CBSE Class 10 Answered

Extraction of Aluminium - Electrolysis Cell.

                 
The steel container is coated with carbon (graphite)
and this is used as the negative electrode (cathode).

Aluminium oxide (Al₂O₃) is an ionic compound.
When it is melted the Al³⁺ and O^2- ions are free to move
and conduct electricity.
Electrolysis of the alumina/cryolite solution
gives aluminium at the cathode and oxygen at the anode.

4Al³⁺  +  12e^-       4Al   (aluminium metal at the (-)cathode) reduction.
6O^2-  -  12e^-       3O₂   (oxygen gas at the (+)anode) oxidation.

Aluminium is more dense than the alumina/cryolite solution
and so it falls to the bottom of the cell
where it can be tapped off as pure liquid metal.

The overall reaction is

aluminium oxide       aluminium + oxygen.
2Al₂O_3(l)                 4Al_(l)   +     3O_2(g)

Oxygen is given off at the positive carbon anode.
Carbon dioxide is also given off at the carbon anode because
hot oxygen reacts with the carbon anode to form carbon dioxide gas.
carbon + oxygen    carbon dioxide.
C_(s)  +    O_2(g)             CO_2(g)

The carbon anodes slowly disappear
because each molecule of carbon dioxide which is given off
takes a little piece of carbon away with it.
The carbon anodes need to be replaced when they become too small.