What is the color of $PCl_5$?

$A$ pungent smelling gas $A$ gives dense white fumes with conc. $HCl$. When $A$ reacts with an alkaline solution of a colourless reagent $B$,a reddish-brown precipitate $C$ is formed. The compounds $A, B, C$ respectively are:

Which of the following, when subjected to thermal decomposition, will liberate dinitrogen?
$(i)$ Sodium nitrate
$(ii)$ Ammonium dichromate
$(iii)$ Barium azide

Give the correct characteristic colours of the products $(P, Q, R, S)$ formed in the following reactions:
$(A)\ NO + NO_2 \xrightarrow{< 250 K} P$
$(B)\ 2 NO_2 \longleftrightarrow Q$
$(C)\ Pb(NO_3)_2 \xrightarrow{673 K} R + PbO$
$(D)\ 4 HNO_3 + P_4O_{10} \longrightarrow S + 4 HPO_3$

The number of $P-O-P$ bridges in the structures of phosphorus pentoxide $(P_4O_{10})$ and phosphorus trioxide $(P_4O_6)$ are respectively ....

Nitrogen forms only $NCl_3$,whereas phosphorus forms both $PCl_3$ and $PCl_5$. Why?