Alum is an inorganic compound composed of Water molecules, Aluminum, other Metal than Aluminium, and Sulphates. Alum is a double salt in the hydrated form. Alum exists in different forms: Potash Alum, Soda Alum, ammonium Alum, and chrome Alum. The general chemical formula for Alum is XAl(SO4)2·12H2O.
They are colourless, odourless, and exist as a white crystalline powder. Alums are generally soluble in hot water, and they can be readily precipitated from aqueous solutions to form large octahedral crystals.