Alum refers to a class of chemical compounds, specifically hydrated double sulfate salts of aluminum, with the general formula XAl(SO₄)₂·12H₂O, where X is a monovalent cation such as potassium or ammonium. Common examples include potassium alum (KAl(SO₄)₂·12H₂O) and chrome alum (KCr(SO₄)₂·12H₂O).
Historically, alum has been used in dyeing, water treatment, and medicine, while its chemical characteristics—solubility, astringency, and crystalline structure—have been studied extensively. In literature, the term "alum" has also been used metaphorically, as seen in works by Frederick P. Keppel and Sinclair Lewis, who discuss the role of alumni in education and society, emphasizing their impact on institutional growth and professional connections.