6.2.4Unit 6. Practical Skills in Chemistry II
A-Level Qualitative Analysis — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 14 Unit quiz
Part 1 of 5Tests for Anions
Always add acid first to remove carbonates that would interfere.
Carbonate
- Adding dilute acid gives effervescence, and the gas turns limewater milky, showing carbon dioxide.
Sulfate
- Adding dilute hydrochloric acid and then barium chloride solution gives a white precipitate of barium sulfate.
Chloride, bromide, iodide
- Acidify with dilute nitric acid and add silver nitrate: white, cream and yellow precipitates form respectively.
Ammonia solution test
- The chloride precipitate dissolves in dilute ammonia, the bromide in concentrated ammonia and the iodide does not dissolve.
| Halide ion | Precipitate with AgNO₃ | Effect of ammonia |
|---|---|---|
| Cl⁻ | White | Dissolves in dilute NH₃ |
| Br⁻ | Cream | Dissolves in concentrated NH₃ |
| I⁻ | Yellow | Insoluble |
Part 2 of 5Tests for Cations
Tests for Cations
Flame tests
- Sodium gives yellow, potassium lilac, calcium brick-red and barium apple-green flame colours.
Sodium hydroxide
- Copper(II) gives a blue precipitate, iron(II) a green one that turns brown, and iron(III) a brown precipitate.
Amphoteric hydroxide
- Aluminium hydroxide forms a white precipitate that dissolves in excess sodium hydroxide.
Ammonium ions
- Warming with sodium hydroxide releases ammonia, which turns damp red litmus paper blue.
Part 3 of 5Organic Functional Group Tests
Alkene
- Bromine water changes from orange to colourless when shaken with an alkene.
Aldehyde or ketone
- 2,4-dinitrophenylhydrazine gives a bright orange precipitate.
Aldehyde only
- Warming with Tollens' reagent gives a silver mirror, and Fehling's solution gives a red precipitate.
Carboxylic acid
- Sodium carbonate gives effervescence, while a primary or secondary alcohol turns acidified dichromate from orange to green.
Part 4 of 5Reporting Observations
1
Describe what you see, not what you think, such as 'white precipitate' rather than 'barium sulfate'.
2
Record colour of solutions, precipitates, gases and any change after excess reagent is added.
3
Use test paper or a lit splint only to identify gases that you can name, with the result.
4
Write an ionic equation with state symbols, such as Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
5
State a conclusion linking the observation to the ion or functional group.
Part 5 of 5Exam Checklist & Tips
6.2.4 – Key Points for the Exam
- Precipitate An insoluble solid formed when two solutions react.
- Observation A description of what is seen, heard or smelled.
- Flame test Heating a compound in a flame and observing the characteristic colour.
- Amphoteric hydroxide A hydroxide that dissolves in both acid and excess alkali.
Common Mistakes
- Writing 'it turns blue' without stating if a solution or a precipitate forms.
- Testing for sulfate without acidifying first, so carbonate gives a false positive.
- Mixing up the ammonia solubility for chloride and bromide precipitates.
- Using sulfuric acid to acidify before the barium test, which itself contains sulfate.
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