1. A compound is said to be amphoteric if it can behave both as an acid and as a base. For example, with strong acids aluminum hydroxide acts as a base, Al(OH)3, forming aluminum salts. With strong bases it behaves as an acid, H3AlO3, forming salts containing the ion AlO33-. The formation of amphoteric hydroxides is a characteristic of metalloids.
2. Compounds are also classified as amphoteric if they contain both basic and acidic functional groups. See, for example, the amino acid glycine.