Question -
Answer -
When a cation of higher valence is added to anionic solid as an impurity to it, the cation of higher valence replaces morethan one cation of lower valence so as to keep the crystal electricallyneutral. As a result, some sites become vacant. For example, when Sr2+ isadded to NaCl, each Sr2+ ion replaces two Na+ ions.However, one Sr2+ ion occupies the site of one Na+ ionand the other site remains vacant. Hence, vacancies are introduced.