Acidified potassium permanganate ($\text{KMnO}_4$) is a strong oxidising agent. It oxidises sulphur dioxide ($\text{SO}_2$) gas to sulphate ($\text{SO}_4^{2-}$) or hydrogen sulphate ($\text{HSO}_4^-$) ions, while it itself gets reduced from the highly coloured $\text{MnO}_4^-$ (purple/pink) to the colourless $\text{Mn}^{2+}$ ion . Thus, $\text{SO}_2$ readily decolourises acidified $\text{KMnO}_4$ solution. The reaction can be represented as: $2\text{MnO}_4^- + 5\text{SO}_2 + 2\text{H}_2\text{O} \rightarrow 2\text{Mn}^{2+} + 5\text{SO}_4^{2-} + 4\text{H}^+$ Other gases like $\text{CO}_2$ and $\text{P}_2\text{O}_5$ cannot be further oxidised as carbon and phosphorus are already in their highest stable oxidation states (+4 and +5, respectively).