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Sodium hydroxide reacts with aluminium oxide to produce sodium aluminate, NaAlO2. This is a compound that, like all pure aluminium compounds, is a white solid at room temperature. However, when hydrated, it forms a variety of different species. These are all known as aluminates and are usually referred to by their formulas: NaAlO2NaAlOX2, NaAl(OH)4(NO3)2, NaAl(OH)3(NO3)4 and NaAl(OH)2[n]2[n]3. The names used depend on the particular aluminate produced, for example the hydrate of aluminium oxide with the lowest molecular weight is commonly known as sodium tetrahydroxoaluminate.
The reaction between aluminium oxide and aqueous sodium hydroxide is an example of an acid-base reaction. It is also an amphoteric reaction because aluminium has both basic and acidic sides to its character. This is why it can react with acids such as hot dilute hydrochloric acid to give aluminium chloride solution – the basic side of its character – and with bases such as sodium hydroxide to make different aluminates – the acidic side of its character.
Aluminium oxide is a well-known industrial compound that has many uses, including the production of aluminium metal. A key process in its production is the ‘anodizing’ of aluminium articles. This involves etching the aluminum with sodium hydroxide solution to remove its existing oxide layer and making it the anode in an electrolysis of dilute sulphuric acid to generate oxygen gas, which reacts with the remaining oxide on the aluminum surface to build up a new layer of aluminium oxide up to 0.02 mm thick. This is an acid-reactive material that can be made porous so that it will readily take up dyes to colour the finished article.