Identification of Anions (Cambridge International IGCSE Science)

Identification of anions

• Negatively charged non-metal ions are known as anions

• You must be able to describe the tests for the following ions:

  • Carbonate ions, CO₃^2–
  • Halide ions, Cl^– , Br^– , I^–
  • Sulfate ions, SO₄^2–

Test for carbonate ions (CO₃^2-)

• Carbonate compounds contain the carbonate ion, CO₃^2-

Test

• Add dilute acid to the sample
• A gas is produced
• Bubble the gas through limewater

Result

• Limewater turns cloudy if carbon dioxide is present
• This happens because carbon dioxide reacts with limewater to form a white precipitate of calcium carbonate

Key observation

• Fizzing / effervescence when acid is added
• Limewater turns cloudy

Limewater turns milky in the presence of carbon dixoide caused by the formation of insoluble calcium carbonate

Exam Hint

• 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 (Cl^-, Br^-, I^-)

• Halide ions are negative ions (anions) formed by Group 7 elements.

Test

• Acidify the sample with dilute nitric acid
• Add aqueous silver nitrate

Result

• A silver halide precipitate forms. The colour of the precipitate identifies the halide ion:

  • Chloride (Cl⁻): white precipitate (silver chloride)
  • Bromide (Br⁻): cream precipitate (silver bromide)
  • Iodide (I⁻): yellow precipitate (silver iodide)

• 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)

Each silver halide produces a precipitate of a different colour

Exam Hint 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 (SO₄^2-)

• Sulfate compounds contain the sulfate ion, SO₄²⁻.

Test

• Acidify the sample with dilute nitric acid
• Add aqueous barium nitrate

Result

• A white precipitate of barium sulfate forms if sulfate ions are present

Ba²⁺(aq) + SO₄^2-(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 Hint Nitric is added first to remove any carbonates which may be present which would also produce a precipitate and interfere with the results.

Exam Hint For qualitative inorganic analysis, there will be one test for the metal cation and another test for the non-metal anion.

If you are an extended level student you may be asked to write balanced ionic equations for cation and anions tests, so make sure you know the formulae of all the ions and precipitates formed.