Alum

For alum meaning "graduate," see Alumn.

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Alum, in chemistry, is a term given to the crystallized double sulfates of the typical formula M2SO4·MIII2(SO4)3·24H2O, where M is the sign of an alkali metal (lithium, sodium, potassium, rubidium, caesium, or francium), and MIII denotes one of the trivalent metals (aluminium, chromium, or ferric iron). These salts are employed in dyeing and various other industrial processes. They are soluble in water; have an astringent, acid, and sweetish taste; react acid to litmus; and crystallize in regular octahedra. When heated they liquefy; and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains.

Related Topics:
Chemistry - Sulfate - Lithium - Sodium - Potassium - Rubidium - Caesium - Francium - Aluminium - Chromium - Iron - Salt - Water - Acid - Litmus - Crystal - Octahedra

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Potash alum is the common alum of commerce, although both soda alum and ammonium alum are manufactured.

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