5.2 describe and explain the extraction of aluminium from purified aluminium oxide by electrolysis, including: i the use of molten cryolite as a solvent and to decrease the required operating temperature ii the need to replace the positive electrodes iii the cost of the electricity as a major factor

Aluminium oxide has a very high melting point and hence it is dissolved in molten cryolite to make the electrolyte. This mixture has a much lower melting point and is also better conductor of electricity than molten aluminium oxide.

The electrodes are made of graphite (carbon) and they need to be replaced regularly because of the hot temperatures and the carbon anodes burn with oxygen to create carbon dioxide. This means that they need to be replaced regularly and this adds to the cost of the extraction.

Large amounts of energy are needed to produce aluminium, this is why using molten cryolite saves quite a lot of money, as it has a reasonably low melting point and acts as a solvent for aluminium oxide.