Explain why potassium iodide conducts electricity when dissolved in water but does not conduct electricity when it is a solid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Potassium iodide is an ionic compound used in some medical treatments. It is a white crystalline solid at room temperature.
Model answer (4 marks)
Potassium iodide is made up of potassium ions (K⁺) and iodide ions (I⁻). In the solid state the ions are held in a fixed lattice and cannot move. When the solid is dissolved in water the ions are released from the lattice and become free to move in solution. Moving ions carry charge, so a current can flow and the solution conducts electricity.
Examiner tips
- Use the word "ions" and specify K⁺ and I⁻
- Explain that ions are fixed in the solid and free in solution
- Show that free ions carry charge and allow current
Common mistakes
- Saying the solid conducts electricity (ignoring ion immobility)
- Forgetting to mention that ions are released into solution
- Using vague terms like "particles" instead of "ions"
Mark scheme (4 marks)
- Potassium iodide is made up of / contains ions (potassium ions and iodide ions)
- In the solid state, the ions are held in fixed positions / cannot move
- When dissolved in water, the ions are free to move / are released from the lattice
- Moving ions carry charge / allow a current to flow, so the solution conducts electricity
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →