Which of the following reactions involves both oxidation and reduction of the same element?

Solution

1. In ${\text{Cl}}_2+2{\text{OH}}^{-}\to {\text{Cl}}^{-}+{\text{ClO}}^{-}+{\text{H}}_2\text{O}$, the oxidation state of chlorine changes from $0$ to $-1$ in ${\text{Cl}}^{-}$ and from $0$ to $+1$ in ${\text{ClO}}^{-}$.
2. This is a classic disproportionation reaction where the same element is both oxidized and reduced.
3. Hence the answer is (A).