Lesson overview
Paper chromatography: What is hiding in the green dye?
Draw the start line, spot a green dye beside two known dyes and decide how deep the water goes. Predict, lift the paper when you choose, mark the front, then read the ruler and work out Rf. Finally, find the substances in a sample that looks colourless.
Suggested sequence
- Lesson 1 · Can one colour hide several substances? Prepare and develop the mystery dye. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · What can the pattern tell you? Compare spots and question purity. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Measure travel, then reveal what you cannot see. Measure Rf and locate colourless substances. 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 one colour hide several substances?
- Explain and repair a valid coloured paper chromatography method
Part 2: What can the pattern tell you?
- Interpret unknown/reference matches and purity evidence under stated conditions
Part 3: Measure travel, then reveal what you cannot see
- Calculate and interpret Rf from a shared baseline and equal units
- Describe colourless chromatography and locating after development
Misconceptions to listen for
- “The sample spots should start submerged.” — Only the lower paper edge touches the reservoir. Solvent rises to the spots; submerging them can wash sample off.
- “The cover keeps the pencil baseline in place.” — Pencil graphite stays in place in the chosen solvent. The cover instead reduces solvent evaporation.
- “Dry the paper before marking the solvent front.” — The wet boundary may disappear as solvent evaporates. Mark it immediately after lifting, before drying.
- “No movement means the pigment is pure.” — A stationary spot may mean an unsuitable solvent or an insoluble pigment. It can’t establish purity or count the components.
- “One visible spot guarantees purity.” — One spot may contain overlapping substances or conceal invisible components. Two separated spots establish a mixture.
- “The same colour identifies the same substance.” — Colour alone is insufficient. Different spot heights on the same paper under the same conditions don’t support a match.
- “Rf uses distances from the bottom paper edge, or front divided by spot.” — Subtract the baseline offset from both edge readings. Divide spot-centre travel by front travel with equal units; the ratio is unitless.
- “The locating agent is the solvent that separates the substances.” — The suitable solvent separates substances during development. A locating agent reveals their positions on the removed dry paper afterwards.
Scope
0620 Extended12.3.1–4: suitable solvent, coloured and colourless paper chromatography, known/unknown comparison, purity evidence and Rf. Bounded12.4.2 method choice. Illustrative data; no real practical or chemical identity certification.