The decreasing order of the number of ionisable hydrogen atoms in the following molecules is:
$I$. $H_3PO_4$
$II$. $H_3PO_3$
$III$. $H_3PO_2$
$IV$. $H_4P_2O_6$

Phosphorus has three allotropic forms:
$(i)$ white phosphorus
$(ii)$ red phosphorus and
$(iii)$ black phosphorus.
Write the difference between white and red phosphorus on the basis of their structure and reactivity.

$Cu +$ conc. $HNO_3$ (hot) $\rightarrow Cu(NO_3)_2 + X$ (oxide of nitrogen);
then $X$ is

$A$ metal $(X)$ dissolves in both dilute $HCl$ and dilute $NaOH$ to liberate $H_2$ gas. Addition of $NH_4Cl$ and excess $NH_4OH$ to an $HCl$ solution of $(X)$ produces $(Y)$ as a precipitate. $(Y)$ is also produced by adding $NH_4Cl$ to the $NaOH$ solution of $(X)$. The species $(X)$ and $(Y)$,respectively,are:

The reaction of $PCl_3$ with water produces ..........

The orange solid on heating gives a colourless gas and a green solid which can be reduced to metal by aluminium powder. The orange and the green solids are,respectively