Molten Magic: Extract Aluminium
Electrolyze alumina dissolved in cryolite to get pure aluminium.
What this lesson covers
Why it matters
Aluminium clings to its oxygen so tightly that no chemical can pull it free — only electricity can. Pure alumina melts above 2000 °C, but dissolving it in molten cryolite drops that to about 950 °C, so a current can split it into aluminium and oxygen.
The idea in plain words
Balance the cell reaction: Al³⁺ ions gain electrons at the cathode while O²⁻ ions give them up at the anode — 2Al₂O₃ splits into aluminium and oxygen.
Predict first
Why do we dissolve alumina in molten cryolite instead of melting pure alumina?
Pure alumina has a very high melting point. Cryolite lowers it significantly, making the process economically viable.
- Cryolite acts as a catalyst to speed up the reaction
- To lower the melting point and save energy — correct
- Alumina does not conduct electricity in any state
What you do
Switch on the current. The molten alumina (dissolved in cryolite) splits — aluminium forms at the cathode and oxygen at the anode.
Check yourself
In the Hall–Héroult cell, what forms at the cathode (the negative electrode)?
Al³⁺ ions move to the cathode and gain three electrons each (Al³⁺ + 3e⁻ → Al), forming molten aluminium that sinks to the bottom and is tapped off.
Why is aluminium extracted by electrolysis rather than by heating its oxide with carbon?
Aluminium sits above carbon in the reactivity series, so carbon cannot reduce Al₂O₃. Only an electric current can pull the aluminium out.
What is the balanced equation for the electrolysis of alumina?
Two units of Al₂O₃ give 4 Al atoms and 6 O atoms → 4Al + 3O₂.
- Molten aluminium — correct
- Oxygen gas
- Carbon dioxide
- The alumina stays unchanged
- It is too reactive to be displaced from its oxide by carbon — correct
- It is less reactive than carbon
- Its ore cannot be obtained as a solid
- 2Al₂O₃ → 4Al + 3O₂ — correct
- Al₂O₃ → 2Al + O₂
- Al₂O₃ → Al₂ + 3O