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Electrolysis is the process of using electricity to split compounds. For example, it is used to extract metals from their oxides or to release gases such as carbon dioxide.
Aluminium is extracted from bauxite, an ore which contains aluminium oxide. The aluminium oxide is melted and used as an electrolyte to make the electricity work.
This is done in a cell that has two electrodes called the cathode and the anode. The cathode is made of graphite, a form of carbon with a high melting point and which conducts electricity. The aluminium oxide melts and sinks to the bottom of the cell where it is tapped off and collected.
The oxygen is produced at the cathode, gaining 3 electrons. This forms neutral aluminium atoms, which are then drained off as liquid aluminium.
During the electrolysis, oxygen is also formed. This is a by-product of the reaction that breaks the bonds between the atoms of the aluminium ions. The oxygen ions are then attracted to the anode, where they lose 2 electrons.
At the anode, this oxygen combines with the carbon of the graphite to form carbon dioxide, which burns away and adds to the cost of the process. This is why the anode needs to be replaced regularly.
To reduce the energy needed to extract aluminium, the powdered bauxite ore is dissolved in molten cryolite - an ionic compound that has a lower melting point than aluminium oxide. This enables the aluminium to be extracted at a lower temperature and saves money. However, the ionic compound requires energy to melt, so it still has to be kept molten for the electricity to work.