Molten – IGCSE Chemistry Definition and Electrolysis Context

“Molten” is not just a vocabulary word — it is a key condition in electrolysis that determines what ions are present and therefore what products form. Confusing molten with aqueous is one of the most common errors on Paper 4 electrolysis questions.

The 0620 definition

Molten describes a substance that has been heated above its melting point and exists as a liquid. A molten substance is a pure liquid — it has not been dissolved in water or any other solvent. The state symbol is (l).

Example: sodium chloride melts at 801 °C. Below this temperature it is NaCl(s). Above it, NaCl(l) — molten sodium chloride.

Why molten matters in electrolysis

Ionic compounds conduct electricity only when their ions are free to move. In the solid state, ions are locked in a fixed lattice and cannot carry charge. Two conditions give ions freedom of movement:

1. Melting the compound — the ionic lattice breaks down and the ions become free to move in the molten liquid.
2. Dissolving the compound in water — the ions separate and move freely in aqueous solution.

The products of electrolysis differ depending on which condition is used:

Electrolysis of molten lead(II) bromide

Only two ions are present: Pb²⁺ and Br⁻.

• Cathode: Pb²⁺ + 2e⁻ → Pb (lead metal forms)
• Anode: 2Br⁻ → Br₂ + 2e⁻ (bromine gas forms)

Electrolysis of aqueous lead(II) bromide

Four ions are present: Pb²⁺, Br⁻, H⁺ (from water), and OH⁻ (from water).

The products may differ because H⁺ and OH⁻ compete with the original ions for discharge. Which ion is discharged depends on position in the electrochemical series and concentration.

This is why Paper 4 questions always specify whether the compound is molten or aqueous — the answer depends on it.

Molten versus liquid versus aqueous

Term	State symbol	Meaning	Example
Solid	(s)	Fixed lattice	NaCl(s) at room temperature
Molten / liquid	(l)	Pure substance heated above melting point	NaCl(l) at 801 °C+
Aqueous	(aq)	Dissolved in water	NaCl(aq) — salt water
Gas	(g)	Particles far apart	HCl(g)

Water itself is H₂O(l) at room temperature — it is a liquid, not molten, because it is already below its melting point in everyday conditions. Technically, “liquid” and “molten” describe the same physical state, but in chemistry “molten” emphasises that a substance normally solid has been heated to become liquid.

Which substances are commonly electrolysed when molten?

Substance	Formula	Melting point	Products
Lead(II) bromide	PbBr₂	371 °C	Lead + bromine
Sodium chloride	NaCl	801 °C	Sodium + chlorine
Aluminium oxide	Al₂O₃	2072 °C	Aluminium + oxygen
Zinc chloride	ZnCl₂	290 °C	Zinc + chlorine

The extraction of aluminium uses molten aluminium oxide dissolved in molten cryolite to lower the operating temperature from over 2000 °C to about 950 °C.

Particle-level explanation

In a molten ionic compound, the ions have enough kinetic energy to overcome the strong electrostatic forces holding them in the lattice. They can now move freely toward the electrodes when a potential difference is applied:

• Positive ions (cations) move toward the negative electrode (cathode).
• Negative ions (anions) move toward the positive electrode (anode).

This movement of ions is what constitutes the flow of electric current through the molten electrolyte.

Common exam mistakes

1. Writing (aq) when the question says “molten”. If the compound is molten, the state symbol is (l), and only the compound’s own ions are present.
2. Including H⁺ and OH⁻ ions in the electrolysis of a molten compound. Water ions are only present in aqueous solutions.
3. Saying ionic compounds “conduct electricity in the solid state”. They do not — the ions are fixed and cannot move.
4. Confusing “molten” with “dissolved”. A molten substance has been heated to its liquid state. A dissolved substance has been mixed with a solvent (usually water).