Acidified ${\mathrm{KMnO}}_4$ can be decolourised by

Acidified ${\mathrm{KMnO}}_4$ solution is easily decolored by SO2. The colour ${\mathrm{KMnO}}_4$of  is purple. While ${\mathrm{KMnO}}_4$ is an oxidising agent and decolorizes when exposed to sulphur dioxide gas, the latter works as a reducing agent.