Qualitative analysis
Tests for cations, anions and gases, the reagents they use, and how to turn observations into inferences for G3 Pure and O-Level Chemistry.
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Before you begin
Given an unknown salt or gas, how do you find out what it is? This topic starts with the method every test shares: fresh portions, the exact reagent and condition, what you observe, and the conclusion it supports. It then applies it to the tests for seven cations, five anions and six gases.
Be comfortable with: ions and ionic bonding, writing chemical equations, and acids, bases and salts.
Learning goals
- Use aqueous sodium hydroxide and aqueous ammonia to identify aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc ions
- Describe the tests for carbonate, chloride, iodide, nitrate and sulfate ions
- Describe the tests for ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide
Syllabus statements covered
- describe the use of aqueous sodium hydroxide and/or aqueous ammonia to identify the following aqueous cations through the formation of precipitates (if any) and their subsequent solubility: aluminium, ammonium (together with evolution of ammonia gas upon warming), calcium, copper(II), iron(II), iron(III) and zinc (formulae of complex ions are not required)
- describe tests to identify the following anions: carbonate (by the addition of dilute acid and subsequent use of limewater); chloride (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate); iodide (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate); nitrate (by reduction with aluminium in aqueous sodium hydroxide to ammonia and subsequent use of damp red litmus paper) and sulfate (by reaction of an aqueous solution with nitric acid and aqueous barium nitrate)
- describe tests to identify the following gases: ammonia (using damp red litmus paper); carbon dioxide (using limewater); chlorine (using damp litmus paper); hydrogen (using a burning splint); oxygen (using a glowing splint) and sulfur dioxide (using acidified potassium manganate(VII)).
Lessons
Work through them in order.
Method
Cations, anions and gases
- Qualitative analysis: tests for cationsIdentify seven cations from the precipitates they form with aqueous sodium hydroxide and aqueous ammonia, and why each test works.
- Qualitative analysis: tests for anionsTest for carbonate, chloride, iodide, nitrate and sulfate, and why the acid must come first.
- Qualitative analysis: gas testsThe tests for six gases, why each one works, and how to report the result.
Practise and check
Or choose
Topic reference
Quick reference
| Task | Method and evidence |
|---|---|
| Answer order | Observation → inference (include reagent + condition in the observation) |
| What to write | Precipitate colour + soluble/insoluble in excess; the named gas test and its visible result |
| Cations (aq) | Add NaOH(aq) dropwise, then in excess; use a fresh portion for NH₃(aq) |
| Anions | Follow the prescribed sequence; chloride, iodide and sulfate tests require acidification first |
| Acidify rule | Use dilute HNO₃(aq); don’t use HCl(aq) before AgNO₃(aq); don’t use H₂SO₄(aq) before sulfate tests |
| Chloride / iodide | Acidify with dilute HNO₃(aq), then add AgNO₃(aq) |
| Sulfate | Acidify with dilute HNO₃(aq), then add Ba(NO₃)₂(aq) |
| Carbonate | Add dilute acid → effervescence; CO₂(g) turns limewater milky (a white precipitate forms) |
| Ammonium (NH₄⁺) | Add NaOH(aq), warm, test NH₃(g) with damp red litmus (turns blue) |
| Nitrate (NO₃⁻) | Add NaOH(aq) + aluminium foil, warm, then test for NH₃(g); check for ammonium interference with a no-aluminium control |
| Gas tests | Use the correct named test (and use damp litmus paper) |
Core knowledge to remember
- An observation is what you see; an inference is the ion or gas it shows. Write both.
- A precipitate is an insoluble solid formed when two solutions are mixed.
- The cations: aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc, identified with aqueous sodium hydroxide and aqueous ammonia, each added a few drops at a time and then in excess.
- The anions: carbonate, chloride, iodide, nitrate and sulfate. Acidify with dilute nitric acid before the chloride, iodide and sulfate tests.
- The gases: ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide, each with its own named test.
- Use a fresh portion of the unknown for each test.
Common mistakes
- Writing the ion or gas with no observation.
- Leaving out what happens when excess reagent is added.
- Leaving out the colour of a precipitate.
- Acidifying with hydrochloric acid before a chloride test, or sulfuric acid before a sulfate test.
- Forgetting to warm in the ammonium and nitrate tests.
- Using dry litmus paper, or writing that limewater “turns white”.
- Mixing up the gas tests: hydrogen, lighted splint, pop; oxygen, glowing splint, relights.