Topic 8 · page 28 of 29
Silver nitrate test and hydrogen halides
Acidified silver nitrate gives a white, cream or yellow precipitate with chloride, bromide or iodide ions, which dissolve in ammonia in a set order. Hydrogen halides make white smoke with ammonia and strong acids with water.
Read first: Halides with concentrated sulfuric acid
Silver nitrate and halide ions 8.278.27(ii)
Silver ions precipitate chloride, bromide and iodide ions from solution. The three silver halides are insoluble, and each has a different colour.
MethodTest for halide ions
- Add dilute nitric acid to the solution of the sample.
- Add silver nitrate solution.
- Note the colour of the precipitate.
- Add dilute ammonia solution, and if the precipitate remains, concentrated ammonia solution.
| Halide ion | Precipitate | Colour | Dilute NH₃(aq) | Concentrated NH₃(aq) |
|---|---|---|---|---|
| Cl⁻ | AgCl | white | dissolves | dissolves |
| Br⁻ | AgBr | cream | stays (or dissolves very little) | dissolves |
| I⁻ | AgI | yellow | stays | stays (insoluble) |
When a precipitate dissolves, a colourless solution of a complex ion forms, for example . You are not asked to know the complex ion's formula at this stage, only that the precipitate dissolves.
Why nitric acid first? Silver ions also form precipitates with carbonate and hydroxide ions, for example silver carbonate, which is also pale. Adding dilute nitric acid first removes them (carbonate is destroyed with fizzing), so that only the halide precipitates are seen. Do not use hydrochloric acid, which would add chloride ions, or sulfuric acid, whose sulfate gives a slightly soluble precipitate with silver ions.
Hydrogen halides with ammonia and water 8.278.27(iii)
The hydrogen halides, HCl, HBr and HI, are colourless gases that give steamy fumes in moist air.
With ammonia gas, they give a white solid, an ammonium halide, seen as white smoke:
The white smoke is fine crystals of an ionic solid, containing NH₄⁺ and X⁻. This is how a glass rod dipped in concentrated hydrochloric acid detects ammonia (page 17).
With water, the hydrogen halide dissolves and ionises completely, forming a strong acid:
The same equation with HBr or HI gives hydrobromic acid and hydroiodic acid. All three are strong acids: they are fully ionised in water.
Worked exampleObservations for two reactions of hydrogen bromide
Describe what you would see when hydrogen bromide gas is mixed with ammonia, and when it is bubbled into water.
- With ammonia: white smoke of solid ammonium bromide, .
- In water: the gas dissolves and forms a colourless solution that is strongly acidic (universal indicator turns red): .
Check yourself
- Describe the test that distinguishes chloride, bromide and iodide ions, with the observations for each.
- Write the ionic equation, with state symbols, for the formation of the precipitate with iodide.
- Why is the sample acidified with dilute nitric acid before silver nitrate is added?
- A halide ion gives a cream precipitate that dissolves in concentrated ammonia but not in dilute ammonia. Name the ion and the precipitate.
- Write the equation for hydrogen chloride gas with ammonia gas, and state what you would see.
- Explain why the reaction of hydrogen chloride with water is not a redox reaction.
Answers cover these until you have tried the questions
- Add dilute nitric acid, then silver nitrate solution. Cl⁻: white precipitate, dissolves in dilute ammonia. Br⁻: cream precipitate, dissolves in concentrated ammonia only. I⁻: yellow precipitate, insoluble in ammonia.
- To remove carbonate (and hydroxide) ions, which would also give a precipitate with silver ions (silver carbonate), giving a false positive. Nitric acid is used because it adds no halide or sulfate ions.
- Bromide, Br⁻; the precipitate is silver bromide, AgBr.
- . White smoke forms, as tiny crystals of ammonium chloride.
- HCl is hydrogen +1, chlorine −1. In water it forms H⁺ (+1) and Cl⁻ (−1). No oxidation number changes: it is an acid–base reaction, in which HCl gives H⁺ to water.