Potassium bromide is an ionic compound with a giant lattice structure. When potassium bromide is melted, it conducts electricity, but in its solid state it does not. Explain why solid potassium bromide does not conduct electricity, and explain why molten potassium bromide does conduct electricity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Potassium bromide is an ionic compound. It has a high melting point of 734 °C.
Model answer (4 marks)
In solid potassium bromide the ions are fixed in a giant lattice by strong electrostatic forces, so the ions cannot move.
Because the ions cannot move, they cannot carry charge and the solid does not conduct electricity.
When potassium bromide is melted the electrostatic forces between the oppositely charged ions are overcome, and the ions become free to move.
The free‑moving ions carry charge through the liquid, so the molten salt conducts electricity.
Because the ions cannot move, they cannot carry charge and the solid does not conduct electricity.
When potassium bromide is melted the electrostatic forces between the oppositely charged ions are overcome, and the ions become free to move.
The free‑moving ions carry charge through the liquid, so the molten salt conducts electricity.
Examiner tips
- Use the word ‘fixed’ or ‘immobile’ to describe ions in the solid. Mention that movement of ions is required for conduction. Explain that melting breaks the lattice and frees the ions. Show the link between ion mobility and conductivity.
Common mistakes
- Confusing the role of electrons with ion movement. Forgetting to state that ions are immobile in the solid. Saying the solid conducts because of electrons instead of ions.
Mark scheme (4 marks)
- In solid potassium bromide, the ions are held in fixed positions (in the giant lattice) by strong electrostatic forces
- Because the ions cannot move, they cannot carry charge / current through the solid
- When potassium bromide is melted, the strong electrostatic forces between the oppositely charged ions are overcome / broken down, and the ions become free to move
- The free-moving ions carry charge through the liquid, so it conducts electricity
Key terms in this question
ionic compound · giant lattice
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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