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S4 Chemistry: Separation and Purification Paper 2 (distillation, chromatography and purity)
S4 Chemistry: Separation and Purification Paper 2 (distillation, chromatography and purity)
This paper concentrates on calculations: Rf, concentration change, recovery and purity, each written out clearly.
Quick answer Rf 0.40; fractions collected by boiling point; simple distillation for pure water; a magnet separates iron powder; 0.30 to 0.48 g/cm3; 0.40 and 0.60, 3 cm apart; 94.0%; 33%; 5.6 cm and 10.4 cm; brine cannot be filtered.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | 25 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry: Separation and Purification |
| Level | S4 |
02 Questions (10 items)
- FoundationA spot lies 2.8 cm from the start line and the solvent front is 7.0 cm from it. What is the Rf value?
- FoundationOn what property are the fractions of petroleum collected in order?
- FoundationWhich method obtains pure water from sea water?
- FoundationWhat is the most direct way to separate a mixture of iron powder and sulphur powder?
- IntermediateAn 80 cm3 solution contains 24 g of solute. Heating removes 30 cm3 of water, leaving 50 cm3. What are the concentrations before and after, in g/cm3?
- IntermediateOn one chromatogram the solvent front is 15.0 cm from the start. Spot A is 6.0 cm and spot B 9.0 cm from the start. What are their Rf values and their separation in cm?
- IntermediateA 12.5 g sample contains 0.75 g of impurity. What is its purity, to one decimal place?
- ChallengeFractional distillation of 800 cm3 of a liquid mixture collects 264 cm3 of distillate. What is the recovery?
- ChallengeA chromatogram runs 16.0 cm from the start to the solvent front and a spot has Rf 0.35. How far is the spot from the start and from the front?
- Error redoA student writes that filtration separates the salt from brine. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 0.40 | Two point eight divided by 7.0 gives 0.40, and Rf is always below 1. |
| 2 | Boiling point, from lowest to highest | The top of the column is coolest, so gases and petrol with low boiling points come off first, then kerosene, diesel and heavy oil. |
| 3 | Simple distillation | Heating vaporises the water and leaves the salt behind; the vapour is condensed to distilled water, whereas filtration only removes insoluble solids. |
| 4 | Use a magnet to lift out the iron powder | Iron is magnetic and sulphur is not, so a magnet separates them because the components are not chemically combined. |
| 5 | From 0.30 to 0.48 g/cm3 | The solute mass is unchanged, so 24 over 80 is 0.30 and 24 over 50 is 0.48. |
| 6 | A is 0.40, B is 0.60, 3.0 cm apart | Six over 15.0 gives 0.40 and 9.0 over 15.0 gives 0.60, while the spots are 9.0 minus 6.0, that is 3.0 cm, apart. |
| 7 | 94.0% | The pure mass is 12.5 minus 0.75, that is 11.75 g, which over 12.5 times 100 gives 94.0%. |
| 8 | 33% | Two hundred sixty-four over 800 times 100 gives 33%, with the rest left in the flask or lost. |
| 9 | 5.6 cm from the start and 10.4 cm from the front | Sixteen times 0.35 gives 5.6 cm, and 16.0 minus 5.6 gives 10.4 cm, since Rf is measured from the start line. |
| 10 | The salt is dissolved and passes through the filter paper with the water; evaporating or crystallising gives the salt back, and distillation gives the water. | Filtration only separates an insoluble solid from a liquid; dissolved salt and water form one phase that filter paper cannot split. |
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.
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