Identify halide ions with silver ions and ammonia
Use precipitate colors and ammonia solubility to distinguish chloride, bromide and iodide ions.
Section 11 · Group 17
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
What color is silver chloride precipitate?
What you will learn
- State the AgX precipitate colors
- Use dilute and concentrated ammonia to distinguish precipitates
- Keep the test sequence free of halide contamination
Silver ions form characteristic silver halide precipitates
For a solution test, first acidify the sample with dilute nitric acid, then add aqueous silver ions (commonly silver nitrate). Chloride gives white AgCl, bromide gives cream AgBr, and iodide gives yellow AgI. Fluoride remains in solution because AgF is soluble.
Do not acidify with hydrochloric acid: it adds chloride ions and can create a false positive. Use the same controlled reagent sequence for each unknown.
Ammonia solubility separates the silver halides
AgCl dissolves in dilute aqueous ammonia. AgBr does not dissolve in dilute ammonia but dissolves in concentrated ammonia. AgI remains insoluble in both dilute and concentrated ammonia.
The syllabus does not require the formula of the silver–ammonia complex. Record the observed precipitate color and its behavior at each ammonia concentration; one color alone is weaker evidence than the full sequence.
Pause and explain
Which silver halide dissolves in dilute aqueous ammonia?
Worked example
An acidified solution gives a cream precipitate with aqueous silver ions. The precipitate dissolves in concentrated, but not dilute, ammonia. Identify the halide.
Show the worked solution
- Cream precipitate is consistent with AgBr.
- AgBr is insoluble in dilute ammonia.
- Its dissolution in concentrated ammonia confirms bromide in this test pattern.
Answer The halide is bromide, Br⁻; AgBr is cream, insoluble in dilute ammonia and soluble in concentrated ammonia.
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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 pale yellow precipitate remains insoluble in concentrated ammonia. Identify the halide ion.
Give me a hint
Use both the precipitate color and the concentrated-ammonia result.
Compare my reasoning
- Yellow precipitate indicates AgI.
- AgI remains insoluble in concentrated ammonia.
- The halide is iodide.
Iodide, I⁻; yellow AgI remains insoluble in concentrated ammonia.
Look for these in your work
- Color matched
- Solubility behavior matched
- Ion identified
Why must the sample be acidified with dilute nitric acid rather than hydrochloric acid before adding Ag⁺?
Give me a hint
Check which anion hydrochloric acid adds to the sample.
Compare my reasoning
- HCl adds Cl⁻ ions.
- Cl⁻ would form AgCl with the test reagent.
- That could produce a false positive for chloride.
HCl adds chloride ions that form AgCl, so it can create a false positive; dilute HNO₃ avoids adding a halide ion.
Look for these in your work
- Contaminating ion named
- AgCl formation linked
- Correct acid choice explained
A white precipitate dissolves in dilute ammonia. State the likely halide and one reason to use the complete test sequence.
Give me a hint
White AgCl dissolves in dilute ammonia; compare with the other silver halides.
Compare my reasoning
- The precipitate is consistent with AgCl.
- This indicates chloride.
- The color plus ammonia response is stronger evidence than color alone, with controls still needed for a complex sample.
Chloride is indicated: white AgCl dissolves in dilute ammonia. Use the full controlled sequence because a color alone is not unique proof in an unknown mixture.
Look for these in your work
- Likely ion identified
- Ammonia response used
- Evidence limitation stated
Common mistakes
- Using HCl to acidify the sample
- Calling AgBr white or AgI cream
- Testing only with dilute ammonia and assuming every insoluble precipitate is iodide
- Claiming a silver–ammonia complex formula is required
What can you explain now?
What color is silver chloride precipitate?
Compare with the explanation
White
AgCl forms a white precipitate.
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Make it stick
What are the AgCl/AgBr/AgI colors, and how does ammonia concentration distinguish them?
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