O-Level and SEC G3 Chemistry K324
C6: Qualitative Analysis
Select a test, state the reagent and condition, record the observation, and make only the supported inference.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Qualitative Analysis identifies ions and gases from a sequence of reagents, observations, and confirmatory tests. Marks come from exact reagent names, precipitate colours, solubility in excess reagent, gas conditions, and a conclusion that follows only from the observations.
Cation tests
Add aqueous sodium hydroxide or aqueous ammonia dropwise, then in excess when required. Record whether a precipitate forms, its colour, and whether it dissolves in excess. Aluminium and zinc can both give white precipitates, so the excess-reagent pattern helps distinguish them. Calcium, copper(II), iron(II), and iron(III) have their stated K324 observation patterns.
For ammonium ions, add aqueous sodium hydroxide and warm. Ammonia gas is released and turns damp red litmus paper blue. Do not report a precipitate test for ammonium, and do not hold litmus inside a hot liquid where splashes can give a false result.
Anion tests
Carbonate ions release carbon dioxide with dilute acid; pass the gas through limewater. For chloride and iodide, acidify with nitric acid before adding aqueous silver nitrate, then use precipitate colour. For sulfate, acidify with nitric acid and add aqueous barium nitrate to obtain the specified white precipitate.
For nitrate, warm with aluminium in aqueous sodium hydroxide. The ammonia produced turns damp red litmus blue. Use the exact reagents and conditions in the question. Acidifying removes interfering ions, so replacing the named acid casually can introduce the ion being tested.
Gas tests and observation discipline
Ammonia turns damp red litmus blue; carbon dioxide turns limewater milky; chlorine bleaches damp litmus after its initial acid effect; hydrogen gives a pop with a burning splint; oxygen relights a glowing splint; sulfur dioxide decolourises acidified potassium manganate(VII).
A test has four parts: reagent or test material, condition, observation, and inference. Colourless is an observation; clear means transparent; a precipitate is a solid formed from solution. Do not infer identity from smell, and do not use unsafe direct inhalation.
Formulae and relationships
| Relationship | Use |
|---|---|
| Explain gas release in the carbonate test. | |
| Represent the chloride precipitate test. | |
| Represent the sulfate precipitate test. |
Worked examples
Example 1: An unknown solution gives a pale blue precipitate with aqueous sodium hydroxide. The precipitate is insoluble in excess. What ion is indicated, and what must be reported?
- Name the reagent: aqueous sodium hydroxide.
- Record both observations: pale blue precipitate and insoluble in excess.
- Match the complete observation pattern to the K324 cation table.
Answer: Copper(II) ions are indicated.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- State the reagent and condition before the observation and inference.
- Use precipitate colour, gas behaviour, or flame evidence precisely, including any change with excess reagent.
- Combine tests only when each step leaves the sample suitable for the next inference.
Official outcome coverage
K324 C6: 3 mapped outcomes, references C6(a), C6(b), C6(c). Check the official K324 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Practise a four-column record: test, reagent and condition, observation, inference. Never replace an observation with the name of the ion.
Exam traps and retrieval check
Avoid these traps
- Giving only the ion name without reagent and observation.
- Using dry litmus paper for a gas test that requires damp litmus.
- Calling a colourless solution clear when the distinction matters.
Check from memory
What confirms ammonium ions?
Warm with aqueous sodium hydroxide; ammonia turns damp red litmus blue.
Which gas relights a glowing splint?
Oxygen.
Why acidify before the silver nitrate test?
To remove ions that could form misleading precipitates.
Pure versus Combined scope
Combined Chemistry uses the same top-level topic label but a narrower outcome set. Use the Combined component checklist before removing or adding detail.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K324. Open the related IP explanation.