Explain why ionic compounds such as sodium chloride have high melting points but do not conduct electricity when solid, yet do conduct electricity when melted.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium chloride is an ionic compound used widely in food preservation and the chemical industry. Its electrical behaviour changes dramatically depending on its physical state.
Model answer (4 marks)
Ionic compounds are held together by strong electrostatic forces between oppositely charged ions. A large amount of energy is required to overcome these forces, giving ionic compounds high melting points. In the solid state the ions are fixed in a lattice and cannot move, so charge cannot be carried and the solid does not conduct electricity. When the compound is melted the lattice breaks down, the ions become free to move and can carry charge, allowing electrical conduction.
Examiner tips
- Use the phrase "strong electrostatic forces" and "fixed positions" to hit the key points.
- Show the link between energy required to break the lattice and the high melting point.
- Explain that conduction requires mobile charge carriers.
- Keep the answer concise – 4 marks, so one sentence per point.
Common mistakes
- Confusing the reason for high melting point with the reason for non-conduction in the solid.
- Saying the solid conducts because ions are present, ignoring that they are immobile.
- Using vague terms like "strong bonds" without specifying electrostatic forces.
Mark scheme (4 marks)
- Ionic compounds are held together by strong electrostatic forces (between oppositely charged ions)
- A large amount of energy is needed to overcome these strong electrostatic forces, hence the high melting point
- In the solid state the ions are held in fixed positions and cannot move, so charge cannot be carried
- When melted the ions are free to move and can carry charge, allowing electrical conduction
Key terms in this question
ionic compound · ion · melting point
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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