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2.3.3Physical Chemistry

Testing for Halide Ions — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 62 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4The Silver Nitrate Test

Silver halides are insoluble, so a precipitate forms when silver ions meet halide ions.

1
Add dilute nitric acid to the sample, which removes carbonate and hydroxide ions that would form interfering silver precipitates.
2
Add a few drops of aqueous silver nitrate and observe the colour of any precipitate that forms.
3
The ionic equation for the precipitation is Ag⁺(aq) + X⁻(aq) → AgX(s), where X is Cl, Br or I.
4
Confirm the identity by adding aqueous ammonia and noting whether the precipitate dissolves.
5
Never acidify with hydrochloric acid or sulfuric acid, because they supply chloride or sulfate ions that give false results.

Part 2 of 4Results Table

Chloride
  • Chloride AgCl is white and dissolves in dilute ammonia to form a colourless solution of the diamminesilver(I) complex.
Bromide
  • Bromide AgBr is cream and dissolves only in concentrated ammonia, so dilute ammonia leaves it unchanged.
Iodide
  • Iodide AgI is yellow and does not dissolve in dilute or concentrated ammonia.
HalidePrecipitate colourEffect of dilute NH₃Effect of concentrated NH₃
ChlorideWhiteDissolvesDissolves
BromideCreamInsolubleDissolves
IodideYellowInsolubleInsoluble

Part 3 of 4Why the Ammonia Test Works

Why the Ammonia Test Works
Solubility differences
  • Solubility differences The solubility of the silver halides decreases from chloride to iodide, so only the most soluble dissolve in dilute ammonia.
Complex ion formation
  • Complex ion formation Ammonia reacts with silver ions to form [Ag(NH₃)₂]⁺, which removes silver ions from the equilibrium and dissolves the solid.
Colour clue
  • Colour clue Colours alone can be hard to judge, so the ammonia test provides evidence that separates similar precipitates.
Fluoride
  • Fluoride Silver fluoride is soluble, so it gives no precipitate, which is why the test is used for Cl⁻, Br⁻ and I⁻ only.

Part 4 of 4Exam Checklist & Tips

2.3.3 – Key Points for the Exam

  • Precipitate An insoluble solid formed when two solutions are mixed.
  • Halide ion A halogen atom that has gained one electron, forming X⁻.
  • Ionic equation An equation showing only the ions that react to form the precipitate.
  • Complex ion An ion in which a central metal ion is bonded to ligands, such as [Ag(NH₃)₂]⁺.

Common Mistakes

  • Using hydrochloric acid to acidify the sample, which adds chloride ions.
  • Mixing up the precipitate colours of chloride, bromide and iodide.
  • Omitting the dilute nitric acid step, so carbonate ions could give a false positive.
  • Writing the ionic equation without state symbols, such as AgCl(s).

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