$NaCl$ (solid) $+ K_2Cr_2O_7$ (solid) $+$ conc. $H_2SO_4 \xrightarrow{\text{warm}} X$. Reddish brown fumes of $X$. The oxidation state of central atom in compound $X$ is

$K_2Cr_2O_7 +$ conc. $H_2SO_4 + NaCl \rightarrow$ red orange vapours. Here red orange vapours are of :

Identify the weakest halogen acid from the following.

The reaction of fluorine with dilute $NaOH$ and concentrated $NaOH$ produces,respectively,........

$Br_2 + NaOH \xrightarrow{R.T.} Y + Z$
If $Y$ gives a precipitate with $AgNO_3$,then $Z$ does not undergo a reaction with:

The colour of the $X_2$ molecules of group $17$ elements changes gradually from yellow to violet down the group. This is due to
[$A$] the physical state of $X_2$ at room temperature changes from gas to solid down the group
[$B$] decrease in $HOMO-LUMO$ gap down the group
[$C$] decrease in $\pi^*-\sigma^*$ gap down the group
[$D$] decrease in ionization energy down the group