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S4 Chemistry: Separation and Purification Paper 1 (filtration, distillation and chromatography)
S4 Chemistry: Separation and Purification Paper 1 (filtration, distillation and chromatography)
The choice of separation method depends on solubility, boiling point and adsorption on the stationary phase.
Quick answer Rf 0.40; a 22 degree gap; sand and water by filtration; copper sulphate by crystallisation; 24.0%; 0.50 g/cm3; A and B share an Rf; 0.40 and 0.60; 36%; Rf is not upside down.
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)
- FoundationIn a chromatogram a spot moves 3.6 cm from the start line while the solvent front moves 9.0 cm. What is its Rf value?
- FoundationEthanol boils near 78 C and water near 100 C. Distillation exploits a boiling point gap of how many degrees?
- FoundationWhich method separates a mixture of sand and water?
- FoundationWhich method recovers blue crystals from copper sulphate solution?
- IntermediateA 25.0 g mixture contains 6.0 g of sand and the rest soluble salt. After filtration, washing and drying, 6.0 g of sand remains. What is the percentage of sand?
- IntermediateA 100 cm3 solution contains 20 g of solute. Heating removes 60 cm3 of water, leaving 40 cm3. What are the concentrations before and after, in g/cm3?
- IntermediateOn one chromatogram sample X gives a single spot whose Rf equals that of pure substance A. What follows?
- ChallengeA solvent front moves 12.0 cm. Spot A is 4.8 cm from the start and spot B is 2.4 cm above A. What are their Rf values?
- ChallengeFractional distillation of 500 cm3 of a liquid mixture collects 180 cm3 of distillate. What is the recovery?
- Error redoA student defines Rf as solvent front distance divided by spot distance. What goes wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 0.40 | Rf is the spot distance divided by the solvent front distance, so 3.6 over 9.0 gives 0.40. |
| 2 | 22 degrees | One hundred minus 78 gives 22 degrees, so ethanol vaporises first and is condensed and collected. |
| 3 | Filtration | Sand is insoluble, so it stays on the filter paper while water passes through as the filtrate. |
| 4 | Crystallisation | Gentle heating evaporates the solvent until the solution is saturated and crystals form, which are relatively pure. |
| 5 | 24.0% | Six divided by 25.0 times 100 gives 24.0%, and washing removes adhering salt. |
| 6 | From 0.20 to 0.50 g/cm3 | The mass of solute is unchanged, so 20 over 100 is 0.20 and 20 over 40 is 0.50. |
| 7 | X is very likely A, but another solvent should confirm it. | An equal Rf is evidence of identity, yet two substances can share an Rf in one solvent. |
| 8 | A is 0.40 and B is 0.60 | A is 4.8 over 12.0, giving 0.40; B is 7.2 cm from the start, giving 0.60. |
| 9 | 36% | One hundred eighty over 500 times 100 gives 36%, with the rest left in the flask or lost. |
| 10 | Inverting it gives a value above 1; Rf must be spot distance over solvent front distance, so it is always below 1. | Rf is a ratio whose numerator never exceeds the denominator, so inverting it is meaningless. |
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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