Lesson overview
Mixtures and separation: The salt disappears. Has it gone?
Follow a mixture from the first stir to the final separation. Discover what dissolves, what stays behind and how to get a substance back.
Suggested sequence
- Lesson 1 · Where did the salt go? Solvent, solute and solution. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · What gets through the filter? Residue, filtrate and dissolved salt. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · How can the salt return? Recovery and choosing a solvent. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 4 · Which separation should come first? Build and explain a 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. 6 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. 8 questions, 15 min, lesson closed.
Learning goals
Part 1: Where did the salt go?
- 12.1.3 — Solvent, solute, solution, saturation, residue and filtrate
Part 2: What gets through the filter?
- 12.1.3 — Solvent, solute, solution, saturation, residue and filtrate
- 12.4.1 — Selective solvent, filtration, crystallisation, simple/fractional distillation
Part 3: How can the salt return?
- 12.1.3 — Solvent, solute, solution, saturation, residue and filtrate
- 12.4.1 — Selective solvent, filtration, crystallisation, simple/fractional distillation
Part 4: Which separation should come first?
- 12.1.3 — Solvent, solute, solution, saturation, residue and filtrate
- 12.4.1 — Selective solvent, filtration, crystallisation, simple/fractional distillation
- 12.4.2 — Selecting and sequencing separation methods
Misconceptions to listen for
- “Salt is the solvent.” — Salt is the substance that became dissolved. The solvent is the medium it dissolved in: water.
- “Water is the solvent because every solvent is water.” — Water is the solvent in this case because salt dissolves in it. Some solutions use a different solvent.
- “The salt has become water.” — Dissolving changes how the salt is distributed. Water and the dissolved salt are still different components.
- “Invisible means absent.” — Our eyes don’t detect the dispersed salt particles individually. That limits what we can conclude from appearance.
- “There is no solvent because there is no water.” — The question tells you that iodine dissolves in ethanol. That gives ethanol the solvent role.
- “Iodine alone is the solution.” — Iodine is one component. A solution includes both the solvent and the dissolved solute.
- “Water made new salt when it was removed.” — No source of new salt is provided. The original salt remained in the solution and became visible again as water was removed.
- “It melted because crystals vanished.” — The observations describe salt dissolving in water. Losing visible crystals alone doesn’t show a solid becoming a pure liquid.
- “Every liquid mixture is a solution.” — A solution has dissolved material mixed throughout. C contains a separate undissolved solid.
- “B is pure because the sugar is invisible.” — B still contains sugar and water. Invisible dissolved material is still a component.
Scope
Solvent, solute, solution, residue and filtrate; selective dissolution, filtration and bounded method choice. The experiment includes magnetic collection and salt recovery. Saturation, full crystallisation, distillation and chromatography need later lessons.