3.2.4Unit 3. Practical Skills in Chemistry I
AS Qualitative Analysis — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 10 of 10 Unit quiz
Part 1 of 5Testing for Anions
Solid samples are dissolved in deionised water before testing.
Carbonate
- Add dilute acid; effervescence of CO₂ that turns limewater milky shows CO₃²⁻ or HCO₃⁻ is present.
Sulfate
- Acidify with dilute HCl and add barium chloride solution;
- a white precipitate of BaSO₄ shows sulfate ions: Ba²⁺ + SO₄²⁻ → BaSO₄.
Halides
- Add acidified silver nitratewhite AgCl, cream AgBr and yellow AgI form, and dilute then concentrated ammonia help to distinguish them.
Why acidify
- The acid removes carbonate ions that would otherwise give a white precipitate with barium or silver ions and cause a false positive.
Part 2 of 5Testing for the Ammonium Ion and Flame Tests
Testing for the Ammonium Ion and Flame Tests
Ammonium
- Warm the sample with sodium hydroxide; ammonia gas turns damp red litmus paper blue: NH₄⁺ + OH⁻ → NH₃ + H₂O.
Flame test method
- Clean a nichrome wire in concentrated acid, then dip it in the solid and hold it at the edge of a blue Bunsen flame.
Colours
- Lithium gives red, sodium yellow, potassium lilac, calcium brick-red and barium pale green flames.
Limitations
- A mixture of metal ions produces mixed colours, and sodium impurities can mask weaker colours, so the test works best for one ion.
| Ion | Flame colour |
|---|---|
| Li⁺ | Red |
| Na⁺ | Yellow |
| K⁺ | Lilac |
| Ca²⁺ | Brick-red |
| Ba²⁺ | Pale green |
Part 3 of 5Testing Organic Functional Groups
Alkene
- Shaking with bromine water decolourises it from orange to colourless as the C=C double bond reacts.
Alcohol
- Adding PCl₅ gives steamy white fumes of HCl, showing an –OH group is present.
Aldehyde
- Warming with Tollens’ reagent gives a silver mirror, and warming with Fehling’s solution gives a red precipitate.
Carboxylic acid
- Adding sodium carbonate solution gives effervescence of carbon dioxide because acids react with carbonates.
Part 4 of 5Recording and Reporting
Good qualitative practice relies on careful observations.
1
State the test carried out, giving the reagent and any conditions such as warming or acidifying.
2
Record the observation using colours and states, for example white precipitate, rather than words like positive.
3
Write the formula of the ion or group deduced from each observation.
4
Include the balanced or ionic equation with state symbols where the question asks for it.
5
Combine all of the tests to give a final conclusion that explains each observation.
Part 5 of 5Exam Checklist & Tips
3.2.4 – Key Points for the Exam
- Anion A negatively charged ion such as chloride or sulfate.
- Precipitate An insoluble solid that forms when two solutions are mixed.
- Flame test A test in which a metal ion gives a characteristic colour to a Bunsen flame.
- Effervescence Fizzing caused by gas bubbles escaping from a liquid.
Common Mistakes
- Writing precipitate colours as positive or negative instead of recording the actual colour seen.
- Forgetting to acidify before adding barium chloride, so carbonate ions may give a false precipitate.
- Letting the flame test wire glow red hot so that its colour is mistaken for a flame colour.
- Using red litmus without wetting it, so ammonia gas has no effect.
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