Explain why sodium chloride has a high melting point but does not conduct electricity when solid.

Edexcel GCSE Chemistry (1CH0) — 1.7 Types of substance · Explain · 4 marks · View as Markdown

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

Sodium chloride is an ionic compound formed from sodium ions and chloride ions arranged in a giant lattice structure.

Model answer (4 marks)

Sodium chloride has a giant ionic lattice structure.
The strong electrostatic forces of attraction between the oppositely charged Na⁺ and Cl⁻ ions require a large amount of energy to break, giving it a high melting point.
In the solid state the ions are held in fixed positions and cannot move freely.
Electrical conductivity requires charged particles to move, so solid sodium chloride does not conduct electricity.

Examiner tips

  • Use the word ‘ionic lattice’ and ‘electrostatic forces’ to show understanding of structure. Explain that high melting point comes from the energy needed to break the lattice. Mention that ions are fixed in the solid, so no charge carriers. Link the lack of ion mobility directly to no conductivity.

Common mistakes

  • Forgetting to state that ions are fixed in the solid. Using ‘covalent bonds’ instead of ‘ionic lattice’. Saying the compound conducts electricity in the solid state.

Mark scheme (4 marks)

  1. Sodium chloride has a giant ionic lattice structure
  2. There are strong electrostatic forces of attraction between oppositely charged ions that require a large amount of energy to break, giving a high melting point
  3. In the solid state the ions are held in fixed positions / cannot move freely
  4. Electrical conductivity requires charged particles to move, so solid sodium chloride does not conduct electricity

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