Hoop's process of purification of aluminium involves formation of layers during electrolysis. It involve:

the three layers have same densities but different materials.

the three layers have different densities.

the bottom layer is of impure aluminium which acts as an anode and middle layer consists of cryolite and ${\mathrm{BaF}}_2$

the upper layer is of pure aluminium which acts as a cathode.