Explain with an example how impurity defect develops in `NaCl` crystal.

Explanation

`SrCl_(2)` is added to molten `NaCl` and crystallised. Some `Na^(+)` sites will be occupied by `Sr^(2+)`. Each `Sr^(2+)` replaces two `Na^(+)` ions and thus cationic vacancy is produced for every `Sr^(2+)` ion added.