Name the gas that can readily decolourise acidified KMnO 4 solution.

${\mathrm{SO}}_2$ readily decolourises acidified ${\mathrm{KMnO}}_4.$
$\mathrm{K}\stackrel{+7}{\mathrm{Mn}}{\mathrm{O}}_4+\stackrel{+4}{\mathrm{S}}{\mathrm{O}}_2\to \underset{\mathrm{Colourless}}{{\mathrm{MnSO}}_4}+{\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{K}}_2{\mathrm{SO}}_4$

Potassium permagnate gives the colouless solution with sulphurdioxide.