Lithium fluoride is an ionic compound with a giant lattice structure. Explain why lithium fluoride dissolves readily in water yet does not conduct electricity as a solid.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.6 Ionic compounds · Explain · 4 marks · View as Markdown

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

Lithium fluoride is widely used in specialist optical equipment. It is an ionic compound in which lithium ions (Li⁺) and fluoride ions (F⁻) are held together by strong electrostatic forces in a giant lattice structure.

Model answer (4 marks)

In the solid, the Li⁺ and F⁻ ions are fixed in a giant lattice, so they cannot move.

Because the ions cannot move, no charge can be carried and the solid does not conduct electricity.

When the solid is placed in water, the lattice breaks down and the ions separate.

The free Li⁺ and F⁻ ions in solution can move, carrying charge and allowing the solution to conduct electricity.

Examiner tips

Mention the fixed lattice in the solid and the lack of ion mobility. Explain that dissolution breaks the lattice, freeing ions. Show that free ions carry charge in solution.

Common mistakes

Confusing the solid with the solution; forgetting that the solid is non-conductive. Failing to state that the lattice breaks down in water. Using vague terms like "electrons" instead of "ions".

Mark scheme (4 marks)

In the solid state, the ions are held in fixed positions within the giant lattice (and so cannot move)
Without free-moving ions, charge cannot be carried / conducted
When dissolved in water, the giant lattice breaks down / the ions separate and become free to move through the solution
The free ions in solution can carry electrical charge, allowing the solution to conduct electricity

Key terms in this question

ionic compound · giant lattice structure