When melted or dissolved in water, ionic compounds conduct electricity…

The idea

Ionic compounds form giant lattices of alternating positive and negative ions held by strong electrostatic attraction. In the solid state these ions vibrate about fixed positions, so although each ion carries charge, no ion can travel through the lattice and no current flows. Melting supplies enough energy to overcome the attractions, releasing the ions so they can slide past one another. Dissolving in water has a similar effect: water molecules pull ions away from the lattice and surround them, forming a solution in which the ions are mobile. In both cases the charged particles themselves move, so charge can flow and the compound conducts electricity.

For example: Solid sodium chloride does not light a bulb, but molten sodium chloride and sodium chloride solution both conduct because Na⁺ and Cl⁻ ions are free to move.

What earns the marks

• Solid ionic compounds do not conduct because their ions are held in fixed positions in the giant lattice and cannot move through the structure.
• Melting provides energy to overcome the strong electrostatic attractions between oppositely charged ions, so the ions become free to move.
• When an ionic compound dissolves, water molecules separate the ions from the lattice and surround them, so the ions are free to move in solution.
• Conduction occurs because the moving ions carry charge through the liquid or solution, completing the circuit.
• The charge carriers in an ionic compound are the ions themselves, not electrons as in a metal.

Common mistakes and how to put them right

• Saying that solid ionic compounds conduct because ions are charged.

  Put it right: Charge alone is not enough; the ions must be free to move, and in a solid lattice they are not.

• Stating that electrons flow through molten or dissolved ionic compounds.

  Put it right: The mobile charge carriers are ions, so the current is carried by moving positive and negative ions.

• Claiming that melting destroys the ions or that dissolving breaks ions into atoms.

  Put it right: The ions remain intact; melting and dissolving only free them from the lattice so they can move.

Exam tips

• Link the state of the substance to the mobility of its ions in every explanation: solid means fixed ions, molten or aqueous means mobile ions.
• Use the phrase free to move and so charge can flow when explaining conduction, and name the charge carriers as ions.
• Compare a solid sample with the same compound when molten or dissolved to show that the difference is ion mobility, not a change in the ions themselves.