Identifying Unknown Substances
Systematic qualitative analysis approach for IGCSE Chemistry 0620, covering cation tests, anion tests, gas tests, and flame tests for Paper 6.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Why this skill matters
Qualitative analysis forms a substantial portion of Paper 6 and also appears as structured questions on Papers 3 and 4. The tests are systematic: if you know the procedure and the expected observations, you can identify any ion the syllabus requires. These marks are among the most predictable on the exam.
Method and technique
Cation identification (metal ions in solution)
Add sodium hydroxide (NaOH) solution to the unknown solution:
| Metal ion | Precipitate with NaOH(aq) | Effect of excess NaOH |
|---|---|---|
| Cu2+ | Blue precipitate | Insoluble (does not dissolve) |
| Fe2+ | Green precipitate | Insoluble |
| Fe3+ | Red-brown precipitate | Insoluble |
| Al3+ | White precipitate | Dissolves (soluble in excess) |
| Ca2+ | White precipitate | Insoluble |
| Zn2+ | White precipitate | Dissolves (soluble in excess) |
| Mg2+ | White precipitate | Insoluble |
Key distinction: Al3+ and Zn2+ both give white precipitates that dissolve in excess NaOH. To distinguish them, use ammonia solution: Al(OH)3 is insoluble in excess ammonia, but Zn(OH)2 dissolves.
Anion identification
Carbonate (CO3 2-): Add dilute acid. Effervescence (bubbles). The gas turns limewater milky. Conclusion: CO2 produced, confirming carbonate.
Sulfate (SO4 2-): Add dilute nitric acid, then barium nitrate solution. A white precipitate (BaSO4) forms that is insoluble in acid.
Halides (Cl-, Br-, I-): Add dilute nitric acid (to remove carbonate interference), then silver nitrate solution.
- Cl-: white precipitate (AgCl)
- Br-: cream precipitate (AgBr)
- I-: yellow precipitate (AgI)
Nitrate (NO3-): Add sodium hydroxide solution and aluminium foil (or Devarda’s alloy), warm gently. Ammonia gas is produced, which turns damp red litmus paper blue.
Flame tests
Dip a clean nichrome wire in concentrated hydrochloric acid, then into the solid sample, and hold in a blue Bunsen flame:
| Ion | Flame colour |
|---|---|
| Li+ | Crimson / red |
| Na+ | Yellow / orange |
| K+ | Lilac / purple |
| Ca2+ | Orange-red / brick red |
| Cu2+ | Blue-green |
Gas tests
See the separate testing for gases page for complete gas test methods.
A systematic approach
When faced with an unknown on Paper 6:
- Note the appearance: colour of the solid or solution gives initial clues (blue solution suggests Cu2+, green suggests Fe2+ or Ni2+).
- Flame test: quick screen for Group I/II metals and copper.
- Dissolve in water if possible. If insoluble, test the solid directly.
- Add NaOH(aq): precipitate colour and behaviour in excess identifies the cation.
- Test for anions using the appropriate reagent.
Common errors and how to avoid them
Confusing precipitate colours: Green is Fe2+, red-brown is Fe3+, blue is Cu2+. Mixing these up is the most common mistake.
Forgetting to add acid before barium nitrate: The acid prevents false positives from carbonate or sulfite ions, which also form white precipitates with barium ions but dissolve in acid.
Not cleaning the flame test wire: Sodium contamination gives a persistent yellow colour that masks other flame colours. Clean the wire in HCl and heat until no colour is seen before testing the next sample.
Marking points examiners look for
- Correct reagent named for each test
- Specific observations described (precipitate colour, gas test result)
- Conclusion correctly matched to the observation
- Distinction between ions that give similar results (Al3+ vs Zn2+ using excess NaOH or NH3)
- Flame test wire cleaned between tests
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