Heating $(NH_4)_2Cr_2O_7$ liberates a gas. The same gas is obtained by:

Given below are two statements :
Statement $I$: In group $13$, the stability of $+1$ oxidation state increases down the group.
Statement $II$: The atomic size of gallium is greater than that of aluminium.
In the light of the above statements, choose the most appropriate answer from the options given below:

Why are pentahalides of $P$,$As$,$Sb$,and $Bi$ more covalent than their trihalides?

How many allotropic forms of black phosphorus exist?

Which of the following is used in the industrial production of nitric acid by the Ostwald process?

In the Ostwald's process for the manufacture of $HNO_3$,the catalyst used is