Q.KCl crystals develop violet colour when heated in an atmosphere of potassium vapour. Explain.

KCl turns violet when heated in potassium vapour because extra K atoms deposit on the surface, Cl- ions diffuse out to combine with them, leaving behind anion vacancies that trap electrons (F-centres) which absorb light and produce colour.

This is an example of a metal excess defect due to anion vacancies. When crystals of an alkali metal halide like KCl are heated in an atmosphere of the corresponding alkali metal vapour (here, potassium vapour):

1. Potassium atoms deposit on the surface of the crystal.
2. Cl- ions diffuse from the interior of the crystal to the surface and combine with these K atoms to form new KCl, so the K atom loses its electron to become K+ (which occupies a normal lattice site).
3. The electron released diffuses back into the crystal and occupies the anion vacancy that was left behind (since a Cl- ion has effectively moved to the surface without a corresponding cation). …