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P3 General Studies: Dissolving and Separation (Paper 3 (challenge))
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P3 General Studies: Dissolving and Separation (Paper 3 (challenge))
Paper three trains multi-step reasoning: separate then calculate, concentration after evaporation, unchanged concentration after pouring half away, crystallisation from a solubility gap, and the division of labour between separation methods.
Direct answers Write the three lines for solute, solvent and solution masses before substituting into any formula, so the solution mass is never mistaken for the mass of water.
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: P3 General Studies: Dissolving and Separation |
| Level | parts of a closed circuit, conductors against insulators, series against parallel, adding up battery voltage, and electrical safety |
02 Questions (10 items)
- FoundationWhat is the first step to separate a mixture of sand and salt solution?
- FoundationWhat is the difference between dissolving and melting?
- FoundationWhat is a saturated solution?
- IntermediateAt 20 degrees Celsius, 100 g of water dissolves at most 36 g of salt. If 20 g of salt is added to 50 g of water, how much stays undissolved?
- IntermediateA 300 g salt solution contains 45 g of salt. After 100 g of water evaporates with no salt crystallising out, what percentage of the solution is salt?
- IntermediateAt 20 degrees Celsius, 136 g of saturated salt solution contains 36 g of salt. Half is poured away; what percentage of the remaining solution is salt?
- ChallengeSalt makes up 15 percent of a 200 g mixture of sand and salt. After dissolving, filtering and evaporating, how much salt is collected?
- ChallengeSalt makes up 3.5 percent of 400 g of sea water. If all the water is evaporated, how much salt is left?
- ChallengeA salt dissolves 20 g per 100 g of water at 10 degrees and 60 g at 60 degrees. An 80 g water saturated solution at 60 degrees is cooled to 10 degrees. How much salt crystallises out?
- Error redoA classmate says a saturated solution can dissolve nothing else. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Filter it, so the sand stays on the filter paper. | Sand does not dissolve, so filtering removes it and leaves a clean salt solution. |
| 2 | Dissolving spreads particles through a solvent; melting changes a solid into a liquid by heating. | Dissolving needs a solvent, while melting only changes the state through heat. |
| 3 | A solution that cannot dissolve any more of the same solute at that temperature. | Adding more of the same solute only leaves it at the bottom; the concentration no longer rises. |
| 4 | 2.0 g | Fifty grams of water dissolves only 18 g, so twenty minus eighteen leaves 2.0 g. |
| 5 | 22.5 percent | The solution is now 200 g, so forty-five divided by two hundred gives 22.5 percent. |
| 6 | 26.5 percent | The remainder is 50 g of water plus 18 g of salt, so eighteen over sixty-eight gives 26.5 percent. |
| 7 | 30.0 g | Two hundred times zero point one five is 30.0, and the separation steps do not change the mass of salt. |
| 8 | 14.0 g | Four hundred times zero point zero three five equals 14.0. |
| 9 | 32.0 g | The drop is forty grams per 100 g of water, so 80 g of water gives 32.0 g. |
| 10 | It is saturated only for that solute at that temperature; other solutes can still dissolve and heating allows more. | Saturation refers to one solute at one temperature, not to every substance. |
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.
Paper four clears the conductor, insulator and battery direction misconceptions.Tutor Registration →
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