Qualitative analysis: ions and tests

Qualitative analysis is detective work: you identify the ions in an unknown substance from the colours, precipitates and gases its reactions produce. The notes are printed in the exam, so the skill is careful observation and logical deduction — not memorising every result.

Testing for cations

Add sodium hydroxide, then ammonia, and watch the precipitate. Different metal ions give precipitates of different colours, and a key discriminator is whether the precipitate redissolves in excess — aluminium and zinc hydroxides do, while others (like copper or iron) do not. Iron(II) gives a green precipitate, iron(III) a red-brown one, copper(II) a blue one.

Testing for anions

Each anion has a signature test: carbonate fizzes with acid, releasing carbon dioxide; chloride gives a white precipitate with silver nitrate; iodide a yellow one; sulfate a white precipitate with barium nitrate; and nitrate is confirmed by warming with aluminium and alkali to release ammonia.

Testing for gases

Identify gases by a quick test: carbon dioxide turns limewater milky; hydrogen gives a squeaky pop with a lit splint; oxygen relights a glowing splint; ammonia turns damp red litmus blue; chlorine bleaches damp litmus. (K324 does not test sulfur dioxide identification in the practical paper.)

Deduce, don't guess

The exam reward is method: record exactly what you observe (colour, precipitate, gas), then reason to the ion using the printed notes. Students lose marks by deciding the ion first and bending the observation to fit. Work one test at a time and let the evidence lead.