Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.

Explanation

Let us condiser an ionic solid sodium chloride `(Na^(+)Cl^(-))` to which a small amount of strontium chloride `(SrCl_(2))` has been added to act as impurity. Since the crystal as a whole is to remain electrically neutral, two `Na^(+)` ions have to leave their sites to create two vacancies. Out of these, one will occupied by `Sr^(2+)` ion while the other will be valcant. Thus, vacancies will be created in the ionic solid.