Aluminium oxide is an ionic compound with a giant lattice structure. It does not conduct electricity as a solid, but it does conduct electricity when it is melted. Explain why aluminium oxide behaves in these two different ways.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.6 Ionic compounds · 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 the solid, the Al³⁺ and O²⁻ ions are fixed in a giant lattice and cannot move.
Because the ions are immobile, no charged particles are free to carry charge, so the solid does not conduct.
When the compound is melted, the electrostatic forces between the ions are overcome and the ions become free.
The free ions carry charge through the molten liquid, allowing electrical conduction.

Examiner tips

  • Use the word ‘fixed’ or ‘immobile’ to show ions cannot move in the solid.
  • Explain that no free charge carriers means no conduction.
  • Show that melting breaks the lattice, freeing ions.
  • Mention that the free ions carry the current in the molten state.

Common mistakes

  • Confusing the role of electrons with ionic conduction.
  • Saying the solid conducts because of electrons instead of ions.
  • Failing to mention that melting breaks the lattice structure.

Mark scheme (4 marks)

  1. In the solid state, the ions are held in fixed positions (within the giant lattice) and cannot move
  2. Because the ions cannot move, there are no charged particles free to carry charge / current
  3. When melted (molten), the strong electrostatic forces between the oppositely charged ions are overcome / broken, so the ions are free to move
  4. The free-moving ions carry charge through the molten liquid, allowing electrical conduction

Key terms in this question

ionic compound · giant lattice

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