Thiosulphate \( {S_2O_3}^{2-} \) is oxidized to ________ in presence of permanganate ion.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \({SO_4}^{2-}\)
The permanganate ion is a strong oxidising agent whose product depends on the medium. In neutral or faintly alkaline solution it is reduced to \( \ce{MnO2} \) (Mn +7 to +4, three electrons), and it oxidises thiosulphate almost completely to sulphate.
The oxidation state of sulphur rises from an average of +2 in \( \ce{S2O3^2-} \) (2x + 3(−2) = −2, x = +2) to +6 in \( \ce{SO4^2-} \): each \( \ce{S2O3^2-} \) loses 2 × 4 = 8 electrons, and each \( \ce{MnO4-} \) gains 3. Balancing 24 electrons (3 thiosulphate, 8 permanganate):
\( \displaystyle \ce{8MnO4^- + 3S2O3^2- + H2O -> 8MnO2 + 6SO4^2- + 2OH^-} \)
So thiosulphate is oxidised to sulphate.
Why the other options are incorrect
Option (1) \(S^{2-}\) — Incorrect. Sulphide (S = −2) has a lower oxidation state than thiosulphate; forming it would be a reduction, not an oxidation.
Option (2) \({SO_3}^{2-}\) — Incorrect. Sulphite (S = +4) is itself a reducing agent and is oxidised further by permanganate, so it does not remain as the product.
Option (4) \({SO_2}^{2-}\) — Incorrect. There is no such stable ion formed here.
NCERT (2026-27 edition):
In Neutral or Faintly Alkaline Solutions
