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2.8.4Atoms, Elements and Compounds

Tests for Anions — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 21 of 26 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Carbonate Ions

Carbonates react with acid to release carbon dioxide, which is then identified with limewater.

1
Add dilute acid to the sample, which produces effervescence if a carbonate is present.
2
Bubble the gas released through limewater using a delivery tube.
3
Limewater turns cloudy white if the gas is carbon dioxide, which confirms the carbonate ion.
4
Use a separate portion of the sample for this test, so the added acid does not interfere with the halide or sulfate tests.

Part 2 of 5Halide Ions

Halide ions form silver halide precipitates, and the colour of each precipitate identifies the halide.

Preparation
  • Preparation Acidify the sample with dilute nitric acid first, which removes carbonate ions that would also form a precipitate.
Chloride
  • Chloride Silver nitrate gives a white precipitate of silver chloride, AgCl.
Bromide
  • Bromide Silver nitrate gives a cream precipitate of silver bromide, AgBr.
Iodide
  • Iodide Silver nitrate gives a yellow precipitate of silver iodide, AgI.
HalidePrecipitate with acidified AgNO₃Ionic equation
Chloride, Cl⁻WhiteAg⁺(aq) + Cl⁻(aq) → AgCl(s)
Bromide, Br⁻CreamAg⁺(aq) + Br⁻(aq) → AgBr(s)
Iodide, I⁻YellowAg⁺(aq) + I⁻(aq) → AgI(s)

Part 3 of 5Sulfate Ions

Sulfate ions form a white precipitate of barium sulfate with barium ions.

Sulfate Ions
Preparation
  • Preparation Acidify the sample with dilute hydrochloric acid, not sulfuric acid, which would add sulfate ions.
Reagent
  • Reagent Add aqueous barium chloride solution to the acidified sample.
Observation
  • Observation A white precipitate of barium sulfate forms if sulfate ions are present.
Equation
  • Equation The ionic equation is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s), which forms a white solid.

Part 4 of 5Worked Example

Question

  • A sample of an unknown sodium salt is acidified with nitric acid, then aqueous silver nitrate is added, giving a cream precipitate.
  • Identify the anion and write the ionic equation.

  1. Acidification with nitric acid is correct for halide tests.
  2. A cream precipitate with silver nitrate is the test result for bromide ions.
  3. Ionic equationAg⁺(aq) + Br⁻(aq) → AgBr(s).

Answer

  • Bromide ion, Br⁻.
  • Ionic equation: Ag⁺(aq) + Br⁻(aq) → AgBr(s).

Part 5 of 5Exam Checklist & Tips

2.8.4 – Key Points for the Exam

  • Anion A negatively charged ion, such as Cl⁻, Br⁻ or SO₄²⁻.
  • Halide An ion of a Group 7 element, such as Cl⁻, Br⁻ or I⁻.
  • Acidify Add a dilute acid to remove interfering ions before the test is carried out.
  • Silver halide An insoluble silver salt formed in the halide test.

Common Mistakes

  • Acidifying with sulfuric acid for the sulfate test, which gives a false positive by adding sulfate ions.
  • Omitting the acid in the halide test, so carbonate ions give a white precipitate and cause confusion.
  • Confusing the cream precipitate of bromide with the yellow precipitate of iodide.
  • Testing for sulfate without acidifying first, since carbonate ions also give a white precipitate with barium ions.

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