The shape of the lesson
By the end of the lesson, learners can
- Running a chromatogram: Every detail of the set-up has a reason.
- Rf values and locating agents: Extended.
- Baseline in pencil; solvent level below it.
- One spot = pure; several spots = mixture.
- Same height as a known substance = same substance.
- Extended: Rf = spot distance ÷ solvent front distance; locating agent shows colourless spots.
One picture every idea returns to
In paper chromatography spots of the samples are placed on a pencil baseline, and the paper stands in a solvent whose level is below the baseline. The solvent moves up the paper and carries each substance a different distance, depending on its solubility in the solvent and its attraction to the paper. A pure substance gives one spot; a mixture gives several. An unknown is identified when its spots match those of known substances at the same heights. Extended: colourless substances are made visible with a locating agent, and each substance has an Rf value = distance moved by the substance ÷ distance moved by the solvent front.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Separate the colours, identify the spots.. A spot of ink on paper, a little solvent, and ten minutes later the mixture has spread into its separate components. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Running a chromatogram | Every detail of the set-up has a reason. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Rf values and locating agents | Extended. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Rf values and locating agents, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Measure both distances from the baseline, not from the bottom of the paper, and measure to the centre of the spot. | Rf is spot ÷ solvent front, so it can never be bigger than 1: if you get 2.5, you divided the wrong way round. A substance that gives one spot in one solvent may still be a mixture; a second solvent can split it. |
Chromatography: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Give three precautions when setting up paper chromatography, with a reason for each.
A food colouring gives three spots. Two of them are at the same heights as spots from the permitted dyes E102 and E133. What can you conclude?
(Extended) On a chromatogram the solvent front is 8.5 cm from the baseline. Spot S is 3.4 cm from the baseline. Calculate the Rf value of S.
(Extended) Amino acid T has an Rf value of 0.35 in a particular solvent. In a run with the same solvent, the solvent front moves 6.0 cm. How far from the baseline will the spot for T be?
(Extended) Amino acids are colourless. Explain how their positions on a chromatogram are found.
Explain why different substances in a mixture travel different distances up the paper.
Answer key and marking notes
- Draw the baseline in pencil, because ink would dissolve in the solvent.
- Keep the solvent level below the baseline, so the samples do not dissolve into the solvent in the beaker.
- Cover the beaker, so the solvent does not evaporate from the paper (or use small spots so they do not merge).
- The food colouring is a mixture of at least three substances.
- It contains E102 and E133, plus one other substance that is not either of these.
- Rf = distance moved by substance ÷ distance moved by solvent front
- = 3.4 ÷ 8.5 = 0.40
- Distance = Rf × solvent front distance
- = 0.35 × 6.0 = 2.1 cm
- The dried chromatogram is sprayed with a locating agent.
- The locating agent reacts with the amino acids to form coloured spots that can be seen and measured.
- Each substance has a different solubility in the solvent and a different attraction to the paper.
- More soluble substances (less attracted to the paper) are carried further by the solvent.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The chromatogram shows an unknown mixture U and three known dyes P, Q and R, run in the same solvent. Which dyes does U contain?
- AP and R
- BP and Q
- CQ and R
- DP, Q and R
Why is the starting line on a chromatogram drawn in pencil rather than ink?
- Apencil does not dissolve in the solvent, but ink would separate and move up the paper
- Bpencil helps the solvent rise up the paper
- Cpencil reacts with the dyes to make them visible
- Dink would stop the solvent reaching the spots
When setting up paper chromatography, why must the solvent level be below the baseline?
- Aotherwise the samples would dissolve into the solvent in the beaker
- Botherwise the solvent would not move up the paper
- Cotherwise the paper would tear
- Dotherwise the solvent front would be too high
A sample of a drug is run on a chromatogram and gives one spot. A second sample of the same drug gives three spots. What can be concluded?
- Athe first sample may be pure; the second sample is impure
- Bboth samples are pure
- Cthe first sample is impure; the second is pure
- Dthe second sample contains three different drugs that are all pure
In a chromatogram of a black ink, one spot stays on the baseline. What does this show?
- Athat substance is insoluble in the solvent used
- Bthat substance is the most soluble in the solvent
- Cthat substance is pure
- Dthe solvent front has not moved
Use the chromatogram. The solvent front is 7.0 cm from the baseline, and the higher spot of U is 4.9 cm from the baseline. What is the Rf value of this component?
- A0.70
- B1.4
- C0.30
- D4.9
Amino acid X has an Rf value of 0.30 in a certain solvent. On a chromatogram using this solvent, the solvent front is 6.0 cm from the baseline. How far from the baseline is the spot for X?
- A1.8 cm
- B20 cm
- C5.7 cm
- D4.2 cm
A mixture of colourless amino acids is separated by paper chromatography. What must be done before the spots can be seen?
- Aspray the dried chromatogram with a locating agent
- Brun the chromatogram with water as the solvent
- Cdraw the baseline in ink
- Dheat the paper until the amino acids melt
Quiz key
U has two spots: one at the same height as P and one at the same height as R, so U contains P and R. No spot of U lies at the height of Q, so Q is absent, which rules out every option that includes Q. Matching heights only works because all four samples were run on the same paper with the same solvent.
Graphite in pencil is insoluble in the solvent, so the line stays put. Ink contains soluble dyes that would dissolve and run up the paper, adding spots that confuse the result. Pencil does not affect how fast the solvent rises, does not react with the dyes, and ink would not block the solvent.
If the spots are under the solvent, the samples dissolve into the bulk solvent in the beaker instead of being carried up the paper, and the chromatogram is spoiled. The solvent still rises by soaking up the paper either way. The solvent level has nothing to do with tearing, and the solvent front is decided by how long the paper is left, not the starting level.
A pure substance gives a single spot, so the first sample is consistent with being pure; the second sample gives three spots, so it contains at least three substances and is impure. Three spots cannot come from a pure sample, and one spot is not a sign of impurity. A mixture of three substances is not a pure sample, whatever the substances are.
A substance moves up the paper only if it dissolves in the solvent, so a spot left on the baseline is insoluble in that solvent (a different solvent might move it). The most soluble substance travels furthest, not least. Staying on the baseline says nothing about purity. The other spots moved, so the solvent did move.
Rf = 4.9 ÷ 7.0 = 0.70. 1.4 divides the solvent distance by the spot distance; Rf can never be more than 1. 0.30 is 1 − 0.70, which is the fraction of the distance the spot did not travel. 4.9 is just the distance moved, in cm, not the ratio.
Distance moved by X = Rf × distance moved by solvent = 0.30 × 6.0 = 1.8 cm. 20 cm divides 6.0 by 0.30, which is impossible because the spot cannot pass the solvent front. 5.7 cm subtracts 0.30 from 6.0. 4.2 cm is (1 − 0.30) × 6.0, the gap between the spot and the front.
A locating agent reacts with colourless substances such as amino acids to form coloured spots that can be seen and measured. Changing the solvent alters how far the spots move but does not make them visible. An ink baseline would add extra coloured spots, ruining the result. Heating to melt the amino acids would not make them coloured.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.