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

When a cation of higher valence is added to an ionic solid as an impurity to it, the cation
of higher valence replaces more than one cation of lower valence so as to keep the
crystal electrically neutral. As a result, some sites become vacant. For example, when
Sr2+ is added to NaCl, each Sr2+ ion replaces two Na+ ions. However, one Sr2+ ion
occupies the site of one Na+ ion and the other site remains vacant. Hence, vacancies are
introduced.