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

 To explain this, let us take an example of NaCl doped with SrCl, impurity when SrCl2 is added to NaCl solid as an impurity, two Na+ ions will then be replaced and one of their site will be occupied by Sr2- while the other will remain vacant. Thus, we can say that when a cation of higher valance is added as an impurity to an ionic soil, two or more cations of lower valency are replaced by a cation of higher valence to maintain electrical neutrality. Hence, some cationic vacancies are created.