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S4 Chemistry: Ion Tests, Flame Tests and Precipitation (Paper 2: Intermediate)
S4 Chemistry: Ion Tests, Flame Tests and Precipitation (Paper 2: Intermediate)
Ten items on telling zinc from aluminium, avoiding false positives by acidifying, and calculating dilution concentration and precipitate mass.
Direct answers Answers in order: use ammonia solution, it does not dissolve, warming releases ammonia, it would add sulphate, remove carbonate, 0.020 mol dm-3, iron(III), chloride and bromide, 2.92 g, no.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry: Ion Tests, Flame Tests and Precipitation |
| Level | S4 |
02 Questions (10 items)
- FoundationZinc ions and aluminium ions both give a white precipitate with sodium hydroxide. How are they distinguished?
- FoundationWhat happens to the white precipitate when excess sodium hydroxide is added to calcium ions?
- FoundationWhy is warming necessary when testing for ammonium ions?
- IntermediateWhy is dilute hydrochloric acid used rather than dilute sulphuric acid when testing for sulphate ions?
- IntermediateWhy acidify with dilute nitric acid before adding silver nitrate solution?
- IntermediateWhat is the concentration after 10.0 cm3 of 0.50 mol dm-3 solution is diluted to 250 cm3?
- ChallengeA solution gives a reddish-brown precipitate with sodium hydroxide and a blood-red colour with potassium thiocyanate. Which ion is present?
- ChallengeTwo halides give a white and a pale yellow silver salt; the white one dissolves in dilute ammonia and the pale yellow one only in concentrated ammonia. What are they?
- ChallengeWhat mass of barium sulphate forms when 0.0125 mol of sulphate is completely precipitated? (molar mass of BaSO4 is 233.4 g mol-1)
- Error redoDoes a white precipitate with sodium hydroxide prove that calcium ions are present?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Both precipitates dissolve in excess sodium hydroxide; with ammonia solution the zinc precipitate dissolves in excess but the aluminium one does not. | Zinc hydroxide is amphoteric and also forms a complex with ammonia, whereas aluminium hydroxide does not dissolve in excess ammonia. |
| 2 | It does not dissolve and remains. | Calcium hydroxide is not amphoteric, so excess sodium hydroxide does not dissolve it. |
| 3 | Warming drives ammonia gas out so that the colour change on the litmus paper can be seen. | At room temperature most ammonia stays in solution, so warming is needed to release it as a gas. |
| 4 | Dilute sulphuric acid would itself introduce sulphate ions and give a false positive. | The acid added already contains sulphate, so a sample without sulphate would also give a white precipitate. |
| 5 | To remove interfering ions such as carbonate that would also give a precipitate with silver ions. | Silver carbonate is also white, and acidifying destroys it so that only silver halide remains. |
| 6 | 0.020 mol dm-3 | The number of moles is unchanged: 0.010 dm3 times 0.50 gives 0.0050 mol, divided by 0.250 dm3 gives 0.020 mol dm-3. |
| 7 | Iron(III) ions | The reddish-brown hydroxide and the blood-red thiocyanate complex are both characteristic of iron(III). |
| 8 | The white one is a chloride and the pale yellow one is a bromide. | Silver chloride dissolves readily in dilute ammonia while silver bromide needs a more concentrated solution. |
| 9 | 2.92 g | Mass equals moles times molar mass: 0.0125 times 233.4 gives 2.92 g. |
| 10 | No, zinc, aluminium and lead ions also give white precipitates. | A white precipitate is a shared observation, so solubility behaviour or a flame test is still needed. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
When an item is wrong, write down which reagent should be added and what should be seen.Tutor Registration →
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