Plan reagent choice, observations and confirmatory tests. Explain why the acid matters and distinguish a precipitate from a coloured solution.
A complete halide test
Use a fresh portion of the aqueous unknown. Acidify with dilute nitric acid, then add aqueous silver nitrate. A silver-halide precipitate can form. Record its colour, then use ammonia to distinguish similar-looking precipitates.
Nitric acid removes carbonate interference: otherwise silver carbonate could precipitate. Do not use hydrochloric acid, because it introduces Cl⁻ and can produce a false chloride result. Keep the sulfate test on a separate portion; its hydrochloric acid and barium chloride both introduce chloride.
AgF is soluble, so fluoride does not give the usual silver-halide precipitate. No precipitate alone is not proof of fluoride: it may mean that no detectable halide is present.
| Ion | Silver nitrate result | Dilute NH₃(aq) | Concentrated NH₃(aq) |
|---|---|---|---|
| Cl⁻ | White AgCl precipitate | Dissolves | Dissolves |
| Br⁻ | Cream AgBr precipitate | Does not appreciably dissolve | Dissolves |
| I⁻ | Yellow AgI precipitate | Does not dissolve | Does not dissolve |
Why ammonia is a useful second test
White, cream and pale yellow can be difficult to distinguish in a small sample. Ammonia adds an independent observation: whether the solid dissolves. When AgCl dissolves, the silver is present mainly as the soluble [Ag(NH₃)₂]⁺ complex.
“The solution goes colourless” is not enough if the liquid was already colourless: say that the precipitate dissolves to form a colourless solution. If two possible ions are specified, use the smallest set of tests that distinguishes that pair.
Diagram placeholder
Halide-test decision diagram
Labels to include:
- Fresh unknown → dilute nitric acid → silver nitrate
- Three solid colours: white, cream, yellow
- Dilute ammonia: AgCl dissolves
- Concentrated ammonia: AgBr dissolves; AgI remains
- Use text labels as well as colours; do not depict a precipitate as a clear coloured solution
The colour narrows the identity; ammonia confirms it. Nitric acid is added before silver nitrate to remove interfering carbonate.
Required Practical 4: use separate portions
Label fresh portions of the unknown before adding reagents. A reagent used in one test can introduce ions or neutralise a species needed in another. Record reagent, observation and inference separately, with full names or formulae for reagents.
The related Group 2 pages explain hydroxide and sulfate precipitation. The table below joins these tests to the other RP4 ions. In an unknown mixture, results require interpretation; an alkaline indicator result alone does not uniquely identify hydroxide because other ions can also make solutions alkaline.
| Target | Reagent / method | Positive observation |
|---|---|---|
| Halide | Dilute HNO₃, then AgNO₃; ammonia confirmation | Characteristic precipitate and ammonia solubility |
| SO₄²⁻ | Dilute HCl, then BaCl₂ solution | White BaSO₄ precipitate |
| CO₃²⁻ | Dilute acid; test the gas with limewater | Effervescence; gas makes limewater cloudy |
| NH₄⁺ | Add aqueous NaOH and warm gently | Gas turns damp red litmus blue (NH₃) |
| OH⁻ in a simple unknown | Indicator / pH test | Alkaline response; confirm against the possible identities |
| Group 2 cations | Compare hydroxide/sulfate precipitation under specified conditions | Use solubility pattern, not a single white precipitate as unique proof |
Do not confuse three different halogen experiments
With silver nitrate, you are looking for a solid silver halide. With an aqueous halogen displacement, you are observing the colour of a newly formed free halogen. With concentrated sulfuric acid and solid halide, you may see fumes, vapours or solid reduction products.
The 2023 Q06.2 report highlights confusion between the silver nitrate and concentrated sulfuric acid experiments. Start every answer by reading the reagent and physical state of the starting halide.
Use goggles and the prescribed small-scale method. Ammonia vapour is irritating; concentrated ammonia needs suitable ventilation. Collect silver-containing waste as instructed. Do not smell gases directly.
Using precipitation for an amount calculation
If all chloride in a sample is precipitated, one mole Cl⁻ gives one mole AgCl. For example, 0.287 g dry, pure AgCl with Mr = 143.5 corresponds to 0.00200 mol AgCl and therefore 0.00200 mol Cl⁻. In an original 25.0 cm³ aliquot, [Cl⁻] = 0.00200 ÷ 0.0250 = 0.0800 mol dm⁻³.
Excess AgNO₃ helps collect all the halide. Filter, wash and dry before weighing. Do not use the total mass of a mixed AgCl/AgBr/AgI precipitate as if it had one molar mass unless the composition is known or another measurement resolves it.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Why are nitric acid and hydrochloric acid not interchangeable in the silver nitrate test?Show answer
Nitric acid removes carbonate without supplying a halide. HCl supplies Cl⁻, which forms AgCl and may give a false positive.
Q2. A cream precipitate remains in dilute ammonia but dissolves in concentrated ammonia. Identify the ion.Show answer
Br⁻. The precipitate is AgBr. The concentration-dependent ammonia result distinguishes it from AgCl and AgI.
Q3. Write the ionic equation for the iodide test and state the observation.Show answer
Ag⁺(aq) + I⁻(aq) → AgI(s). A yellow precipitate forms; it remains in concentrated ammonia.
Q4. Why should a sample already used for the acidified BaCl₂ sulfate test not be reused for a chloride test?Show answer
The added BaCl₂ and usually HCl introduce chloride ions. Silver nitrate could then produce AgCl from those reagents rather than from the original unknown.
Q5. 0.4305 g dry AgCl is obtained from 20.0 cm³ of a chloride solution. Calculate [Cl⁻], given Mr(AgCl) = 143.5 and complete precipitation.Show answer
n(AgCl) = 0.4305 ÷ 143.5 = 0.003000 mol. n(Cl⁻) is the same. [Cl⁻] = 0.003000 ÷ 0.0200 = 0.150 mol dm⁻³.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise — AQA 2.3 Halogens (January 2022) — Primary coverage checklist, pp1–3; original explanations and questions.
- AQA 7405 specification — 3.2.3.1–3.2.3.2 and Required Practical 4.
- AQA 7404/1 2021 mark scheme (November archive) — Q04.1–04.3, p14: precipitation, ammonia gas and dilute ammonia confirmation.
- AQA 7404/1 June 2019 mark scheme — Q08.2, p15: acid and carbonate observations.
- AQA 7404/1 June 2019 examiner report — Q08, pp5–6.
- AQA 7404/1 June 2023 examiner report — Q06.2, p4: separate acid reactions from silver nitrate tests.
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