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 AlO₃^3-. 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.