Which is used for sterilization of water in the water supply system of cities?

$NaCl$ (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

In chromyl chloride test,the oxidation state of $Cr$ changes from

Although the electron gain enthalpy of fluorine is less negative compared to chlorine, fluorine is a stronger oxidizing agent than chlorine. Why?

Which halogen can be oxidised by concentrated nitric acid?

Bleaching action of chlorine is due to