When crystals of KCl are heated in an atmosphere of Potassium vapours the colour imparted is _______ and it is due to __________ defect

In such cases, Cl-  ions diffuse to the surface;To combine with these Cl-  ions the Potassium atoms of the vapour  lose electrons to form K+ and thus  form KCl. The electrons lost by Potassium atoms occupy anionic sites.This crystal now has anion vacancies. It is a metal excess defect. The anionic sites occupied by  unpaired electrons are called F-centres. They impart lilac colour to the crystal.