Answers · Crystallisation
Crystallisation — final check key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. CK1 · Final check
Model answer. No. Concentrated describes relatively much solute per amount of solution. Test whether more of that solute can dissolve, with stirring and time at fixed temperature. If added solute dissolves completely, it was unsaturated; if some remains after enough stirring and time, it is saturated at that temperature.
A full answer includes
- I rejected the claim that concentrated always means saturated.
- I described adding solute, stirring and allowing time at fixed temperature.
- I interpreted complete dissolving versus remaining added solid.
Watch for
- “A clear solution must be unsaturated.” — Clear means transparent; it doesn’t tell us whether any more can dissolve.
2. CK2 · Final check
Model answer. Stop heating and leave the main solution to cool and stand. Less of this salt can remain dissolved as it cools; some forms crystals and some stays in liquid solution. Keeping liquid allows growth from solution; strong heating can damage hydrated crystals. The cooled drop is a readiness test, not an exact hot saturation measurement. Some water left as vapour while the salt stayed in the basin, making the remaining solution more concentrated.
A full answer includes
- I stopped heating and let the main solution cool and stand.
- I explained crystal formation while some salt remains in liquid solution.
- I kept liquid for growth and avoided damage from strong heating.
- I treated the drop as a readiness test, not an exact hot saturation measurement.
- I accounted for water lost and salt retained during concentration.
Watch for
- “The main basin must be heated until no liquid remains.” — Keep liquid for crystal growth, then separate it by filtering.
3. CK3 · Final check
Model answer. Gently evaporate some water to concentrate the solution; test a cooled drop. When it forms crystals, stop heating and let the main solution cool and stand. Filter the cooled crystals/solution mixture: solid crystals stay on paper, while water and still-dissolved salt form the filtrate. Blot crystals between fresh papers to remove surface solution, preserving water of crystallisation; avoid direct strong heating.
A full answer includes
- I described gentle concentration, a cooled-drop test, stopping heat, then cooling and standing.
- I filtered crystals as residue and kept water with dissolved salt in the filtrate.
- I blotted surface solution while preserving water of crystallisation.
Watch for
- “All salt is collected and the filtrate is pure water.” — Some salt remains dissolved and passes with water; only the solid crystals are retained.