Why is the bond angle in ${\mathrm{PH}}_3$ is lesser than that in ${\mathrm{NH}}_3$ molecule?
 

Give the correct characteristic colours of the products $(\mathrm{P}, \mathrm{Q}, \mathrm{R}, \mathrm{S})$ formed in the following reactions.

A) $\mathrm{NO}+{\mathrm{NO}}_2\stackrel{<250 \mathrm{K}}{⟶}\mathrm{P}$

B) $2{\mathrm{NO}}_2⟷\mathrm{Q}$

C) $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\stackrel{673 \mathrm{K}}{⟶}\mathrm{R}+\mathrm{PbO}$

D) ${\mathrm{HNO}}_3+{\mathrm{P}}_4{\mathrm{O}}_{10}⟶\mathrm{S}+4{\mathrm{HPO}}_3$