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Aluminium oxide is the most common compound of aluminium and is found in the Earth’s crust. It incorporates about 8% oxygen in the form of hydroxyl groups, which is why it has such a high refractive power (it makes clear glasses). It is also resistant to corrosion and fracture under pressure, making it perfect for spectacle frames that need to be put on and taken off frequently. It’s even used by optometrists to make lenses that can withstand frequent exposure to water.
Aluminium is a reactive metal that reacts with many other metals and oxides. The most important reaction is with iron(III) oxide, which produces a dazzling white flame called the thermite reaction. Thermite reactions are extremely exothermic – they release a lot of energy. The only things that are needed for a thermite reaction to occur are a metal, a metal oxide and enough heat. Thermite reactions can also be initiated by magnesium, zinc, titanium, boron and silicon oxides and other metals such as chromium, copper(II) and lead (II).
Aluminum readily reacts with dilute acids to produce aluminum sulfate Al2SO4, aluminum chloride AlCl3 and aluminum nitrate AlNO3. It does not react with concentrated sulfuric or hydrochloric acid. Sodium hydroxide, however, does bind with aluminium to form the solid sodium aluminate, Al(OH)3. Aluminium oxide is insoluble in nitric acid but does dissolve in sulfuric acid, which forms an aluminum sulfate solution, Al2SO4. Exposure to this sulfate solution can cause irritation to lungs and eyes and has been associated with chronic industrial bronchitis known as pneumoconiosis.