Alum is a type of chemical compound that is typically a hydrated double sulfate salt of aluminum combined with a monovalent cation such as potassium or ammonium. Its general chemical formula is XAl(SO₄)₂·12H₂O, where X is the monovalent ion. The most common form is potassium alum (KAl(SO₄)₂·12H₂O)

. Alum occurs naturally and has been used historically and across various industries for its astringent, antibacterial, and preservative properties. It is used in water purification, cosmetics (such as aftershave and deodorants), food preservation, and medicinal applications. In cosmetics, alum acts as an astringent that causes skin tissues to shrink and has bacteriostatic effects, helping to reduce bacterial growth

. In water treatment, alum (often referring to aluminum sulfate) helps clarify water by binding phosphorus and reducing algae growth in lakes

. In traditional and household uses, alum is known as phitkari in India and is valued for its antimicrobial and astringent qualities

. In summary, alum is a versatile aluminum-based compound widely used for its chemical and physical properties in industries ranging from water treatment to personal care and food preservation