Liquids and gases have the tendency to flow, i.e., their molecules can move freely from one place to another. Therefore, they are known as fluids. e.g., glass panes fixed to windows or doors of old buildings are sometimes found to be thicker at bottom. This is due to ability of glass to flow.

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.

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.

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.

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.