Sodium chloride is an ionic compound. Explain why sodium chloride has a high melting point and does not conduct electricity when solid, but does conduct electricity when molten.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium chloride is formed when sodium, a metal, reacts with chlorine, a non-metal. The resulting compound is used widely in food preservation and the chemical industry.
Model answer (5 marks)
Sodium chloride is held together by strong electrostatic forces between the oppositely charged Na⁺ and Cl⁻ ions.
These forces are very strong, so a large amount of energy is required to break them apart – this gives NaCl a high melting point.
In the solid state the ions are fixed in a crystal lattice and cannot move.
Because charge cannot be carried through the solid, it does not conduct electricity.
When molten, the ions are free to move and can carry charge, so it conducts electricity.
These forces are very strong, so a large amount of energy is required to break them apart – this gives NaCl a high melting point.
In the solid state the ions are fixed in a crystal lattice and cannot move.
Because charge cannot be carried through the solid, it does not conduct electricity.
When molten, the ions are free to move and can carry charge, so it conducts electricity.
Examiner tips
- Use the term ‘electrostatic forces’ or ‘ionic bonds’ to show understanding of the lattice structure.
- Explain that the high melting point is due to the energy needed to overcome these forces.
- Mention that in the solid the ions are fixed, so no charge transport occurs.
- State that in the molten state ions are mobile, allowing conductivity.
Common mistakes
- Saying the compound is covalent instead of ionic.
- Claiming the melting point is low or that it conducts in the solid state.
- Omitting the reason for lack of conductivity in the solid (fixed ions).
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
- When solid, the ions are held in fixed positions (in a lattice) and cannot move
- Therefore, charge cannot be carried through the solid, so it does not conduct electricity
- When molten, the ions are free to move and can carry charge, so it 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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