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What happens when aluminium oxide reacts with sodium hydroxide?
Aluminium oxide, known as Al2O3, is amphoteric in that it can be treated as both a base and an acid. It reacts with a strong base to give a salt and water, and with an acid to produce hydrogen gas. The reaction with NaOH is highly exothermic (it gives off more energy than it absorbs).
Bauxite ore, from which aluminium is extracted, contains a mixture of other metal oxides as well as aluminium oxide. These are separated from the molten metal in a process called Bayer's reaction, by treating it with hot concentrated solution of sodium hydroxide. The result is a mixture of various aluminates, such as sodium aluminosilicate and sodium tetrahydroxoaluminate, which are called "red mud". This is stored in huge lagoons and used as the basis for further processing into alumina.
Alumina is a strong, durable metal that resists corrosion by the formation of a thin layer of oxide on its surface. This is strengthened further by anodising, which involves etching the article with sodium hydroxide solution to remove the existing oxide layer and then placing it as the anode in an electrolysis cell filled with dilute sulfuric acid. This produces a layer of oxide up to 0.02 mm thick.
For this reason, pure aluminium is usually alloyed with copper or silicon to improve its strength and hardness. It also expands more than most other metals when heated, so it is often used in the manufacture of electrical conductors.