Alums are classified as double salts. Double salts are compounds that contain two different cations and one anion, held together by ionic bonds. In the case of alums, they are double salts of a monovalent cation (such as potassium or ammonium), a trivalent cation (such as aluminum or chromium), and a sulfate anion. The general formula for alums is M^+M^3+(SO4)2·12H2O, where M^+ is a monovalent cation and M^3+ is a trivalent cation. Alums are commonly used as mordants in dyeing and in various industrial processes.