Alum, a chemical compound of two sulfates and water. (A sulfate is a compound of sulfur, oxygen, and one or more other elements.) One of the two sulfates usually contains aluminum, but it can be any other metal with a valence of 3, such as chromium or iron. The other sulfate usually contains an alkali metal, such as potassium, but can instead contain either another metal with a valence of 1 or ammonium (a group of atoms formed by combining ammonia and hydrogen). At ordinary temperatures, all alums are solids.

The most important commercial alums are potassium alum and ammonium alum. Other alums include sodium alum (soda alum), chrome alum, rubidium alum, and cesium alum. Potassium and ammonium alum are used in tanning leather, in the manufacture of paper, and in dye making. They are also ingredients of some baking powders. Certain types of fire extinguishers contain alum. Alums are also used in the purification of water and in the treatment of sewage. Alum is sometimes used to produce hardness and crispness in pickles (but too much alum will make the pickles bitter). It has also been used as a preservative.

Alum is an emetic (an agent that causes vomiting) and is used for this purpose to treat certain types of poisoning. However, large quantities of alum taken internally are poisonous. Alum is also used as an astringent (a substance that causes shrinkage of tissue).

The general formula for alum is M2(SO4)3 X2SO424H2O, where M is aluminum or some other trivalent metal, and X is an alkali metal, some other univalent metal, or ammonium. Thus, potassium alum is Al2(SO4)3K2SO424H2O; ammonium alum is Al2(SO4)3(NH4)2SO424H2O.