JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Hydrogen

Oxygen is at $-1$ in ${\mathrm{H}}_2{\mathrm{O}}_2$, midway between its states in water and in dioxygen, so the peroxide oxidises when that oxygen falls to $-2$ and reduces when it rises to $0$. Whenever ${\mathrm{O}}_2$ is set free the peroxide has itself been oxidised, which is what happens with chlorine, with periodate and with iodine in alkali. In the reaction of iodide in acid no oxygen is evolved: iodide at $-1$ is raised to $0$ in ${\mathrm{I}}_2$ while the peroxide oxygen ends as water at $-2$. The oxidising nature is shown by $2{\mathrm{I}}^{-}+{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to {\mathrm{I}}_2+2{\mathrm{H}}_2\mathrm{O}$.