Aluminium compounds and uses
Al₂O₃ is a white, very hard, hard-to-melt substance; in nature it is the mineral corundum. It is amphoteric: it reacts with both acid and alkali: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O and Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]. Al(OH)₃ is insoluble in water; we make it by adding alkali to a solution of an aluminium salt: AlCl₃ + 3NaOH → Al(OH)₃↓ + 3NaCl (a white gelatinous precipitate). With an excess of alkali the precipitate dissolves: Al(OH)₃ + NaOH → Na[Al(OH)₄], and with acid it gives a salt: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O. On heating the hydroxide decomposes: 2Al(OH)₃ → Al₂O₃ + 3H₂O. Alum KAl(SO₄)₂·12H₂O is used in dyeing fabrics, tanning leather and purifying water; corundum with Cr³⁺ impurity is ruby, with Fe and Ti ions it is sapphire.
Teacher demonstration: first a little and then excess NaOH is added to AlCl₃ solution and the formation and dissolving of the precipitate is observed. The teacher does it in goggles and gloves; do not work with alkali at home. You write the “precipitate” and “solution” stages in your notebook with equations.