Breaking Free — The Prison Break of Ions

📖 Breaking Free — The Prison Break of Ions

Imagine ions as prisoners trapped in a crystal jail. In solid ionic compounds, every single ion is locked in a fixed position by powerful electrostatic forces — they can't move, can't escape, can't do anything. Try to pass electricity through? Nothing happens. The ions are frozen in place! But heat them up until they melt... suddenly the prison walls collapse, the ions are FREE to roam, and electricity can flow!

This is the fundamental reason why solid ionic compounds don't conduct electricity but molten (liquid) ionic compounds DO conduct. The ions must be free to move to carry electrical charge from one electrode to the other.

What happens when we electrolyse molten ionic compounds?

Positive ions (cations) are attracted to the negative cathode
At the cathode, they GAIN electrons → This is REDUCTION
The cations become neutral metal atoms → METAL forms at the cathode
Negative ions (anions) are attracted to the positive anode
At the anode, they LOSE electrons → This is OXIDATION
The anions become neutral atoms/molecules → NON-METAL forms at the anode

The beautiful simplicity of molten electrolysis: There are only TWO types of ion present (the metal cation and the non-metal anion), so predicting products is straightforward:

CATHODE: Always the METAL
ANODE: Always the NON-METAL

Example — Molten Lead Bromide (PbBr₂):

Ions present: Pb²⁺ and Br⁻

At cathode: Pb²⁺ + 2e⁻ → Pb (silvery lead metal forms)
At anode: 2Br⁻ → Br₂ + 2e⁻ (orange/brown bromine vapour)

Observations: Silvery liquid metal at cathode, orange/brown fumes at anode

Memory trick for electrodes:
• CATions go to the CAThode (positive ions → negative electrode)
• REDuction at Cathode (both have the letter C in "ReDuCtion"!)
• ANions go to the ANode (negative ions → positive electrode)
• Oxidation at Anode (both are vowels!)

Hotspot diagram

Electrolysis of molten lead bromide — click each part to zoom in and explore.

Electrolysis cell for molten lead bromide showing DC power supply, carbon electrodes, crucible with glowing molten electrolyte on a Bunsen burner tripod. Click each part to explore.

1
DC power supply —
2
Cathode (negative) —
3
Anode (positive) —
4
Molten electrolyte —

Figure 1: Electrolysis of molten lead bromide — click each part to zoom in and explore.

Spotlight

How It Works: Ion Migration in Molten Electrolysis

In a molten ionic compound, the ionic lattice has broken down due to the high temperature. This means the positive metal ions (cations) and negative non-metal ions (anions) are no longer fixed in position — they can move freely through the liquid.

Exam tip

Earn the mark scheme marks

🧠 Memory Aids

CATions go to the CAThode, ANions go to the ANode

Both "CATion" and "CAThode" start with "CAT". Both "ANion" and "ANode" start with "AN". This makes it easy to remember which ion goes where.

OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)

At the Anode = Oxidation (A and O are both vowels!)

At the Cathode = Reduction (both contain the letter C!)

For predicting molten products: "Metal at the Minus, Non-metal at the Plus" — cathode is negative (minus) → metal; anode is positive (plus) → non-metal.

Now try it yourself

Quiz · Question 1 of 21

Which condition is required for electrolysis to occur with an ionic compound?

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This topic in real past papers

Every real exam question we've found on electrolysis of molten compounds, with a full worked answer.

AQA Paper 1

Predict or explain the products formed at each electrode during electrolysis of an aqueous solution, using the rules for which ion is discharged preferentially.

4 marks

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Electrolysis of Aqueous Solutions

Electrolysis of Molten Compounds