Explain why potassium iodide (KI) dissolves readily in water but does not conduct electricity as a solid.

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).

Model answer (4 marks)

KI is an ionic compound consisting of K⁺ and I⁻ ions held together by strong electrostatic attractions in a lattice.
Water molecules are polar and attract the ions, breaking the lattice apart so the ions become dispersed and hydrated in solution.
In solution the ions are free to move, allowing charge to be carried – KI solution conducts electricity.
In the solid state the ions are fixed in the lattice and cannot move, so solid KI cannot conduct electricity.

Examiner tips

  • Use the word ‘ionic compound’ and mention the lattice. Explain hydration of ions by polar water. State that free ions in solution give conductivity. Contrast with fixed ions in the solid.
  • common_mistakes
  • :
  • Forgetting to mention the ionic lattice. Saying the solid conducts because of ions, ignoring that they are immobile. Using vague terms like ‘electrons’ instead of ‘ions’.

Mark scheme (4 marks)

  1. KI is an ionic compound consisting of K⁺ and I⁻ ions held together by strong electrostatic attractions in a lattice
  2. Water molecules (polar/with partial charges) attract and surround the ions, breaking the ionic lattice apart — the ions become dispersed/hydrated in solution
  3. In solution the ions are free to move, allowing charge to be carried — KI solution conducts electricity
  4. In the solid state the ions are fixed/held rigidly in the lattice and cannot move, so solid KI cannot conduct electricity

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