Identifying ions
GCSE Chemistry (8462) · Required practical 7 — method, variables, the marks examiners report students losing.
Identify the ions in unknown compounds using flame tests, metal-hydroxide precipitates, and tests for carbonate, sulfate and halide ions.
Apparatus
- Nichrome / platinum wire loop and Bunsen burner
- Test tubes and rack
- Dilute sodium hydroxide solution
- Dilute hydrochloric acid and limewater
- Barium chloride solution; silver nitrate solution with dilute nitric acid
Method
- 1Flame test: clean the wire loop in acid, dip it in the sample and hold it in a blue Bunsen flame; note the colour.
- 2Metal-hydroxide test: add sodium hydroxide solution and note the precipitate colour; test whether it dissolves in excess.
- 3Carbonate test: add dilute acid — carbonates fizz, and the carbon dioxide turns limewater cloudy.
- 4Sulfate test: add dilute hydrochloric acid, then barium chloride solution — a white precipitate shows sulfate.
- 5Halide test: add dilute nitric acid, then silver nitrate solution — white (chloride), cream (bromide) or yellow (iodide).
Results & processing
- Flame colours: lithium red, sodium yellow, potassium lilac, calcium orange-red, copper green.
- Hydroxide precipitates: copper(II) blue, iron(II) green, iron(III) brown; aluminium, calcium and magnesium white (aluminium hydroxide redissolves in excess NaOH).
Where students lose marks
Confusing the flame colours, especially sodium (yellow) and potassium (lilac).
Fix: View a potassium flame through blue cobalt glass to mask any yellow sodium colour; learn the full set.
Forgetting which white hydroxide dissolves in excess sodium hydroxide.
Fix: Aluminium hydroxide (white) redissolves in excess NaOH; calcium and magnesium hydroxides (white) do not.
Not acidifying before the sulfate or halide test.
Fix: Add dilute acid first (HCl for sulfate, HNO3 for halide) to remove carbonate ions that would give a false positive precipitate.
Improve the method
- Clean the flame-test loop between samples so colours aren't carried over.
- Do the anion tests in a set order (carbonate, then sulfate, then halide) so reagents don't interfere.
Try it — exam-style
Adding sodium hydroxide solution to a solution gives a white precipitate, which dissolves when more sodium hydroxide is added. Identify the metal ion present.
Describe the test for sulfate ions and the positive result.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on identifying ions?
Ion tests are pure recall with an order that matters — I drill the sequence and the results, and your first lesson is free.