How to Test for Anions (Edexcel IGCSE Chemistry (Modular))
Revision Note
Tests for anions
Negatively charged non-metal ions are known as anions
You must be able to describe the tests for the following ions:
Carbonate ions, CO32–
Halide ions, Cl– , Br– , I–
Sulfate ions, SO42–
Test for carbonate ions
Carbonate compounds contain the carbonate ion, CO32-
The test for the carbonate ion is:
Add dilute acid
Bubble the gas released through limewater
Limewater turns cloudy if the carbonate ion is present
If a carbonate compound is present then fizzing / effervescence should be seen as CO2 gas is produced, 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)
The white precipitate turns limewater cloudy
Testing for carbonate ions
Limewater turns milky in the presence of carbon dixoide caused by the formation of insoluble calcium carbonate
Examiner Tip
If you are asked to describe the test for carbonate ions, make sure that you say:
Bubble the gas produced through limewater, which turns cloudy if the carbonate ion is present
Just saying that limewater turns cloudy is not enough
This isn't describing the test, it is stating the result
Test for halide ions
Halide ions are the negative ions / anions formed by the elements in Group 7
The test for the halide ions is:
Acidify the sample with nitric acid
Add silver nitrate solution, AgNO3,
A silver halide precipitate forms if a halide ion is present
The precipitate is indicated by the state symbol (s)
The colour of the silver halide precipitate depends on the halide ion:
The chloride ion forms a white precipitate of silver chloride
potassium chloride + silver nitrate → potassium nitrate + silver chloride
KCl (aq) + AgNO3 (aq) → KNO3 (aq) + AgCl (s)
The bromide ion forms a cream precipitate of silver bromide
potassium bromide + silver nitrate → potassium nitrate + silver bromide
KBr (aq) + AgNO3 (aq) → KNO3 (aq) + AgBr (s)
The iodide ions forms a yellow precipitate of silver iodide
potassium iodide + silver nitrate → potassium nitrate + silver iodide
KI (aq) + AgNO3 (aq) → KNO3 (aq) + AgI (s)
Testing for halide ions
Each silver halide produces a precipitate of a different colour
Examiner Tip
The acidification step in the halide ion test must be done with nitric acid rather than hydrochloric acid.
HCl contains the chloride ion which would interfere with the results.
Test for sulfate ions
Sulfate compounds contain the sulfate ion, SO42-
The test for the sulfate ion is:
Acidify the sample with dilute hydrochloric acid
Add a few drops of barium chloride solution
A white precipitate of barium sulfate is formed, if the sulfate ion is present
Ba2+ (aq) + SO42- (aq) → BaSO4 (s)
The test can also be carried out with barium nitrate solution
Testing for sulfate ions
A white precipitate of barium sulfate is a positive result for the presence of sulfate ions
Examiner Tip
HCl is added first to remove any carbonates which may be present which would also produce a precipitate and interfere with the results.
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