question_answer 94)

  An excess of potassium ions makes KCl crystals appear violet or lilac in colour since........... (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

Answer:

                  (a, d) When KCl crystals are heated it leads the diposition of potassium ion on surface of KCl. The CI- ions diffuse to the surface of crystal and loss electron by potassium atom to form K'1' ion released electron occupies anionic site which is known as F- center and impart colour to the crystal.