Iron(III) chloride (FeCl₃) is an ionic compound. When solid iron(III) chloride is dissolved in water, the resulting solution conducts electricity. Explain, in terms of particles, why solid iron(III) chloride does not conduct electricity but its aqueous solution does.

Edexcel A-Level Chemistry (9CH0) — Topic 1 Foundations in chemistry · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Iron(III) chloride is formed when iron reacts with chlorine. It exists as a white-yellow crystalline solid at room temperature.

Model answer (5 marks)

Solid FeCl₃ is an ionic lattice of Fe³⁺ and Cl⁻ ions held in a fixed arrangement. In the solid the ions are locked in place and cannot move, so no charge carriers are available and the material does not conduct electricity.

When FeCl₃ is dissolved in water the lattice breaks down and the compound dissociates into Fe³⁺ and Cl⁻ ions. These ions are now free to move throughout the solution. The movement of the charged ions carries electrical charge, allowing the aqueous solution to conduct electricity.

Examiner tips

  • Use the word "lattice" and "dissociation" to show understanding of solid vs. solution behaviour.
  • Explain that ions are fixed in the solid and mobile in solution – this is what the mark scheme rewards.
  • Mention the specific ions (Fe³⁺ and Cl⁻) to show particle knowledge.
  • Keep the answer concise – 5 points can be covered in 3–4 short sentences.

Common mistakes

  • Saying the solid conducts because it contains ions – ignores that ions are immobile.
  • Forgetting to mention that the lattice breaks down or that the compound dissociates in water.
  • Using vague terms like "charged particles" without specifying Fe³⁺ and Cl⁻.

Mark scheme (5 marks)

  1. Solid iron(III) chloride contains ions held in a fixed lattice / regular arrangement
  2. Because the ions cannot move (in the solid), electrical charge cannot be carried
  3. When dissolved in water, the ionic lattice breaks down / the compound dissociates
  4. The ions (Fe³⁺ and Cl⁻) are free to move throughout the solution
  5. Moving ions carry electrical charge through the solution, allowing it to conduct

Key terms in this question

ionic compound · conducts electricity · aqueous solution

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