When Charles Martin Hall was 17, working in a woodshed in Ohio, he discovered the commercial method for producing aluminium metal by the electrolysis of a mixture of bauxite, $\text{Al}_2\text{O}_3$, and cryolite, $\text{Na}_3\text{AlF}_6$. What is the chief purpose of the cryolite?
- A$\text{Al}_2\text{O}_3$ is covalent, and $\text{AlF}_6^{3-}$ ions interact with it to produce $\text{Al}^{3+}$ ions which can be discharged at the cathode.
- BCryolite is a base, forming $\text{NaAlO}_2$ with bauxite, enabling aluminium to be discharged at the anode.
- CCryolite minimises the release of $\text{O}^{2-}$ ions at the graphite anodes, which are otherwise burnt away to CO.
- DCryolite reduces the melting point of the bauxite.