Qualitative Analysis (QA)
Qualitative analysis hub: observation-to-inference method, ion and gas tests, key reagents and exam wording. Practise the topic with its check.
Before you begin
Qualitative analysis uses chemical tests to identify ions and gases. Start with the method: state the reagent and condition, record the observation, then give the inference. The later lessons apply it to cations, anions and gases.
Be comfortable with: ions and ionic bonding, writing chemical equations and acids, bases and salts, for how common test reagents behave.
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)).
Lessons
Work through them in order.
Method
Cations, anions and gases
- Qualitative Analysis: Tests for CationsIdentify the seven required cations from their reactions with aqueous sodium hydroxide and ammonia.
- Qualitative Analysis: Tests for AnionsIdentify the five required anions using the correct reagent sequence and observations.
- Qualitative Analysis: Gas TestsTest for the six required gases and state each positive result.
Practise and check
Recommended nextQualitative Analysis topic check
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Topic reference
What this topic covers
- Cations: aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc.
- Anions: carbonate, chloride, iodide, nitrate and sulfate.
- Gases: ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide.
Quick Reference
Match each reagent to its test
- NaOH(aq) (dropwise, then in excess)
- NH₃(aq) (dropwise, then in excess)
- dilute HNO₃(aq) (acidify first for chloride, iodide and sulfate)
- AgNO₃(aq) (chloride and iodide)
- Ba(NO₃)₂(aq) (sulfate)
- limewater (carbon dioxide)
| 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
- 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 whether the precipitate dissolves.
- Acidify first (when told): prevents false positives, especially from carbonate.
- Gas test wording: “limewater turns milky” and “bleaches damp litmus paper”.
- Cations workflow: sodium hydroxide on one fresh portion, ammonia on another; each dropwise, then in excess.
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 (a white precipitate forms)”.
- Missing the key differentiator (precipitate dissolves vs insoluble in excess).
- Missing precipitate colour (it often distinguishes ions).
- Omitting what happens when excess reagent is added; an initial white precipitate may fit several ions.
- 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).