Lesson overview
Crystallisation: How can a clear solution grow crystals?
Choose a method, test the solution with a glass rod and decide when to stop heating. Let crystals grow, predict what comes through the filter, then get a second batch of crystals from the filtrate.
Suggested sequence
- Lesson 1 · Can any more dissolve? Saturation at a specified temperature. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · Why do crystals appear as it cools? Concentrate, test and cool. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Collect the crystals. Follow the liquid. Filter, blot and explain the method. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 3 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Can any more dissolve?
- Explain saturation at a specified temperature
- Distinguish concentration, saturation and appearance
Part 2: Why do crystals appear as it cools?
- Explain concentration through partial evaporation and salt retention
- Explain cooling, crystal formation and remaining solution for the selected salt
Part 3: Collect the crystals. Follow the liquid.
- Follow solid crystals and dissolved salt through filtration
- Choose and justify a complete crystallisation/collection/drying method
Misconceptions to listen for
- “A clear solution must be unsaturated.” — Clear means transparent; it doesn’t tell us whether any more can dissolve.
- “A dark solution must be saturated.” — A dark solution may still dissolve more solute. Test it at a fixed temperature.
- “Concentrated and saturated are interchangeable.” — Concentration describes an amount per amount of solution; saturation says whether more can dissolve at that temperature.
- “Heating creates more salt.” — The salt was already present. Removing some water changes its relative amount in the solution.
- “Heat until every drop of water is gone.” — Keep liquid so crystals can grow from solution; strong drying can damage hydrated crystals.
- “No crystals proves no salt remains.” — Dissolved salt can be invisible. Concentrate the same solution and try cooling again.
- “The blue crystals are frozen water.” — Cooling forms solid solute crystals; liquid water with dissolved salt remains.
- “Cooling always produces the same result for every soluble solid.” — The temperature dependence belongs to the selected solute. Read the supplied observations.
- “Evaporation and cooling both remove all the water.” — Evaporation removes some water; cooling leaves water liquid while some solute crystallises.
- “The main basin must be heated until no liquid remains.” — Keep liquid for crystal growth, then separate it by filtering.
Scope
Saturation at a specified temperature; partial evaporation, cooling, filtration and blotting hydrated crystals from a prepared copper(II) sulfate solution. Primary12.1.3(d),12.4.1(c); bounded support for12.4.2. No salt synthesis, quantitative solubility or purity measurement.