred to a non-bonding pair of electrons in the other molecules. Releasing a hydrogen ion in this way is characteristic behavior for an acid, and accepting a hydrogen ion is characteristic behavior of a base. This reaction is reversible, and the hydroxide ion can accept a hydrogen ion from a hydronium ion. In the reverse direction, the hydroxide ion is the base, and the hydronium ion is the acid. In pure water, the number and concentration of hydronium ions is the same as that of the hydroxide ions. This equality means that there is no net acidity or basicity in pure water. Pure water is neutral in the acid-base sense.