Calcium chloride is an ionic compound. When calcium chloride dissolves in water, it forms an aqueous solution. Explain why solid calcium chloride does not conduct electricity, but an aqueous solution of calcium chloride does conduct electricity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Calcium chloride, CaCl₂, is an ionic compound used to de-ice roads in winter. It dissolves readily in water.
Model answer (4 marks)
In solid CaCl₂ the Ca²⁺ and Cl⁻ ions are arranged in a rigid lattice and cannot move.
Because the ions are fixed, charge cannot be carried and no electric current flows.
When CaCl₂ dissolves, the lattice breaks down and the ions separate into solution.
The free Ca²⁺ and Cl⁻ ions can move, carrying charge and allowing current to flow, so the solution conducts electricity.
Because the ions are fixed, charge cannot be carried and no electric current flows.
When CaCl₂ dissolves, the lattice breaks down and the ions separate into solution.
The free Ca²⁺ and Cl⁻ ions can move, carrying charge and allowing current to flow, so the solution conducts electricity.
Examiner tips
- State that ions are fixed in the solid lattice and cannot move. Explain that movement of ions is required for conduction. Mention that dissolution breaks the lattice, freeing ions. Show the link between free ions and current flow.
Common mistakes
- Forgetting to mention the fixed lattice in the solid. Saying the solution conducts because of water molecules instead of free ions. Using vague terms like "electrons" instead of "ions".
Mark scheme (4 marks)
- In solid calcium chloride, the ions are held in a fixed lattice / the ions cannot move
- Because the ions cannot move, charge cannot be carried / electrical current cannot flow in the solid
- When dissolved in water, the ionic lattice breaks down / the ions separate and become free to move
- The free ions in solution carry electrical charge / allow current to flow, so the solution 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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