Barium oxide is an ionic compound. When barium oxide is dissolved in water, the resulting solution conducts electricity. Explain why solid barium oxide does not conduct electricity, but the aqueous solution does.

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 state the Ba²⁺ and O²⁻ ions are arranged in a rigid, repeating lattice; the ions are fixed in position and cannot move.

Because the ions cannot move, charge cannot be carried and no electric current can flow.

When the compound is dissolved in water the lattice breaks apart; the Ba²⁺ and O²⁻ ions dissociate and become free to move throughout the solution.

The free‑moving ions carry charge through the solution, allowing electrical conduction.

Examiner tips

  • Use the word ‘fixed’ or ‘rigid lattice’ to show understanding of solid structure. Explain that movement of ions is required for conduction. Show the change from solid to aqueous state and the resulting mobility of ions. Use the phrase ‘free to move’ or ‘mobile ions’ to link to conduction.

Common mistakes

  • Confusing the role of electrons with ions in conduction. Claiming that the solid conducts because of electrons. Forgetting to mention that the ions dissociate in water.

Mark scheme (4 marks)

  1. In solid barium oxide, the ions are held in fixed positions (in a giant lattice) and cannot move freely
  2. Because ions cannot move in the solid, charge cannot be carried / no electrical current can flow
  3. When dissolved in water, the ions separate / dissociate and are free to move throughout the solution
  4. The free-moving ions carry charge through the solution, allowing electrical conduction

Key terms in this question

ionic compound · ion

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