Aluminum Oxide
Formula: Al₂O₃ — Alumina, corundum
Appearance: White powder (synthetic); colorless crystals in pure form
Hazard: Low hazard · Inhalation risk from fine powder
Properties
Extremely hard (Mohs 9, second only to diamond), high-melting (2072 °C), and chemically inert under ordinary conditions. Amphoteric — dissolves in both strong acids and strong bases, unlike most metal oxides. In acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. In base: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄. Pure alumina is colorless, but trace impurities produce spectacular gem colors: chromium gives ruby (red), iron/titanium give blue sapphire. A thin, tightly adherent layer of Al₂O₃ forms spontaneously on all aluminum surfaces in air, providing excellent corrosion resistance.
Historical Context
Corundum — the mineral form of aluminum oxide — has been used as an abrasive for thousands of years. The ancient Greeks ground corundum from the island of Naxos to polish metal and stone. Ruby and sapphire, both forms of corundum, have been prized gemstones since antiquity.
The industrial production of aluminum depended on solving the alumina problem. Alumina (extracted from the ore bauxite by the Bayer process, patented in 1888) is the feedstock for the Hall-Héroult electrolytic process (also 1886) that produces aluminum metal. Every kilogram of aluminum ever made started as Al₂O₃ dissolved in molten cryolite.
Synthetic rubies, produced by flame fusion of aluminum oxide powder with chromium impurities, were the first commercially produced synthetic gemstones (Verneuil process, 1902). The same technology later enabled synthetic sapphires and, crucially, the synthetic ruby used in the world’s first working laser in 1960.
Experiments
Amphoteric Oxide Demonstration: Prepare two test tubes — one with dilute hydrochloric acid, one with dilute sodium hydroxide. Add a small amount of aluminum oxide powder to each and warm gently. The oxide dissolves slowly in both, illustrating that it behaves as both a basic oxide (in acid) and an acidic oxide (in base). Compare with magnesium oxide, which dissolves only in acid.
Polishing: Use fine aluminum oxide powder (the grade sold for lapidary or metallographic polishing) to polish soft metal surfaces, glass, or acrylic. The hardness of the abrasive cuts the softer substrate; finer grades produce higher polish. This demonstrates the practical importance of hardness differences.
Experiments using this chemical:
- Anodizing Colors — Thickening the natural Al₂O₃ layer electrochemically
Safety
Low hazard — inert solid, but avoid inhaling fine powder.
Fine powder can irritate the respiratory tract; wear a dust mask when handling powdered alumina. Fused or calcined alumina is of lower concern than very fine amorphous forms. Not a significant environmental hazard. Dissolves in strong acids and bases — handle resulting solutions appropriately.
Incompatible with: Chlorine trifluoride and other extremely reactive fluorinating agents; strong reducing conditions at high temperature (thermite reactions, though aluminum metal is the reducing agent in practice, not the oxide)