Carbonate and sulphite both turn lime water milky in salt analysis, how do I tell them apart?

7 answers & replies

• [+][–]Pooja Iyer12thSample· 10d ago

  SO2 gas has a sharp smell, like burning sulphur (a match just struck). CO2 has no smell. And SO2 is a reducing agent, so:

  • acidified $K_2Cr_2O_7$ paper turns from orange to green (Cr(VI) reduced to Cr(III))
  • acidified $KMnO_4$ solution loses its pink/purple colour (Mn(VII) to Mn(II))

  CO2 does neither. That's your test. Both do turn lime water milky: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$ and $Ca(OH)_2 + SO_2 \rightarrow CaSO_3 + H_2O$. With excess gas the solid dissolves as the hydrogen salt in both cases.

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  • [+][–]Farhan ShaikhDropperSample· 10d agoAuthor

    Thanks pooja, orange to green for dichromate, pink to colourless for permanganate. Pinning that on my wall.

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• [+][–]Diya Menon11thSample· 10d ago

  omg wait, doesn't CO2 also decolourise KMnO4? I thought any gas that comes from acid + salt does that 😅 i might have mixed it up

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• [+][–]Zoya Khan12thSample· 10d ago

  No, Diya. Permanganate is decolourised only by a reducing gas, and CO2 is not one: carbon is already at its highest oxidation state (+4) in CO2, so there is nothing for permanganate to oxidise. SO2 has sulphur at +4, which can go up to +6, so it acts as a reducing agent and decolourises it. Same logic for the dichromate paper. Reducing gas gives colour change, CO2 gives none.

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  • [+][–]Diya Menon11thSample· 10d ago

    ahh makes sense, reducing gas only. thanks zoya! 💛

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• [+][–]Rahul DubeyDropperSample· 10d ago

  Your list, with what to look for.

  Dilute H2SO4 group

  • Carbonate: CO2, odourless, lime water milky
  • Sulphide: H2S, rotten egg smell, turns lead acetate paper black (PbS)
  • Sulphite: SO2, burning sulphur smell, dichromate paper green
  • Nitrite: reddish-brown fumes of NO2 (colourless NO from the acid turns brown in air)
  • Acetate: vinegar-like smell

  Conc. H2SO4 group

  • Chloride: colourless HCl gas, white fumes with ammonia
  • Bromide: reddish-brown bromine vapours
  • Iodide: violet vapours of iodine
  • Nitrate: brown fumes of NO2, strengthened when Cu turnings are added

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• [+][–]Vivek Mishra12thSample· 10d ago

  Barium chloride alone won't separate carbonate and sulphite. Both give white precipitates, $BaCO_3$ and $BaSO_3$, and both dissolve in dilute HCl with evolution of gas (CO2 in one case, SO2 in the other). Only sulphate gives a white $BaSO_4$ precipitate that does not dissolve in dilute HCl. So BaCl2 is for sulphate. To tell carbonate from sulphite, use the gas tests above.

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