Alum

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Alum, any of a group of chemical compounds, made up of water molecules and two kinds of salts, one of which is usually aluminum sulfate combined in definite proportions. Potassium alum, also known as common alum, is the most important type of alum.

Potassium alum is a colorless substance that forms large octahedral or cubic crystals when potassium sulfate and aluminum sulfate are dissolved together and the solution is cooled. The solutions of potassium alum are acidic.

Potassium alum is soluble in seven times its weight of water at room temperature and is very soluble in hot water. When crystalline potassium alum is heated, some of the water of hydration becomes chemically separated, and the partly dehydrated salt dissolves in this water, so that the alum appears to melt at about 90°C (about 194°F). When heated to about 200°C (about 392°F), potassium alum swells up, loses all the water and some sulfur trioxide, and becomes a basic salt called burnt alum. Potassium alum has a density of 1.725.

Other types of alums made with aluminum sulfate include sodium alum, ammonium alum, and silver alum. Alums are used for flameproofing textiles and in baking powders, mordants for delicate dyeing operations, and medicines. Potassium alum is a powerful

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