Powder of alum is K2(SO4)•Al2(SO4)3•24H2O or KAl(SO4)2•12H2O. Alums are a class of compounds that are formed from potassium and aluminium. They are hydrated double salts or sulphates and contain large amounts of water.
The most widely known alums are potassium aluminum sulphate and ammonium aluminum sulphate. They are soluble in water. Alums are white crystalline solids at room temperature.
Alums have been used since ancient times for purification of water. A paste of alum powder can be used for removing body hair. It is also used as a food preservative. Alum powder is used in fire extinguishers. It is a fire retardant. Hence, it is used to extinguish oil and chemical fires.