Ionic compounds such as sodium chloride do not conduct electricity when solid, but they do conduct electricity when melted or dissolved in water. Explain why ionic compounds behave in this way.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In the solid state the ions are fixed in a crystal lattice and cannot move. Because the ions cannot move, charge cannot be carried, so solid ionic compounds do not conduct electricity. When the compound is melted or dissolved in water the ions are free to move. The free‑moving ions can carry electrical charge, so the compound conducts electricity.
Examiner tips
- Use the word ‘fixed’ or ‘immobile’ for solid ions
- Explain that charge transport requires ion mobility
- Mention both melting and dissolution as states where ions move
Common mistakes
- Forgetting to state that ions are immobile in the solid
- Confusing electrons with ions as charge carriers
- Using vague terms like ‘free’ without linking to conductivity
Mark scheme (4 marks)
- In the solid state, the ions are fixed in place (in a lattice) and cannot move
- Because the ions cannot move, charge cannot be carried, so solid ionic compounds do not conduct electricity
- When melted or dissolved, the ions are free to move
- The free-moving ions can carry electrical charge, so the compound conducts electricity
Key terms in this question
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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