Ncert Exercises & Solutions

An excess of potassium ions makes KCl crystals appear violet or lilac in colour because-

a.	Some of the anionic sites are occupied by an unpaired electron
b.	Some of the anionic sites are occupied by a pair of electrons
c.	There are vacancies at some anionic sites
d.	F-centres are created which impart colour to the crystals

The correct choice among the given is-

1. (a, b)

2. (b, c)

3. (c, d)

4. (a, d)

Hint: The colour due to F-centres created in the crystal

When KCl crystals are heated in an atmosphere of potassium vapour it leads to the deposition of

potassium ions on the surface of KCl.

The Cl^- ions diffuse to the surface of the crystal and combine with K atoms to give KCl. The released electrons diffuse into the

crystal and occupy anionic sites. As a result the crystal now has an excess of sodium. The anionic sites occupied by unpaired

electrons are called F-centres (from the German word Farbenzenter for colour centre)