Lesson Progress
0% Complete
Test for CO32-, SO32- and S2- using dilute acids
| Test | Observation | Inference |
| Dry sample or residue or solution + dil. HCl(aq) or H2SO4(aq) | Sample dissolved. Effervescence occurs, a colourless, odourless gas, which turns moist blue litmus red and lime water milky is liberated. | Acidic gas present Gas is CO2 from CO32- or HCO3– |
| Dry sample or residue or solution + dil. HCl(aq) or H2SO4(aq) | Sample dissolved. Effervescence occurs. Colourless gas with a sharp irritating smell is given off. The gas turns potassium dichromate (IV) paper from orange to green. | SO2(g) (from SO32-) is liberated |
| Dry sample or residue or solution + dil. HCl(aq) or H2SO4(aq) + KMnO4(aq) or K2Cr2O7(aq) | Purple colour of KMnO4(aq) turns colourless or orange colour of K2Cr2O7(aq) turns green | Reducing agent present |
| Dry sample or residue or solution + dil. HCl(aq) or H2SO4(aq) | Sample dissolved. Effervescence occurs. Colourless gas with characteristic rotten egg smell is given off. | H2S(g) from S2- present |
The Brown Ring Test (NO3–)
| Test | Observation | Inference |
| Sample solution + freshly prepared FeSO4(aq) + Conc. H2SO4 solution in drops down the side of the test tube while holding the test tube in a slanting position. | Brown ring formed at the liquid junction | NO3– confirmed |
Dioxonitrate (III) ion, NO2– test
| Test | Observation | Inference |
| Sample solution + freshly prepared FeSO4(aq) + CH3COOH solution in drops | Reaction mixture turns brown | NO2– present |
Additional Confirmatory Test for Some Anions
Hydrogen Trioxocarbonate (IV) ion, HCO3–
| Test | Observation | Inference |
| Sample solution + BaCl2(aq) or MgSO4(aq) solution + heating until boiling | No visible reaction White precipitate formed | HCO3– is confirmed |
| Sample solution + few drops of phenolphthalein + heat | No visible reaction. Solution remains colourless. Solution turns pink. | HCO3– is confirmed |
Trioxosulphate (IV) ions, SO32-
| Test | Observation | Inference |
| Sample solution + H2SO4(aq) + few drops of KMnO4(aq) or few drops of K2Cr2O7(aq) | Purple KMnO4(aq) is decolourisedOrK2Cr2O7(aq) changes from orange to green. | SO2(g) from SO32- which reduces dichromate (VI) to chromate (III). |
| Sample solution + FeCl2(aq) in drops + warm | Yellow solution changes to reddish brown colour. | Oxidizing agent is present. Fe2+ has been oxidized to Fe3+ |
Iodide ion, I–
| Test | Observation | Inference |
| Sample + dil. HNO3(aq) + AgNO3(aq) in excess | Yellow precipitate formed. Precipitate insoluble in concentrated NH3(aq) | I– confirmed. |
Sulphide ion, S2-
| Test | Observation | Inference |
| Sample solution + Pb(CH3COO)2(aq) or Pb(NO3)2(aq) + warming | Black precipitate of lead (II) sulphide formed. | S2- confirmed. |
Responses