Explain why sodium chloride has a high melting point but does not conduct electricity when solid. Include in your answer what happens when sodium chloride is melted.

AQA A-Level Chemistry (7405) — 3.1.3 Bonding · Explain · 5 marks · View as Markdown

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

Sodium chloride is an ionic compound made up of sodium ions and chloride ions arranged in a giant ionic lattice.

Model answer (5 marks)

Sodium chloride has a giant ionic lattice structure, so there are strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is required to overcome these forces, giving a high melting point. In the solid state the ions are held in fixed positions and cannot move, so it does not conduct electricity. When melted the lattice breaks down, the ions become free to move and carry charge, allowing the molten salt to conduct electricity.

Examiner tips

  • Use the term ‘giant ionic lattice’ early to show understanding of structure.
  • Explain that the high melting point is due to strong electrostatic forces.
  • State that in the solid the ions are fixed, hence no conduction.
  • Mention that melting frees the ions, enabling conduction.

Common mistakes

  • Failing to mention the fixed positions of ions in the solid.
  • Confusing the melting point with the boiling point.
  • Using vague terms like ‘strong bonds’ without specifying electrostatic attraction.

Mark scheme (5 marks)

  1. Sodium chloride has a giant ionic lattice structure
  2. There are strong electrostatic forces of attraction between oppositely charged ions
  3. A large amount of energy is needed to overcome these forces, so the melting point is high
  4. When solid, the ions are held in fixed positions and cannot move
  5. When melted, the ions are free to move and carry charge, so it conducts electricity

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