In general, the term alum is given to double sulphates of the type M₂SO₄.M'₂ ( SO4 )3 . 24H₂O where M is a univalent cation like Na⁺, K⁺ and NH₄⁺, M' is a trivalent cation like AI³⁺, Fe³⁺ and Cr³⁺ .

Some important points to be noted about the alums are

(a) General formula is M₂SO₄ .M'₂ ( SO₄ )₃ . 24H₂O

M = Monovalent metal; M' = Trivalent metal

In alum crystals, 6 water molecules are held by monovalent ion, 6 water molecules are held by trivalent ion, 12 water molecules are held in the crystal structure.

(b) All alums are isomorphous. Aqueous solutions of alums are acidic due to cationic hydrolysis of trivalent cation.

(c) Double sulphates of divalent ions and trivalent ions with 24 water molecules in their crystals are known as Pseudoalums. General formula is MSO₄ .M'₂ ( SO₄ )₃ .24H₂O

M = Bivalent metal; M' = Trivalent metal

(d) Pseudoalums are not isomorphous with alums.

(e) Feather alum or ‘Hair-salt’ AI₂SO₄ .18H₂O is a native form of aluminium sulphate.

(f) Potash alum is used for tanning of leather, as mordant in dyeing and calico printing, for sizing paper, as a syptic to stop bleeding and purification of water.

Some important alums are

Potash alum K₂SO₄ .AI₂ (SO₄)₃ .24H₂O

Sodium alum Na₂SO₄ .AI₂ ( SO₄ )₃ .24H₂O

Ammonium alum (NH₄ ) SO₄ .AI₂ ( SO₄ )₃ .24H₂O

Chrome alum K2SO4 .Cr2 ( SO₄ )3 .24H₂O