Answers · Crystallisation
Crystallisation — return questions key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. CR1 · Return
Model answer. Saturated means as much solute is dissolved as can dissolve at the specified temperature. Concentrated means relatively much solute per amount of solution and need not mean saturated.
A full answer includes
- I defined saturation at a specified temperature.
- I distinguished concentration from the ability to dissolve more.
Watch for
- “A clear solution must be unsaturated.” — Clear means transparent; it doesn’t tell us whether any more can dissolve.
2. CR2 · Return
Model answer. Partial evaporation removes some water but keeps the salt, concentrating the solution. Cooling then reduces how much of this salt can stay dissolved, so some crystallises while some remains dissolved.
A full answer includes
- I explained that partial evaporation removes some water while keeping salt.
- I explained that cooling reduces how much of this salt stays dissolved; some crystallises and some remains dissolved.
Watch for
- “Evaporation and cooling both remove all the water.” — Evaporation removes some water; cooling leaves water liquid while some solute crystallises.
3. CR3 · Return
Model answer. Dissolved solute passes through the paper, so first form crystals by concentrating and cooling. Filter the cooled mixture to retain solid crystals, then blot them between fresh papers to remove adhering liquid.
A full answer includes
- I explained that dissolved salt would pass through ordinary paper before crystals form.
- I formed crystals, filtered the cooled mixture and blotted away surface liquid.
Watch for
- “Filtering works before and after crystallisation in the same way.” — Before crystals form the desired salt is dissolved; afterwards a solid portion can be retained.