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S4 Chemistry: Ion Tests, Flame Tests and Precipitation (Paper 3: Challenge)
S4 Chemistry: Ion Tests, Flame Tests and Precipitation (Paper 3: Challenge)
Ten items deducing unknown solutions from flame and precipitate results, writing ionic equations and calculating gas volume and precipitate mass.
Direct answers Answers in order: sodium and chloride, Cu2+ with 2OH- gives Cu(OH)2, CO3^2- with 2H+ gives CO2 and water, zinc, acid gives sulphur dioxide, 240 cm3, screen the yellow, iron(II) sulphate and concentrated acid, 0.430 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)
- FoundationSolution X gives a golden yellow flame, no precipitate with sodium hydroxide, and a white precipitate with acidified silver nitrate. Which ions are present?
- FoundationWrite the ionic equation for the reaction between copper(II) ions and sodium hydroxide solution.
- FoundationWrite the ionic equation for the reaction of a carbonate with dilute acid.
- IntermediateSolution Y gives a white precipitate with sodium hydroxide that dissolves in excess, and the precipitate also dissolves in excess ammonia. Which ion is present?
- IntermediateHow can a sulphite solution be distinguished from a sulphate solution?
- Intermediate0.010 mol of calcium carbonate reacts completely with excess dilute hydrochloric acid. What volume of carbon dioxide forms at room conditions? (molar volume 24 dm3 mol-1)
- ChallengeWhy is blue cobalt glass used when identifying potassium ions by flame test?
- ChallengeWhich reagents are needed for the brown ring test for nitrate, and how is it carried out?
- ChallengeA sample contains 0.00300 mol of chloride ions. What mass of silver chloride forms on complete precipitation? (molar mass of AgCl is 143.3 g mol-1)
- Error redoDoes milky limewater prove that the gas is carbon dioxide?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Sodium ions and chloride ions | The golden yellow flame indicates sodium and the white silver precipitate after acidifying indicates chloride. |
| 2 | Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s) | An ionic equation shows only the ions that actually react, with charge and atoms balanced. |
| 3 | CO3^2-(aq) + 2H+(aq) → CO2(g) + H2O(l) | Carbonate accepts two hydrogen ions to give carbon dioxide and water, and the gas is seen as bubbles. |
| 4 | Zinc ions | Solubility in excess of both reagents is characteristic of zinc, whereas aluminium hydroxide does not dissolve in excess ammonia. |
| 5 | Add dilute hydrochloric acid: the sulphite releases sulphur dioxide while the sulphate shows no clear reaction, and of the barium salts only barium sulphite dissolves in dilute acid. | Sulphite releases sulphur dioxide with acid and can be oxidised, whereas barium sulphate is insoluble in dilute acid. |
| 6 | 240 cm3 | The moles of carbon dioxide are also 0.010 mol, so the volume is 0.010 times 24 dm3, which is 0.24 dm3 or 240 cm3. |
| 7 | It screens out the golden yellow of sodium so that the lilac of potassium can be seen. | Even a trace of sodium gives a strong yellow that hides the weaker lilac, and cobalt glass absorbs the yellow. |
| 8 | Iron(II) sulphate solution and concentrated sulphuric acid, added slowly down the side of the tube after the iron(II) sulphate. | Mixing concentrated acid with water releases heat and reduces nitrate, forming a brown nitroso complex ring at the boundary. |
| 9 | 0.430 g | Mass equals moles times molar mass: 0.00300 times 143.3 gives 0.430 g. |
| 10 | No, sulphur dioxide also turns limewater milky. | Both gases give a white precipitate with calcium hydroxide, so a further test such as acidified dichromate is 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.
For each item write observation, deduction and confirmation in three steps before checking.Tutor Registration →
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