Copper sulfate (CuSO₄) is an ionic compound. Explain how the bonding and structure of copper sulfate account for the following observations: copper sulfate does not conduct electricity when solid, but does conduct electricity when dissolved in water.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Copper sulfate is a blue crystalline solid that is widely used in agriculture and industry. When dissolved in water it forms a blue solution.
Model answer (5 marks)
Copper sulfate is an ionic compound made up of Cu²⁺ and SO₄²⁻ ions held together in a giant ionic lattice. In the solid state the ions are fixed in a regular arrangement and cannot move.
Because the ions cannot move, charge cannot be carried and the solid does not conduct electricity.
When copper sulfate is dissolved in water the ionic lattice breaks down; the Cu²⁺ and SO₄²⁻ ions separate and become free to move through the solution.
The free‑moving ions in solution carry the electrical charge, allowing the solution to conduct electricity.
Because the ions cannot move, charge cannot be carried and the solid does not conduct electricity.
When copper sulfate is dissolved in water the ionic lattice breaks down; the Cu²⁺ and SO₄²⁻ ions separate and become free to move through the solution.
The free‑moving ions in solution carry the electrical charge, allowing the solution to conduct electricity.
Examiner tips
- Use the word "ionic lattice" and mention fixed ions for the solid. Explain that ion mobility is required for conduction. Show the change from solid to solution and the role of free ions.
- common_mistakes
- :
- Saying the solid conducts because of electrons instead of ions. Forgetting to mention that the lattice breaks down in solution. Using vague terms like "charged particles" without specifying ions.
Mark scheme (5 marks)
- Copper sulfate is made up of positive and negative ions (Cu²⁺ and SO₄²⁻) held in a giant ionic lattice / regular arrangement
- In the solid state the ions are fixed / cannot move
- Because the ions cannot move (in the solid), charge cannot be carried / electricity cannot flow
- When dissolved in water, the ionic lattice breaks down / the ions separate and become free to move through the solution
- The free-moving ions in solution carry the electrical charge, allowing the solution to conduct electricity
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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