Phosphorus pentoxide $(P_4O_{10})$ is used as a ........ .

$NH_3 + O_2 \xrightarrow[\Delta]{Pt} A + H_2O$ ;
$A + O_2 \to B$ ;
$B + O_2 + H_2O \to C$
$A, B$ and $C$ are:

Why does the reactivity of nitrogen differ from phosphorus?

$A$ colourless,neutral,paramagnetic oxide of nitrogen '$P$' on oxidation gives reddish brown gas $Q$. $Q$ on cooling gives colourless gas $R$. $R$ on reaction with $P$ gives blue solid $S$. Identify $P, Q, R, S$ respectively.

Which of the following compounds does not produce ammonia upon heating?

Correct statements from the following are: $A$. Nitrogen in oxidation states from $+1$ to $+4$ disproportionates in acid medium. $B$. Nitrogen has the ability to form $d\text{--}p$ multiple bonds with itself and other elements with small size and high electronegativity. $C$. $N$-$N$ single bond is stronger than $P$-$P$ single bond. $D$. Nitrogen has highest density in its group due to small size. $E$. The maximum covalency of nitrogen is four since it has only four valence orbitals for bonding.