In nature, it is found mainly in the form of compounds, the best known ore being bauxite.
The aluminium in compounds always has the valence Al+3. In an acid environment it occurs in the form of aluminium cation; in an alkaline environment as aluminium anion [AlO2].
Aluminium in its pure state is very reactive. However, in the air it quickly becomes covered with a thin layer of aluminium oxide Al2O3 which protects it from further oxidation.
Aluminium has very good solubility in diluted acids. However, a concentrated nitric acid covers it with a passivating film of oxide, as does atmospheric oxygen.
The alkali metal hydroxides also dissolve metal aluminium, creating aluminates (AlO2).
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