Explain why a molten ionic compound conducts electricity but the same compound in its solid state does not.

IB DP Chemistry Standard Level (2023 syllabus) — S2.1 The ionic model · Explain · 4 marks · View as Markdown

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

Ionic compounds such as potassium bromide (KBr) are used in various industrial electrolytic processes, but only when the compound is in the liquid (molten) state or dissolved in water.

Model answer (4 marks)

In the solid state the ions are fixed in a rigid lattice and cannot move.
Because they are immobile there are no charge carriers to carry an electric current.
When the compound melts the lattice breaks down and the ions become free.
The freely moving cations and anions in the molten state act as mobile charge carriers, so an electric current can flow.

Examiner tips

Use the exact terms "fixed in a lattice" and "mobile charge carriers"
Explain the link between ion mobility and conductivity
Keep the answer concise – 4 points, one sentence each

Common mistakes

Confusing the role of electrons with that of ions
Saying the solid conducts because of electrons, ignoring the ionic nature
Omitting the reason why melting disrupts the lattice

Mark scheme (4 marks)

In the solid state, ions are held in fixed positions within the lattice and cannot move freely.
Because the ions cannot move, there is no charge carrier available to carry an electric current in the solid state.
On melting, the regular lattice structure is disrupted/broken down, and the ions become free to move.
The freely moving cations and anions in the molten state act as mobile charge carriers, allowing an electric current to flow.

Key terms in this question

molten