Identify halogens and halides from combined evidence
Use displacement, silver-ion tests and concentrated-acid behavior to make qualified halogen and halide deductions.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State conditions for halogen reactions, distinguish halogen oxidising strength from halide-ion reducing strength, and balance redox equations. Chlorine, bromine, iodine and concentrated acids are hazardous; examples teach chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use molecular halogen formulas X₂, oxidation numbers, ionic equations and electron-transfer ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pair of tests directly distinguishes chloride, bromide and iodide precipitates?
What you will learn
- Combine oxidising and reducing trends
- Select confirmatory observations
- State when evidence does not identify a unique species
Separate evidence about a halogen from evidence about its ion
A halogen displacement test probes oxidising strength of X₂; the Ag⁺/NH₃ test probes halide identity through precipitate color and solubility; concentrated H₂SO₄ probes halide reducing strength.
The direction patterns are related but not interchangeable: oxidising strength X₂ decreases down the group, while reducing strength X⁻ increases down the group.
Build a conclusion from more than one observation
A cream AgX precipitate with dissolution only in concentrated ammonia supports Br⁻. A positive displacement by chlorine also supports bromide or iodide, so it must be combined with the silver-ion pattern to distinguish them.
Color can be affected by concentration, lighting, mixtures or contamination. Acidify the silver test with dilute nitric acid and use the complete prescribed sequence before stating an identity.
Pause and explain
What is the strongest conclusion from chlorine displacing an unknown halide ion?
Worked example
An unknown solution gives a cream silver-halide precipitate that dissolves only in concentrated ammonia. Chlorine also displaces the halide. Identify the ion and explain why the two tests agree.
Show the worked solution
- Cream AgBr dissolves in concentrated but not dilute ammonia.
- Chlorine is stronger as an oxidising agent than bromine and oxidises Br⁻ to Br₂.
- Both observations support bromide.
Answer The ion is Br⁻: the AgBr precipitate is cream and has the stated ammonia behavior, while Cl₂ displaces Br⁻ to form Br₂.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A sample forms white AgCl that dissolves in dilute ammonia. State the likely ion and one suitable follow-up comparison.
Give me a hint
Use the AgCl color and its ammonia behavior; check for contamination if the sample is complex.
Compare my reasoning
- White AgCl is consistent with chloride.
- Its dissolution in dilute ammonia supports the assignment.
- A controlled displacement comparison can provide additional evidence.
The sample likely contains Cl⁻. A controlled halogen displacement test can provide a second line of evidence.
Look for these in your work
- Likely ion identified
- Ammonia response used
- Follow-up specified
Chlorine displaces an unknown halide but bromine does not. Which halide is most consistent with the evidence?
Give me a hint
Chlorine can displace Br⁻ and I⁻, while bromine displaces I⁻ only.
Compare my reasoning
- If bromine does not displace it, iodide is less likely.
- Chlorine displacing it is consistent with bromide.
- The combined pattern supports Br⁻, subject to controlled conditions.
Bromide is most consistent: Cl₂ displaces Br⁻, while Br₂ does not displace Br⁻ from its own salt.
Look for these in your work
- Both observations used
- Species matched to trend
- Conditions qualified
A sample gives a yellow precipitate but the ammonia response is not recorded. What can you conclude and what evidence is missing?
Give me a hint
Yellow is consistent with AgI, but complete the solubility sequence.
Compare my reasoning
- Yellow precipitate suggests AgI.
- Without ammonia results, the full AgCl/AgBr/AgI pattern is incomplete.
- Repeat under controlled conditions and record dilute and concentrated ammonia behavior.
Iodide is suggested, but the missing ammonia results leave the identification incomplete; record the full controlled solubility sequence.
Look for these in your work
- Tentative conclusion stated
- Missing evidence identified
- Next observation specified
Common mistakes
- Using the oxidising order to rank halide reducing strength
- Treating a displacement test as proof of one exact halide
- Ignoring false positives from contaminated silver tests
- Claiming a unique identity from one color observation
What can you explain now?
Which pair of tests directly distinguishes chloride, bromide and iodide precipitates?
Compare with the explanation
Silver ions followed by ammonia
Color and ammonia solubility together distinguish AgX precipitates.
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Make it stick
Which observation tests X₂ oxidising strength, which tests X⁻ identity, and how do you combine them without overclaiming?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.