Explain why electrolysis can occur in a solution of sodium chloride but not in solid sodium chloride.

Cambridge International IGCSE Chemistry (0620) — 4.1 Electrolysis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

In a solution, NaCl dissociates into Na⁺ and Cl⁻ ions, which are free to move.
The ions migrate to the electrodes, carrying charge.
In solid NaCl the ions are fixed in a crystal lattice and cannot move.
Electrolysis needs ion movement to complete the circuit, so it cannot occur in the solid.

Examiner tips

Use the word ‘dissociate’ and mention ions moving to electrodes
Explain that ion movement is required to carry charge
Show the contrast with the solid lattice

Common mistakes

Saying the solid is ‘insoluble’ instead of ‘ions are fixed’
Forgetting to mention that ions must move to carry charge
Using vague terms like ‘electrolyte’ without explaining ion motion

Mark scheme (4 marks)

In solution, the sodium chloride is dissociated / dissolved into ions (Na⁺ and Cl⁻)
These ions are free to move towards the electrodes
In solid sodium chloride, the ions are held in fixed positions in the lattice / cannot move
Electrolysis requires movement of ions to carry charge / complete the circuit; this is not possible in the solid

Key terms in this question

electrolysis