Adding sodium hydroxide solution to a solution gives a white precipitate, which dissolves when more sodium hydroxide is added. Identify the metal ion present.
[1 mark]
Total for this question: 1
Chemistry required practicals
Identifying ions
AQA 4.8 · RP7
GCSE Chemistry (8462) · Required practical 7 — method, variables, the marks examiners report students losing.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
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
Medium
ORIGINAL
Medium
ORIGINAL
Describe the test for sulfate ions and the positive result.
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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Ion tests are pure recall with an order that matters — I drill the sequence and the results. Free intro call, then a free first lesson.