The shape of the lesson
By the end of the lesson, learners can
- Two phases, one separation: Each method below has the same two ingredients.
- Rf values on the plate: An Rf value is a ratio, so it has no units and always lies between 0 and 1.
- Gas-liquid chromatography and peak areas: In GLC the detector draws a chromatogram: one peak per component, plotted against time from injection.
- Rf = spot distance ÷ solvent front distance, both from the baseline.
- TLC: adsorption onto silica or alumina; GLC: partition into a liquid.
- Retention time identifies a GLC peak, under fixed conditions.
- Peak area ÷ total area × 100 gives percentage composition.
One picture every idea returns to
All chromatography separates a mixture between a stationary phase, which stays put, and a mobile phase, which moves through it. A component that interacts more strongly with the stationary phase moves more slowly. In thin-layer chromatography (TLC) the stationary phase is silica or alumina on a plate and separation is mainly by adsorption; each spot has an Rf value = distance moved by the spot ÷ distance moved by the solvent front. In gas-liquid chromatography (GLC) an inert carrier gas carries the vapourised sample through a column coated with a non-volatile liquid, and separation is by partition. Each component has a characteristic retention time, and the area under each peak is proportional to the amount of that component, so relative peak areas give the percentage composition.
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, then measure.. Every chromatography method shares a stationary phase and a mobile phase. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Two phases, one separation | Each method below has the same two ingredients. Project tab 1 of the lab (Chromatography and NMR lab, TLC and Rf) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Rf values on the plate | An Rf value is a ratio, so it has no units and always lies between 0 and 1. Project tab 1 of the lab (Chromatography and NMR lab, TLC and Rf) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Gas-liquid chromatography and peak areas | In GLC the detector draws a chromatogram: one peak per component, plotted against time from injection. Project tab 2 of the lab (Chromatography and NMR lab, gas-liquid chromatography) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 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, 16 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 Gas-liquid chromatography and peak areas, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| Rf is spot distance ÷ solvent front distance, never ÷ the length of the plate, and both are measured from the baseline. | In GLC the amount of a component is given by the peak area, not its height, and the percentage uses the total of all peak areas. A long retention time means strong interaction with the stationary phase, not a large amount of substance. |
Chromatography: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
A mixture is separated by TLC. The solvent front moves 7.5 cm and one spot moves 2.7 cm. Calculate the Rf value of the spot.
State what is meant by the stationary phase and the mobile phase in gas-liquid chromatography, and explain why components leave the column at different times.
A GLC chromatogram of a fuel sample shows three peaks with areas 2.4, 5.6 and 8.0 cm2. Calculate the percentage of each component, assuming equal detector response.
A student runs a TLC plate of a mixture of amino acids. The spots are colourless. Describe two ways of locating them.
A food dye gives two spots with Rf 0.250 and 0.700. Pure dyes X, Y and Z run on the same plate have Rf 0.700, 0.420 and 0.250. Deduce which dyes are present and explain why the result is not absolute proof.
Explain why the solvent level must be below the baseline and why the tank is covered while the plate develops.
Answer key and marking notes
- Rf = distance moved by spot ÷ distance moved by solvent front.
- Rf = 2.7 ÷ 7.5 = 0.360.
- Stationary phase: a non-volatile liquid coated on a solid support inside the column.
- Mobile phase: an unreactive carrier gas such as helium or nitrogen.
- Components partition between gas and liquid; one more soluble in the liquid (or less volatile) spends longer in the stationary phase, so it has a longer retention time.
- Total area = 2.4 + 5.6 + 8.0 = 16.0 cm2.
- Percentages: 2.4 ÷ 16.0 × 100 = 15.0%; 5.6 ÷ 16.0 × 100 = 35.0%.
- 8.0 ÷ 16.0 × 100 = 50.0%.
- Spray with ninhydrin and warm the plate: amino acids show as purple spots.
- Or use a plate with a fluorescent indicator and shine UV light on it: spots appear dark.
- Iodine vapour (brown spots) is a general alternative.
- The spots match X (0.700) and Z (0.250); Y (0.420) is absent.
- Matching Rf values in one solvent suggest identity because each compound has a characteristic Rf for that solvent and stationary phase.
- Two different compounds could share an Rf in one solvent, so a second solvent (or another technique) is needed to confirm.
- If the solvent covers the baseline the sample dissolves into the bulk solvent instead of moving up the plate.
- Covering keeps the air saturated with solvent vapour, so the solvent does not evaporate from the plate and the front moves evenly.
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.
In a TLC experiment the solvent front moves 6.0 cm from the baseline and a spot moves 4.8 cm. What is the Rf value of the spot?
- A0.800
- B1.25
- C0.200
- D4.80
What is the stationary phase in thin-layer chromatography?
- Aa thin layer of silica or alumina on a flat sheet
- Bthe solvent that moves up the plate
- Can inert carrier gas such as nitrogen
- Da non-volatile liquid coated on a solid support in a column
A mixture of amino acids is separated by chromatography. The spots are colourless. Which method makes them visible as purple spots?
- Aspraying with ninhydrin and warming
- Bexposing the plate to iodine vapour
- Cadding acidified potassium dichromate(VI)
- Dadding aqueous silver nitrate
The gas-liquid chromatogram shows three alcohols, P, Q and R, with the peak areas given under each letter. What is the percentage of Q in the mixture, assuming equal detector response?
- A45.0%
- B85.0%
- C33.3%
- D52.9%
What is meant by the retention time in gas-liquid chromatography?
- Athe time between injecting the sample and the component's peak reaching the detector
- Bthe time taken for the carrier gas to pass through the column
- Cthe area under a peak in the chromatogram
- Dthe time for which the sample is heated before injection
A GLC analysis of a solvent mixture gives three peaks with areas 1.2, 3.0 and 1.8 cm2. The third peak is propanone. What is the percentage of propanone in the mixture?
- A30.0%
- B1.80%
- C60.0%
- D42.9%
A mixture of a non-polar hydrocarbon and a polar alcohol is separated by TLC on a silica plate with a non-polar solvent. Which statement explains the result?
- Athe alcohol has the lower Rf because it is adsorbed more strongly onto the polar silica
- Bthe alcohol has the higher Rf because it is more soluble in the solvent
- Cboth have the same Rf because they are separated by partition
- Dthe hydrocarbon has the lower Rf because it has the larger Mr
A student finds that an unknown dye has Rf 0.600 on one plate, and that pure dye X has Rf 0.600 on a different plate run with a different solvent. Which conclusion is valid?
- Ano conclusion: Rf values can only be compared for the same solvent and stationary phase, ideally on the same plate
- Bthe unknown is definitely X, because the Rf values are equal
- Cthe unknown is definitely not X, because different solvents were used
- Dthe unknown is X only if both spots are the same colour
Quiz key
Rf = 4.8 ÷ 6.0 = 0.800. 1.25 is the ratio upside down, which is impossible because a spot can never travel further than the solvent. 0.200 is (6.0 − 4.8) ÷ 6.0, measuring the spot from the front instead of the baseline. 4.80 is just the spot distance, which has units; Rf is a ratio with no units.
In TLC, silica (SiO2) or alumina spread on glass, metal or plastic is the stationary phase, and components separate mainly by adsorption onto it. The solvent is the mobile phase. A carrier gas is the mobile phase in GLC, and a liquid on a solid support is the stationary phase in GLC, not TLC.
Ninhydrin reacts with amino acids to give purple (blue-violet) spots. Iodine vapour is a general locating agent, but it gives brown spots. Acidified dichromate(VI) is a test for oxidisable alcohols and aldehydes, and silver nitrate tests for halide ions; neither is used to locate amino acids.
Total area = 15 + 45 + 40 = 100, so Q = 45 ÷ 100 × 100 = 45.0%. 85.0% reads the peak height instead of the area. 33.3% assumes three components are present in equal amounts just because there are three peaks. 52.9% is 45 ÷ (45 + 40), leaving out peak P.
Retention time is measured from injection to the maximum of a component's peak. Under fixed conditions (column, temperature, gas flow rate) it identifies a component by comparison with a standard. The carrier gas itself is not detected, peak area measures amount, not identity, and heating time is not a defined quantity.
Total area = 1.2 + 3.0 + 1.8 = 6.0 cm2; 1.8 ÷ 6.0 × 100 = 30.0%. 1.80% treats the area as a percentage. 60.0% is 1.8 ÷ 3.0, comparing with the largest peak only. 42.9% is 1.8 ÷ (1.2 + 3.0), leaving propanone out of the total.
The alcohol can hydrogen bond to the polar silica surface, so it is held back and has a low Rf. The non-polar hydrocarbon is barely adsorbed and dissolves well in the non-polar solvent, so it travels near the front. Polar molecules are less soluble in a non-polar solvent, not more. TLC on silica separates mainly by adsorption, and different interactions give different Rf values. Mr is not the main factor here.
An Rf value is characteristic of a compound only for a given solvent and stationary phase, so values from different solvents cannot be compared. The fair test is to spot the unknown and X side by side on the same plate. Equal values from different systems prove nothing either way, and colour alone is not reliable evidence because different dyes can look alike.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
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.