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4.3.5Electrochemistry

Halide Ions — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 7 of 20 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Carrying Out the Test

4.3.5 Halide Ions

The sample is acidified first, then silver nitrate is added to form a precipitate.

1
Dissolve the sample in water, then add dilute nitric acid to make the solution acidic.
2
Add a few drops of silver nitrate solution, and watch for a precipitate forming.
3
If a halide ion is present, a silver halide precipitate forms with a colour that identifies the halide.
4
Record the colour of the precipitate, which tells you whether the ion is chloride, bromide or iodide.
5
Keep a clean test tube and dropper for each sample, because traces of halide from a previous test could give a false precipitate.

Part 2 of 5Silver Halide Precipitates

Chloride, Cl⁻
  • Silver chloride, AgCl(s), forms a white precipitate.
Bromide, Br⁻
  • Silver bromide, AgBr(s), forms a cream precipitate.
Iodide, I⁻
  • Silver iodide, AgI(s), forms a yellow precipitate.
General equation
  • Silver ions react with halide ions to form a precipitate: Ag⁺(aq) + X⁻(aq) → AgX(s), where X is the halogen.
Precipitate colours
  • Because each silver halide has a distinct colour, the colour observed identifies the halide directly without further testing.

Part 3 of 5Why Nitric Acid Is Used

Why Nitric Acid Is Used
Nitric acid choice
  • Nitric acid is used to acidify the sample because it does not add any ions that would interfere with the test.
Hydrochloric acid problem
  • Hydrochloric acid must not be used, because the chloride ions it contains would give a false positive for chloride.
Carbonate removal
  • Acid removes carbonate ions, which would otherwise form a precipitate with silver ions and confuse the result.
Example
  • Potassium chloride reacts with silver nitrateKCl(aq) + AgNO₃(aq) → KNO₃(aq) + AgCl(s).

Part 4 of 5Worked Example

Question

  • A colourless solution is acidified with nitric acid and silver nitrate is added.
  • A cream precipitate forms.
  • Identify the halide ion present and write the ionic equation.

  1. A cream precipitate with silver nitrate identifies bromide ions.
  2. The silver bromide is formed when Ag⁺ ions meet Br⁻ ions.
  3. Write the ionic equationAg⁺(aq) + Br⁻(aq) → AgBr(s).

Answer

  • Bromide, Br⁻;
    • Ag⁺(aq) + Br⁻(aq) → AgBr(s), a cream precipitate.

Part 5 of 5Exam Checklist & Tips

4.3.5 – Key Points for the Exam

  • Halide ion A negatively charged ion of a Group 7 element, such as Cl⁻, Br⁻ or I⁻.
  • Silver nitrate A reagent, AgNO₃, that forms coloured precipitates with halide ions.
  • Anion A negatively charged ion, such as a halide ion.
  • Acidify To add an acid, such as nitric acid, to make a solution acidic before testing.

Common Mistakes

  • Acidifying the sample with hydrochloric acid, which adds chloride ions and gives a false result.
  • Mixing up the colours of the silver halide precipitates, such as cream for bromide and yellow for iodide.
  • Writing the formula of silver chloride as AgCl₂ instead of AgCl.
  • Forgetting the state symbol (s) for the precipitate in the equation.

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