WAEC Chemistry 2013 Paper

Concept:Reduction is the gain of electrons, shown by a decrease in oxidation number.Explanation:In $\mathrm{I}_2$, each iodine atom has oxidation number $0$.In $\mathrm{I}^-$, each iodine atom has oxidation number $-1$.The oxidation number of iodine decreases from $0$ to $-1$, which means iodine gains electrons.Therefore, $\mathrm{I}_2$ is the species that undergoes reduction.The other species, $\mathrm{S}_2\mathrm{O}_3^{2-}$, is oxidised to $\mathrm{S}_4\mathrm{O}_6^{2-}$, but it is not reduced.Answer:$\mathrm{I}_{2(aq)}$ is the species reduced.Correct option: C.