Some molecules or ions such as water are amphoteric [local] or amphiprotic. If water is in combination with an acid, water will act like a base and accept a proton to form the hydronium ion (H3O+); if water is in combination with a base, water can act as an acid to form the hydroxide ion (OH-).
Amphoterism can apply to some hydroxide compounds as well. Oxides of elements that are intermediate in electronegativity may show this property. Stated differently amphoterism is found for a small number of metal oxides or metal hydroxides as the transition of metals that form basic oxides to nometals the form acidic oxides. The following oxides are amphoteric: PbO, Al2O3, Cr2O3, H2O.
Insoluble aluminum hydroxide Al(H2O)3(OH)3 reacts with acid to produce soluble [Al(H2O)6]3+ ions. It also reacts with base to form soluble Al(OH)4- ions.
