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S5 Chemistry: Ionic Precipitation (Paper 2: Intermediate)
S5 Chemistry: Ionic Precipitation (Paper 2: Intermediate)
Ten items that take precipitation further: why acidification is needed in ion tests, hydroxide colours, identifying the limiting reagent, remaining moles and writing net ionic equations, with answers.
Direct answers Answers in order: to remove carbonate, blue copper hydroxide, barium sulphate is extremely insoluble, barium plus sulphate gives barium sulphate with sodium and chloride as spectators, potassium chloride is limiting, 0.144 g, 0.0010 mol, calcium plus carbonate gives calcium carbonate, dilute sulphuric acid gives a false positive, only soluble barium salts are toxic.
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: S5 Chemistry: Ionic Precipitation |
| Level | S5 |
02 Questions (10 items)
- FoundationWhy is dilute nitric acid added before silver nitrate when testing for chloride ions?
- FoundationWhat colour is the precipitate when sodium hydroxide solution is added to copper sulphate solution?
- FoundationWhy can barium sulphate be used as a contrast agent for stomach X-rays?
- IntermediateWrite the net ionic equation for barium chloride solution reacting with sodium sulphate solution and name the spectator ions.
- IntermediateWhen 40.0 cm3 of 0.050 M silver nitrate is mixed with 10.0 cm3 of 0.10 M potassium chloride, which is limiting?
- IntermediateFrom the previous question, what mass of silver chloride forms, given Mr = 143.5?
- ChallengeFrom that mixture, how many moles of silver ions remain after the reaction?
- ChallengeWrite the net ionic equation for sodium carbonate solution mixed with calcium chloride solution.
- ChallengeWhy is dilute hydrochloric acid used rather than dilute sulphuric acid when testing for sulphate?
- Error redoA student claims every barium compound is toxic and should never be used. How should this be corrected?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | The acid removes carbonate interference, since silver carbonate is also a white precipitate but dissolves in acid. | Only a white precipitate that survives acidification is confirmed as silver chloride, so carbonates are not mistaken for chlorides. |
| 2 | A blue precipitate of copper hydroxide. | Copper ions with hydroxide ions give blue copper hydroxide, a common test for copper ions. |
| 3 | Because it is extremely insoluble, so almost no toxic barium ions are released. | Soluble barium salts are the toxic ones; barium sulphate neither dissolves nor is absorbed, so it passes safely through the gut. |
| 4 | Ba2+(aq) + SO4^2-(aq) -> BaSO4(s); sodium and chloride ions are spectators. | Only the barium and sulphate concentrations actually change, the other ions are unchanged. |
| 5 | Potassium chloride, because it supplies only 0.0010 mol of chloride ions against 0.0020 mol of silver ions. | The limiting reagent runs out first; here chloride is short, so it fixes the amount of silver chloride. |
| 6 | 0.144 g | The limiting 0.0010 mol of chloride gives the same moles of silver chloride, 0.0010 times 143.5 equals 0.1435 g. |
| 7 | 0.0010 mol | There were 0.0020 mol of silver ions and 0.0010 mol reacted, so 0.0010 mol stays in solution. |
| 8 | Ca2+(aq) + CO3^2-(aq) -> CaCO3(s) | Calcium carbonate is insoluble and is the main component of limescale and limestone; sodium and chloride are spectators. |
| 9 | Dilute sulphuric acid itself supplies sulphate ions and would give a false positive, while hydrochloric acid does not. | Acidification is only there to destroy carbonates, so the acid chosen must not contain the ion being tested. |
| 10 | Only soluble barium salts are toxic; barium sulphate is insoluble and not absorbed, so it is widely used in laboratories and hospitals. | Toxicity depends on whether barium ions can be freed; insoluble barium sulphate releases no significant concentration of them. |
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.
Pick two items and write both the full equation and the net ionic equation, then circle the spectator ions.Tutor Registration →
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