Potassium aluminum sulfate dodecahydrate, also known as alum, alum, potassium alum, potassium aluminum alum and potassium alum, is a double salt of potassium sulfate and aluminum sulfate containing crystal water.

Colorless cubic crystals are usually octahedral in appearance, or form polygons with cubes and rhombic dodecahedrons.

Alum is a colorless and transparent block crystal or crystalline powder, odorless, slightly sweet and sour.

Weathering in dry air loses crystal water, melting water in humid air, heating to 92.5℃ loses nine crystal water, and losing all crystal water at 200℃ becomes white powder.

Soluble in water, slowly soluble in glycerol, insoluble in ethanol and acetone. Chemical properties: its aqueous solution is acidic, and when it is hydrolyzed with water, it generates colloidal precipitation of aluminum hydroxide.