Why are solids incompressible?
The distance between the constituent particles is very less in solids. On bringing them still closer repulsion will start between electron clouds of these particles. Hence, they cannot be brought further close together and are incompressible.
Inspite of long range order in the arrangement of particles why are the crystals usually not perfect?
Crystals have long range repeated pattern of arrangement of constituent particles but in the process of crystallisation some deviations from the ideal arrangement (i.e., defects) may be introduced, therefore, crystals are usually not perfect.
Why does table salt, NaCl sometimes appear yellow in colour?
Yellow colour in NaCl is due to metal excess defect due to which unpaired electrons occupy anionic sites, known as F-centres. These electrons absorb energy from the visible region for the excitation which makes crystal appear yellow.
Why is $\mathrm{FeO}(\mathrm{s})$ not formed in stoichiometric composition?
In the crystals of FeO , some of the $\mathrm{Fe}^{2+}$ cations are replaced by $\mathrm{Fe}^{3+}$ ions. Three $\mathrm{Fe}^{2+}$ ions are replaced by two $\mathrm{Fe}^{3+}$ ions to make up the loss of positive charge. Eventually there would be less amount of metal as compared to stoichiometric composition.
Why does white Zn 0 (s) becomes yellow upon heating?
On heating ZnO loses oxygen as follows
$$\mathrm{ZnO} \xrightarrow{\text { Heat }} \mathrm{Zn}^{2+}+\frac{1}{2} \mathrm{O}_2+2 e^{-}$$
$\mathrm{Zn}^{2+}$ ions and electrons move to interstitial sites and F -centres are formed which impart yellow colour to $\mathrm{ZnO}(\mathrm{s})$.