JEE Main 2015 — p Block Elements (Group 15, 16, 17 & 18) Question with Solution

Chlorine water loses it's yellow color on standing in sunlight because it forms mixture of hypochlorous acid $\left(\mathrm{HOCl}\right)$  and hydrochloric acid $\left(\mathrm{HCl}\right)$.

$\mathrm{C}{\mathrm{l}}_2\left(\mathrm{g}\right)$ Is soluble in ${\mathrm{H}}_2\mathrm{O}$ and form chlorine water, in presence of sunlight as

$\mathrm{C}{\mathrm{l}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\to \mathrm{H}\mathrm{C}\mathrm{l}+\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}$

This is a disproportionation reaction., where chlorine changes it's oxidation state from zero in ${\mathrm{Cl}}_2$ to -1 in  $\mathrm{HCl}$ and +1 in $\mathrm{HOCl}$. Hypochlorous acid formed in this reaction gives nascent oxygen which is responsible for bleaching action