Qualitative Analysis (QA)
Qualitative analysis hub: observation-to-inference method, ion and gas tests, key reagents and exam wording. Includes quizzes.
Learning goals
- 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)).
Qualitative Analysis (QA) identifies ions and gases using prescribed tests and observations. Strong answers state the reagent and condition, record the observation, and then give the inference.
Use this hub to learn the method, memorise the key tables, then practise exam-style deductions.
Jump to: Prerequisites · Quick Reference · Core knowledge · Sub-topics · Quiz
Prerequisites
- Ions and ionic bonding
- Writing chemical equations
- Acids, bases and salts for the behaviour of common test reagents
Before you start
Recommended order:
Then practise exam-style deductions in the hub quiz and structured questions.
Revision: jump to Quick Reference, Core knowledge, then Common mistakes.
Paper 3 (Practical): use the K324 / 6092 Chemistry Practical Hub.
Section 6(a): use aqueous sodium hydroxide and/or aqueous ammonia to identify aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc ions. Section 6(b): identify carbonate, chloride, iodide, nitrate and sulfate ions. Section 6(c): identify ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide gases.
Quick Reference
- NaOH(aq) (dropwise, then in excess)
- NH₃(aq) (dropwise, then in excess)
- dilute HNO₃(aq) (acidify first)
- AgNO₃(aq) (chloride and iodide)
- Ba(NO₃)₂(aq) (sulfate)
- limewater (carbon dioxide)
| Task | High-scoring rule / sequence |
|---|---|
| Answer order | Observation → inference (include reagent + condition in the observation) |
| What to write | Precipitate colour + soluble/insoluble in excess; gas test result in mark-scheme wording |
| Cations (aq) | Add NaOH(aq) dropwise, then in excess; repeat with 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 (white ppt 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) |
| Gas tests | Use the correct named test (and use damp litmus paper) |
Core knowledge to remember
- Observation: what you see (colour change, precipitate, effervescence, gas test).
- Inference: what the observation means (ion/gas present).
- Precipitate (ppt): insoluble solid formed in a reaction mixture.
- “In excess”: keep adding reagent to see if ppt dissolves.
- Acidify first (when told): prevents false positives, especially from carbonate.
- Gas test wording: “limewater turns milky” and “bleaches damp litmus paper”.
- Cations workflow: NaOH(aq) dropwise → in excess; repeat with NH₃(aq).
Sub-topics
QA Method (Observation → Inference)
How to write answers that score marks.
Tests for Cations
NaOH(aq) / NH3(aq) patterns for the seven required cations.
Tests for Anions
Exact reagent sequences and observations for five required anions.
Gas Tests
The six required gases, named tests and exact observations.
Common mistakes
- Writing the ion/gas with no observation.
- Forgetting “dropwise then in excess”.
- Using the wrong acid (e.g., HCl(aq) before AgNO₃(aq)).
- Using dry litmus paper for gases.
- Writing “limewater turns white” instead of “turns milky (white ppt forms)”.
- Missing the key differentiator (ppt dissolves vs insoluble in excess).
- Missing precipitate colour (it often distinguishes ions).
- Forgetting to state soluble/insoluble in excess (that’s the mark).
- Forgetting to warm for the NH₄ + and NO₃⁻ tests.
- Mixing up gas tests: H₂ pop; O₂ relights; CO₂ limewater; NH₃ red litmus blue; Cl₂ bleaches; SO₂ decolourises acidified manganate(VII).