The appearance of colour in solid alkali metal halides is generally due to

If $NaCl$ is doped with $10^{-4} \ mol \%$ of $SrCl_2,$ the concentration of cation vacancies will be $(N_A = 6.02 \times 10^{23} \ mol^{-1}).$

If $NaCl$ is doped with $10^{-3} \ mol \ \%$ $SrCl_2$,then the concentration of cation vacancies will be

Assertion: In an ionic solid $(MX)$,Schottky defect contains an equal number of cations and anions.
Reason: Equal number of vacancies are present for both cations and anions.

As a result of Schottky defect,the density of the crystal:

What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?