Among the following halogens $F_2$,$Cl_2$,$Br_2$ and $I_2$,which can undergo disproportionation reaction?

The anhydride of the oxoacid $HOCl$ is

With reference to aqua regia,choose the correct option$(s)$.
$(1)$ Reaction of gold with aqua regia produces $NO_2$ in the absence of air.
$(2)$ Aqua regia is prepared by mixing conc. $HCl$ and conc. $HNO_3$ in $3:1$ $(v/v)$ ratio.
$(3)$ Reaction of gold with aqua regia produces an anion having $Au$ in $+3$ oxidation state.
$(4)$ The yellow colour of aqua regia is due to the presence of $NOCl$ and $Cl_2$.

Which is the best reducing agent?

Why are halogens coloured?

$A$ greenish yellow gas reacts with an alkali metal hydroxide to form a halate which can be preferentially used in fire works and safety matches,the gas and halate respectively are