IGCSE Chemistry (0620)

Tests for anions

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

Distinguishing halide ions by the color of the precipitate formed with acidified silver nitrate.
Common mistake: Students often confuse which halide gives white, cream, or yellow, e.g., mixing up bromide (cream) with chloride (white) or iodide (yellow).
Sulfate ions react with barium ions to form insoluble barium sulfate, so the observation to record is a white precipitate, with the nitric acid added first to avoid other precipitates.
Recognise that adding acidified silver nitrate to a solution containing iodide ions produces a yellow precipitate of silver iodide, not a white or cream one, which distinguishes it from chloride or bromide ions.
Recognising that carbon dioxide gas, produced by the reaction of a carbonate with acid, turns limewater milky.
Common mistake: Students who miss this may confuse the gas with others or not recall the specific test for carbon dioxide.
Recognize that only chloride ions (not calcium or nitrate) react with acidified silver nitrate to form a white precipitate of silver chloride, so the observation depends on identifying the correct reacting ion.
To confirm sulfate ions, acid must be added before barium nitrate so that only sulfate (not carbonate or sulfite) forms a white precipitate.
Common mistake: Students who skip acidification may mistake other ions' precipitates for sulfate.

State the observation the learner would record for this test.

Try it first, then show the working

Answer: A cream precipitate forms.

18 exam-style questions on Tests for anions, each marked to the scheme.