Al2O3 is compound of Al and O. O has high affinity to Al atoms resulting in highly ordered crystalline, polymeric structures and polymorphic phases. Each O atom in the crystal structure shares more than 3 Al atoms results more sharing of the edges of octahedral polyhedra, which not susceptible to penetrations any solvents(except HF and H2SO4), simple hydration or any sorts of corrosion on the Al2O3 surface. That is the reason of inertness and high melting point (2,072°C)of Al2O3. Cryolite is also insoluble in water. However, it is soluble in AlCl3 or H2SO4 solution and evolve HF. Now, if we mix Al2O3 and cryolite in aqueous medium containing AlCl3 or acid, the produced HF can easily react with Al2O3 to form water soluble rhombohedral AlF3, which has relatively lower mp (1290°C) and is acting as an additive and reduce the melting point from 2072°C to 1000°C.

Al2O3 + 6 HF → 2 AlF3 + 3 H2O

It is into this molten salt as a whole, aluminium oxide is dissolved and then electrolyzed to give bulk Al metal.