Testing for Carbonate Ions
- Carbonates all contain the carbonate ion, CO32-
- The test for this ion involves adding dilute acid and testing the gas released
- If a carbonate compound is present then effervescence should be seen and the gas produced is CO2 which forms a white precipitate of calcium carbonate when bubbled through limewater:
CO32- (aq) + 2H+ (aq) → CO2 (g) + H2O (l)
CO2 (g) + Ca(OH)2 (aq) → CaCO3 (s) + H2O (l)
Limewater turns milky in the presence of CO2 caused by formation of insoluble calcium carbonate
Exam Tip
Make sure you talk about bubbling the suspected carbon dioxide gas through the limewater.
If describing the practical you’ll need to connect the test tube of the suspected ion to the test tube of limewater quickly so none of the CO2 escapes.
Testing for Sulfate Ions
- Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride
- If a sulfate is present then a white precipitate of barium sulfate is formed:
Ba2+ (aq) + SO42- (aq) → BaSO4 (s)
- The test can also be carried out with barium nitrate solution
A white precipitate of barium sulfate is a positive result for the presence of sulfate ions
Exam Tip
Don't forget to say about adding the HCl first; this is added first to remove any carbonates which may be present. Carbonates would also produce a white precipitate and interfere with the results.
Testing for Halide Ions
- Acidify the sample with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution, AgNO3
- The acidification is done to remove carbonate ions that might give a false positive result
- If a halide ion is present it forms a silver halide precipitate:
Ag+ (aq) + X– (aq) → AgX (s)
- Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion
- Silver chloride precipitate is white, silver bromide precipitate is cream and silver iodide precipitate is yellow
Each silver halide produces a precipitate of a different colour
Exam Tip
The acidification step in the halide ion test must be done with nitric acid rather than hydrochloric acid, as HCl contains chloride ions itself which would interfere with the results to produce a white precipitate!
Make sure you use the colours stated for the precipitates and not deviations such as 'milky' or 'lemon' coloured.
