Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate (potassium alum) with the formula KAl₂·12. More widely, alums are double sulfate salts, with the general formula , where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium or chromium(III). When the trivalent ion is aluminium, the alum is named after the monovalent ion.