Qualitative analysis (QA) is the detective work of O-Level Chemistry: you are given an unknown solution and must identify the ions in it purely from observations. It is tested every year, in the Paper 3/4 practical and in structured deduction questions.
Everything reduces to three routines (cations, anions and gases) plus the discipline of writing observations in exam-approved phrasing. The phrasing itself carries marks, so the reagent, the condition and the exact observation all have to be stated.
What these notes cover
- •Cation tests with NaOH(aq) and NH₃(aq)
- •The master cation table
- •Ionic equations for precipitation
- •Why some precipitates dissolve in excess
- •The test for the ammonium ion
- •Anion tests: carbonate, sulfate, chloride, iodide, nitrate
- •Why acidify with dilute nitric acid
- •Gas tests: the three-step routine
- •The six gases you must know
- •Deduction skills and distinguishing pairs
- •Mistakes that cost marks and the exam-technique checklist
- •The whole topic on one page
Key definitions
- Qualitative analysis
- Identifying the ions in an unknown solution purely from observations, using fixed tests for cations, anions and gases.
- Spectator ions
- Ions that appear unchanged and aqueous on both sides of an equation. They are crossed out to leave the ionic equation.
- Amphoteric hydroxide
- A hydroxide that can react with both acids and alkalis. Zn(OH)₂ and Al(OH)₃ are amphoteric, so they dissolve in excess NaOH(aq) to form a soluble salt and a colourless solution.
- Observation
- Something you can see, hear or smell: the colour, the state (precipitate, effervescence, gas) and what happens in excess. A negative observation such as “no precipitate formed” is still an observation.
- False positive
- A result caused by an ion other than the one being tested for. For example, carbonate or hydroxide ions give white precipitates with Ba²⁺ or Ag⁺, which can be wrongly read as sulfate or chloride.
Key formulas
Precipitation of a metal hydroxide
M²⁺(aq) + 2OH⁻(aq) → M(OH)₂(s)
M is the metal cation. For a 3+ cation: M³⁺(aq) + 3OH⁻(aq) → M(OH)₃(s). The OH⁻ ions come from NaOH(aq) or NH₃(aq).
Ammonium ion with warm NaOH(aq)
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
The ammonia gas turns moist red litmus paper blue. This is also the final step of the nitrate test, after aluminium foil reduces NO₃⁻ to NH₄⁺.
Acid with carbonate
2H⁺(aq) + CO₃²⁻(aq) → CO₂(g) + H₂O(l)
Any dilute acid gives effervescence. The CO₂ must then be confirmed with limewater.
Limewater test for carbon dioxide
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
Limewater is aqueous calcium hydroxide. The white precipitate is calcium carbonate.
Sulfate test
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Acidify with dilute nitric acid, then add aqueous barium nitrate. A white precipitate of BaSO₄ forms. Na⁺ and NO₃⁻ are spectator ions.
Chloride and iodide tests
Ag⁺(aq) + Cl⁻(aq) → AgCl(s); Ag⁺(aq) + I⁻(aq) → AgI(s)
Acidify with dilute nitric acid, then add aqueous silver nitrate. AgCl is a white precipitate and AgI is a yellow precipitate.
Worked example
A colourless solution W gives these results. With NaOH(aq) added slowly, a white precipitate forms. With NaOH(aq) in excess, the precipitate dissolves to give a colourless solution. With NH₃(aq) added slowly to a fresh sample, a white precipitate forms. With NH₃(aq) in excess, the precipitate does not dissolve. Identify the cation in W, explaining how each observation narrows down the candidates.
- 1A white precipitate with NaOH(aq) narrows the candidates to Ca²⁺, Zn²⁺ or Al³⁺. The colour rules out Cu²⁺ (blue), Fe²⁺ (green) and Fe³⁺ (reddish-brown), and the fact that any precipitate formed rules out NH₄⁺.
- 2The precipitate dissolves in excess NaOH(aq), which rules out Ca²⁺ because Ca(OH)₂ is insoluble in excess NaOH(aq), leaving Zn²⁺ and Al³⁺.
- 3The white precipitate with NH₃(aq) is insoluble in excess, which rules out Zn²⁺ because Zn(OH)₂ would have dissolved in excess NH₃(aq) to give a colourless solution.
- 4The only cation consistent with all four observations is Al³⁺. The white precipitate in both tests is aluminium hydroxide: Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s).
Watch out: Never use NaOH(aq) alone to distinguish Zn²⁺ from Al³⁺. Both give a white precipitate that is soluble in excess, so the observations are identical.
Common mistakes
Writing “no reaction”
Wrong: No reaction occurs.
Right: No visible reaction / No observable change. You can only report what you observe, and a negative observation still earns its mark.
Claiming aluminium hydroxide dissolves in excess ammonia
Wrong: The white precipitate of Al(OH)₃ dissolves in excess NH₃(aq).
Right: White precipitate, insoluble in excess NH₃(aq) (for Al³⁺). Al(OH)₃ dissolves only in excess NaOH(aq). Among the white precipitates, only Zn(OH)₂ dissolves in excess of both reagents.
Acidifying with the wrong acid before an anion test
Wrong: Add dilute hydrochloric acid, then aqueous silver nitrate.
Right: Add dilute nitric acid, followed by aqueous silver nitrate. A white precipitate of silver chloride indicates chloride ions.
Questions students ask
How do you test for ammonium ions in O-Level Chemistry?
Add NaOH(aq); no precipitate forms. Warm the mixture gently. A colourless, pungent gas is given off which turns moist red litmus paper blue, confirming ammonia and hence NH₄⁺ ions. Hold the litmus at the mouth of the test tube, not in the solution.
How do you distinguish Zn²⁺ from Al³⁺ in qualitative analysis?
Add aqueous ammonia slowly, then in excess. Both ions give a white precipitate, but only Zn(OH)₂ dissolves in excess NH₃(aq) to give a colourless solution, while Al(OH)₃ stays insoluble. NaOH(aq) cannot separate them because both precipitates dissolve in excess NaOH(aq).
Why is dilute nitric acid added before the chloride and sulfate tests?
It removes carbonate and hydroxide ions, which would otherwise form white precipitates with Ag⁺ or Ba²⁺ and give a false positive. Hydrochloric or sulfuric acid cannot be used because they add the very chloride or sulfate ions being tested for. Nitric acid is safe because all nitrates are soluble.
What is the test for hydrogen and oxygen gas?
Hydrogen extinguishes a lighted splint with a ‘pop’ sound, and oxygen relights a glowing splint. Both gases are colourless, odourless and have no effect on moist litmus, so the splint test is what tells them apart. Swapping the lighted and glowing splint loses the mark.
In the full PDF
- •The complete master cation table for all seven cations, plus the one-page cation grid and memory anchor
- •The full anion test table, including the nitrate test with aluminium foil and the caution when ammonium ions are also present
- •The six-gas table (colour, odour, effect on moist litmus, confirmatory test), including the acidified potassium manganate(VII) test for SO₂
- •Further worked examples on cations, anions, gases and deduction, such as identifying ammonium iodide and planning a test for zinc nitrate vs calcium nitrate
- •The remaining five mark-losing mistakes and the full exam-technique checklist
- •The heated test-tube set-up diagram, the section summaries and the whole-topic summary on one page