Describe the role played in the extraction of aluminium by each of the substances listed.

1. Cryolite
2. Sodium hydroxide
3. Graphite

1. Addition of Cryolite :
   (i) lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
   (ii) enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
   (iii) enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.
2. Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of NaOH under pressure for 2 hrs as a first step in obtaining Al₂O₃. The impurities present in bauxite mainly Fe₂O₃ and SiO₂ remain unaffected with conc. NaOH as impurities are not amphoteric. Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.
3. Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode.