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I learnt at school that when sulfur dioxide gas, $\ce{SO2}$, is combined with potassium permanganate, $\ce{KMnO4}$, the purple solution will become colourless.

What happens in the solution to cause this to happen?

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This is a redox reaction. The permanganate ion (purple) is reduced to manganese(II) ion which is a very pale pink, or almost colourless.

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

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

$$\ce{SO2 + 2H2O -> SO4^2- + 4H+ + 2e-}$$

So, the solution is decolourised. The overall equation is

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

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  • $\begingroup$ But what happen to the potassium ion ? Does it combine with the sulfate ion or still remains as an ion? $\endgroup$ – Simon-Nail-It Nov 13 '14 at 22:04
  • $\begingroup$ Potassium ion is an spectator ion. It doesn't participate in the redox equation. Its role is to keep the solution electrically neutral. $\endgroup$ – Yomen Atassi Nov 14 '14 at 12:30

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