$SO _{2}$ oxides can act both as an oxidising agent as well as reducing agent. It behaves as a reducing agent in moist conditions. It convert iron (III) ions to iron (II) ions and decolourises acidified potassium permanganate (VII) solution.
$2 F e ^{3+}+ SO _{2}+2 H _{2} O \longrightarrow 2 Fe ^{2+}+ SO _{4}^{2-}+4 H ^{+}$
When $SO _{2}$ reacts with $H _{2} S$, it produces sulphur and water. In this reaction, '$SO_2$, act as an oxidising agent.
