Alum is a chemical compound. Alum powder may be used in pickling, to keep the fruits and vegetables crisp. It is also used as an acidic compound in some baking powders. Alum are double sulphate salst which are soluble in water. It is most commonly used for the purification of water. It has antiperspirant and anti bacterial properties therefore it is commonly used in deodorants. It also acts as a spice and is available in several grocery stores.