Cryolite(Sodium aluminium fiuoride)

In industry, aluminium is extracted by electrolysis of molten aluminium oxide.

Cryolite (sodium aluminium fluoride) is added to lower the melting point of the aluminium oxide. Actually, any impurity added would lower the melting point of aluminium oxide. Think conversely. If a solid crystal of aluminium oxide contains an impurity, the crystalline structure is not completely regular as the particles of the impurity are present. Due to the irregularity, the solid lattice would be broken down at a temperature lower than the normal melting point of aluminium oxide. In other words, the melting point of aluminium oxide is lowered on addition of an impurity.

Cryolite (sodium aluminium fluoride) is chosen as the impurity as this ionic impurity would not affect the nature of products (i.e. aluminium and oxygen) in electrolysis.
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