Why does potassium permanganate turn colourless when reacted with sulfur dioxide?
This is a redox reaction. The permanganate ion (purple) is reduced to manganese(II) ion which is a very pale pink, or almost colourless.

$$\ce{MnO4- + 8H+ + 5e- -&gt; Mn^2+ + 4H2O}$$

while sulfur dioxide is oxidized to the sulfate ion, which is colourless.

$$\ce{SO2 + 2H2O -&gt; SO4^2- + 4H+ + 2e-}$$

So, the solution is decolourised. The overall equation is

$$\ce{2MnO4- + 5SO2 + 2H2O -&gt; 2Mn^2+ + 5SO4^2- + 4 H+}$$