The diagram shows a fume test where a gas is bubbled through acidified potassium manganate(VII), which turns from purple to colourless. Which polluting gas ...
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The diagram shows a fume test where a gas is bubbled through acidified potassium manganate(VII), which turns from purple to colourless. Which polluting gas gives this result?
Answer Details
Acidified potassium manganate(VII) is purple because of the MnO4- ion, and it is a strong oxidising agent. It turns colourless only when something reduces the manganese to the colourless Mn2+ ion, so the test is looking for a reducing gas. Sulfur dioxide is that gas: it is oxidised to sulfate, SO2 + 2H2O → SO42- + 4H+ + 2e-, decolourising the solution.
Carbon dioxide is acidic but not a reducing agent, so it leaves the purple colour unchanged; it is identified with limewater instead. Nitrogen and argon are both unreactive and give no result at all. Learn the pairing: sulfur dioxide decolourises acidified manganate(VII), carbon dioxide turns limewater milky.
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