Compounds of Aluminium

         (i) Aluminium oxide or Alumina (Al₂O₃) : It occurs in nature as colourless corundum and several coloured minerals like ruby (red), topaz (yellow), Sapphire (blue), amethyst (violet) and emerald (green). These minerals are taken in use as precious stones (gems).

         (ii) Aluminium chloride (Al₂Cl₆) :  It is prepared by passing dry chlorine over aluminium powder.

         It exists as dimer Al₂Cl₆, in inert organic solvents and in vapour state. It sublimes at 100 ⁰C under vacuum. Dimeric structure disappears when AlCl₃ is dissolved in water.  This is hygroscopic in character and absorbs moisture when exposed to air.

         (iii) Thermite : A mixture of aluminium powder and Fe₂O₃ in the ratio 1:3. It is used in the welding of iron. The reaction between Al and Fe₂O₃ is highly exothermic,

         (iv) Aluminium sulphate [Al₂(SO₄)₃] : It is used for the preparation of alums e.g., potash alum Al₂(SO₄)₃.K₂SO₄.24H₂O. It is also used for making fire proof clothes.

         (iv) Alums :  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 Al³⁺, Fe³⁺ and Cr³⁺.

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

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

         (c) Double sulphates of the divalent ions and trivalent ions with 24 water molecules in their crystals are known as Pseudoalums. Common formula is MSO₄.M^'₂(SO₄)₃.24H₂O

         M= Bivalent metal;  M'= Trivalent metal

         (d)  The pseudoalums are not isomorphous with alums.

         (e) Feather alum or the Hair-salt which can be given as Al₂SO₄.18H₂O is native form of the aluminium sulphate.

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

         Some essential alums are

         Potash alum  K₂SO₄.Al₂(SO₄)₃.24H₂O

         Sodium alum  Na₂SO₄.Al₂(SO₄)₃.24H₂O

         Ammonium alum (NH₄)₂SO₄.Al₂(SO₄)₃.24H₂O

       Chrome alum K₂SO₄.Cr₂(SO₄)₃.24H₂O

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