SEC G3 Combined Science Chemistry component K326/K328
C6: Qualitative Analysis
Apply the official cation, anion, and gas tests through reagent, condition, observation, and 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 an unknown by following an exact reagent and condition, recording the observation, and then making only the inference that observation supports.
Testing cations
With aqueous sodium hydroxide, and form white precipitates soluble in excess; forms a white precipitate insoluble in excess; light blue, green, and red-brown precipitates are insoluble in excess. Warm with sodium hydroxide to release ammonia, which turns damp red litmus blue.
With aqueous ammonia, gives white insoluble precipitate, no precipitate, and a white precipitate soluble in excess. forms light blue precipitate that dissolves in excess to a dark blue solution; and precipitates remain insoluble. Complex-ion formulae are not required.
Testing anions
Carbonate plus dilute acid releases carbon dioxide, confirmed with limewater. For chloride, acidify with dilute aqueous nitric acid, then add aqueous silver nitrate; a white precipitate forms. For sulfate, acidify with dilute aqueous nitric acid, then add aqueous barium nitrate; a white precipitate forms.
For nitrate, warm with aluminium and aqueous sodium hydroxide. Ammonia is produced and turns damp red litmus paper blue.
Testing gases
Ammonia turns damp red litmus blue; carbon dioxide gives a white precipitate with limewater that dissolves in excess carbon dioxide; chlorine bleaches damp litmus; hydrogen gives a pop with a lighted splint; oxygen relights a glowing splint; sulfur dioxide turns aqueous acidified potassium manganate(VII) from purple to colourless.
State both the test and result. A colour change without its starting reagent, condition, or confirmation is incomplete evidence.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: An aqueous sample forms a light-blue precipitate with sodium hydroxide. It is insoluble in excess. What ion is indicated?
- Record the precipitate colour as light blue.
- Record that excess sodium hydroxide does not dissolve it.
- Match both observations to .
Answer: Copper(II) ions are indicated.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Testing cations with the named terms, evidence, and causal links kept distinct.
- Use Testing anions to interpret the evidence given and justify each conclusion.
- Apply Testing gases to a new example, then check the conclusion against the information given.
Official outcome coverage
K326 C6: 3 mapped outcomes, references C6(a), C6(b), C6(c). Check the official K326 syllabus.
K328 C6: 3 mapped outcomes, references C6(a), C6(b), C6(c). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use a four-column record: test, reagent and condition, observation, inference. Add reagents slowly with mixing, state when a change occurs, and record precipitate colour, solubility and gas-test evidence without replacing an observation with an ion name.
Exam traps and retrieval check
Avoid these traps
- Writing an ion name without an observation.
- Using dry litmus for a gas test.
- Omitting warming from the ammonium or nitrate test.
Check from memory
Which gas relights a glowing splint?
Oxygen.
What confirms carbonate after acid is added?
The gas turns limewater milky.
Which cation gives a dark-blue solution in excess ammonia?
Copper(II).
Official Combined Science scope
This shared Combined Chemistry owner serves both K326 and K328. The prescribed cation, anion, and gas tests using reagent, condition, observation, and inference.