IGCSE Chemistry (0620)

Tests for cations

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

Key ideas and common mistakes

Different metal ions produce characteristic flame colours due to their unique emission spectra, so confusing which ion gives which colour (e.g., mistaking sodium's yellow for lithium's red) is a common error.
Calcium hydroxide is not amphoteric, so its white precipitate does not dissolve in excess sodium hydroxide.
Recognising that heating an ammonium salt with an alkali releases ammonia gas, which is alkaline and turns damp red litmus paper blue.
Common mistake: Students often forget the gas produced or its effect on indicators.
Recognise that adding aqueous sodium hydroxide to a solution containing only sodium and nitrate ions produces no visible change, because all possible products (sodium salts, sodium hydroxide) are soluble and do not form a precipitate.
To identify copper(II) ions, add aqueous sodium hydroxide and observe a light blue precipitate that remains insoluble in excess.
Common mistake: The trap is forgetting that some metal hydroxides (like zinc or aluminium) redissolve, but copper(II) does not.
Recognising that adding a small amount of ammonia to Cu2+ forms an insoluble hydroxide precipitate, but adding excess ammonia dissolves it by forming a soluble deep blue complex.
Common mistake: The trap is not realising the precipitate can redissolve in excess reagent.

Name the metal ion that produces a red flame colour in a flame test.

Try it first, then show the working

Answer: lithium

21 exam-style questions on Tests for cations, each marked to the scheme.