amphoteric

Amphoteric describes a substance that can react with both acids and bases. In Bronsted-Lowry terms, an amphoteric compound can behave as an acid (donating a proton) or as a base (accepting a proton) depending on the reaction partner. In many contexts, the term is applied to oxides and hydroxides that dissolve in both acidic and basic solutions. Under the Lewis definition, an amphoteric species can act as either a Lewis acid or a Lewis base, accepting or donating electron pairs.

Water (H2O) is the most familiar example, acting as a base toward strong acids (for example HCl -> Cl- + H3O+) and as an acid toward strong bases (for example NH3 + H2O -> NH4+ + OH-). Metal oxides and hydroxides, such as aluminum oxide Al2O3/Al(OH)3 and zinc oxide ZnO, are classic amphoteric substances. Al2O3 dissolves in acids to give Al3+ and in bases to form aluminate species (for example [Al(OH)4]-). ZnO reacts with acids to form Zn2+ and with bases to form zincate complexes (e.g., [Zn(OH)4]2- or Na2[Zn(OH)4]). Tin oxide SnO2 and other post-transition metal oxides also display amphoteric behavior.

Ambiguity remains for some materials; amphotericity is sometimes distinguished from amphiprotic behavior, which specifically refers to compounds that can both donate and accept protons (a subset of amphoteric behavior). The concept helps explain dissolution, passivation, and catalytic properties of oxides and layered hydrous minerals.