Sodium chloride is an ionic compound. Explain why solid sodium chloride does not conduct electricity, but sodium chloride dissolved in water does conduct electricity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium chloride is widely used in the food industry and in industrial chemical processes. It is composed of sodium ions and chloride ions held together by strong electrostatic forces.
Model answer (5 marks)
In the solid state the Na⁺ and Cl⁻ ions are fixed in a crystal lattice by strong electrostatic forces, so the ions cannot move. Because charge carriers cannot move, no current can flow and the solid does not conduct electricity.
When NaCl dissolves in water the lattice is broken down; the Na⁺ and Cl⁻ ions become surrounded by water molecules and are free to move. These free ions carry charge through the solution, allowing an electric current to flow.
When NaCl dissolves in water the lattice is broken down; the Na⁺ and Cl⁻ ions become surrounded by water molecules and are free to move. These free ions carry charge through the solution, allowing an electric current to flow.
Examiner tips
- Mention the crystal lattice and immobility of ions in the solid
- Explain that dissolution breaks the lattice and frees the ions
- Show that free ions are the charge carriers in solution
Common mistakes
- Saying the solid conducts because of ions, ignoring immobility
- Forgetting that dissolution breaks the electrostatic lattice
- Confusing the role of water as merely a solvent, not a participant in ion separation
Mark scheme (5 marks)
- In solid sodium chloride, the ions are held in fixed positions (in a lattice) by strong electrostatic forces
- Because the ions cannot move, no charge can be carried / no current can flow
- When sodium chloride dissolves in water, the ions separate / become free to move
- The free-moving ions can carry charge through the solution
- The strong electrostatic forces between oppositely charged ions are overcome / broken down when the compound dissolves
Key terms in this question
Related
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