Sodium chloride is an ionic compound used to preserve food. Explain why sodium chloride has a high melting point but does not conduct electricity when solid, yet does conduct electricity when melted.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium chloride is made up of sodium ions (Na⁺) and chloride ions (Cl⁻) arranged in a giant lattice structure. It is widely used in the food industry as a preservative.
Model answer (5 marks)
Sodium chloride is held together by strong electrostatic forces between the Na⁺ and Cl⁻ ions, giving it a high melting point because a large amount of energy is required to overcome these forces.
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 melted the ions become free to move, allowing charge to be carried and the molten salt conducts electricity.
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 melted the ions become free to move, allowing charge to be carried and the molten salt conducts electricity.
Examiner tips
- Use the term "electrostatic forces" and "lattice" to show understanding of ionic bonding. Explain that movement of ions is required for conduction. Mention the high melting point as a consequence of strong ionic bonds.
Common mistakes
- Confusing the role of electrons with ion movement. Saying the solid conducts electricity. Omitting the reason for the high melting point.
Mark scheme (5 marks)
- Sodium chloride is held together by strong electrostatic forces (between oppositely charged ions)
- A large amount of energy is needed to overcome these strong electrostatic forces, so the melting point is high
- In the solid state, the ions are held in fixed positions and cannot move
- Therefore charge cannot be carried and electricity cannot be conducted when solid
- When melted, the ions are free to move and carry charge, so the ionic compound conducts electricity
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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