Ionic compounds such as sodium chloride have high melting points and conduct electricity when molten but not when solid. Explain these two properties in terms of the structure and bonding of ionic compounds.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is required to overcome these forces, giving the compound a high melting point. In the solid state the ions are fixed in a regular lattice and cannot move freely, so the material does not conduct electricity. When the compound is melted the lattice breaks down, the ions become mobile and carry charge, allowing the molten substance to conduct electricity.
Examiner tips
- Use the term ‘electrostatic forces of attraction’ to show understanding of bonding.
- Explain that the high melting point is due to the energy needed to break these forces.
- Mention the fixed lattice in the solid and the mobility of ions in the molten state.
- Show the link between ion mobility and electrical conductivity.
Common mistakes
- Confusing covalent with ionic bonding; forgetting to mention electrostatic forces.
- Saying the compound conducts in the solid state; ignoring the lattice restriction.
- Using vague terms like ‘strong bonds’ without specifying electrostatic attraction.
Mark scheme (4 marks)
- Ionic compounds contain strong electrostatic forces (of attraction) 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 a fixed lattice / are not free to move
- When molten, the ions are free to move (and carry charge), so the compound conducts electricity
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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