Which acid on heating produces phosphine?

How can the brown color of $HNO_3$ be removed?

In case of nitrogen,$NCl_3$ is possible but not $NCl_5$,while in case of phosphorus,$PCl_3$ as well as $PCl_5$ are possible. This is due to:

Two oxides of nitrogen, $NO$ and $NO_2$, react together at $253 \ K$ to form a compound of nitrogen $X$. $X$ reacts with water to yield another compound of nitrogen $Y$. The shape of the anion of $Y$ is:

When $NH_3$ is passed over heated $CuO$,what is it oxidized to?

Rationalize the given statements and provide chemical reactions:
$1$. Lead $(II)$ chloride reacts with $Cl_2$ to give $PbCl_4$.
$2$. Lead $(IV)$ chloride is highly unstable towards heat.
$3$. Lead is known not to form an iodide,$PbI_4$.