Teaching Chromatography

Chromatography is one of the most satisfying topics to teach. The practical is cheap, quick and visually striking; the underlying idea connects to intermolecular forces and polarity; and it links directly to real-world forensic and medical analysis. But students often come away able to calculate an Rf value without really understanding why substances separate. This guide suggests a sequence that builds understanding first and technique second.

Learning goals

By the end of the topic, students should be able to:

1. Explain that chromatography separates substances by their different distribution between a stationary phase and a mobile phase.
2. Carry out paper (and at higher levels, thin-layer) chromatography correctly.
3. Calculate and interpret Rf values.
4. Use chromatograms to identify components of mixtures and assess purity.
5. (Advanced) Relate separation to polarity and intermolecular forces, and describe GC and HPLC.

1. The hook: a mystery

Start with a question rather than a definition. Some options:

• The ransom note: a note written in black felt-tip was found; three suspects’ pens are available. Which pen wrote it? (The technique itself is explained in paper chromatography.)
• Sweet colours: do two brands of coloured sweets use the same dyes?
• Is black really black? Put a dot of black water-soluble ink on wet filter paper and watch the colours spread outwards.

The black ink separating into blue, purple and yellow almost always produces a genuine “wow”, which motivates the explanation that follows.

2. Explaining separation

The central idea: each substance divides its time between sticking to the stationary phase and travelling with the mobile phase. Substances that spend more time in the mobile phase travel further.

Analogies help, provided you discuss their limits:

• The shopping-street analogy: a crowd walks along a street lined with shops. People who love shopping keep stopping and fall behind; people who don’t care about shops keep walking and get further. The street is the mobile phase; the shops are the stationary phase.
• The river and rocks: different items floating down a river snag on rocks to different extents.

Then link to chemistry: whether a substance “stops” depends on how strongly it’s attracted to the stationary phase compared with how well it dissolves in the solvent. At higher levels, connect this to intermolecular forces and polar vs nonpolar molecules.

3. The practical: making it work reliably

Trial the practical with your actual pens and paper first. Tips:

• Use water-soluble felt-tips for water as solvent, or permanent markers with an alcohol-based solvent. Test which inks separate well.
• Chromatography paper gives sharper results than filter paper, but coffee filters work for demonstrations.
• Pencil baseline about 1.5–2 cm from the bottom; solvent depth well below it.
• Small, concentrated spots: dab, dry, dab again, rather than one big blob.
• Lid on the container to saturate the atmosphere with solvent vapour.
• Remove before the solvent reaches the top, and mark the solvent front immediately in pencil.
• Food colourings make good known references for sweet-colour investigations.

Common practical failures and fixes:

Problem	Likely cause	Fix
Spots dissolve into the solvent in the tank	Baseline below solvent level	Lower solvent depth
Streaky, smeared spots	Spots too large or overloaded	Smaller, more concentrated spots
Uneven solvent front	Paper touching the container sides; no lid	Hang paper centrally; cover
Nothing separates	Wrong solvent for the ink	Try an alcohol-water mixture

4. Common misconceptions

Address these explicitly, perhaps as true/false statements at the start of a lesson:

• “The solvent carries the heaviest dyes the shortest distance.” Separation depends on attraction and solubility, not simply mass.
• “The number of colours you see is the number of substances.” Colourless substances are invisible without a locating agent or UV light.
• “One spot means it’s pure.” Two substances could have the same Rf in one solvent; test in a second solvent.
• “Rf depends on how long you leave it.” Rf is a ratio, so it stays constant for a substance under the same conditions, even though distances change over time.
• “The pencil line is just for neatness.” It marks where distances are measured from, and pencil doesn’t dissolve.

5. Rf values

Introduce Rf as the answer to a problem: “Different groups’ chromatograms ran for different times. How can we compare them?” Students realise that a ratio solves the problem.

Rf = distance moved by substance ÷ distance moved by solvent front

Practice builds fluency: see Rf value calculations and the chromatography practice questions.

Emphasise measuring to the centre of each spot, from the baseline, and that Rf has no units and is always less than 1.

6. Extending to TLC and instrumental methods

For older students:

• Thin-layer chromatography on silica plates, visualised under UV light or with iodine, is fast and used for checking reaction progress and product purity. See thin-layer chromatography.
• Column chromatography separates larger quantities for purification. See column chromatography.
• Gas chromatography and HPLC use the same principle with a gas or pressurised liquid as the mobile phase, identifying components by retention time and measuring amounts by peak area. See gas chromatography and HPLC.

A good bridging question: “How is a gas chromatogram like a paper chromatogram turned on its side and measured in time instead of distance?”

7. Real-world context

• Forensic ink and drug analysis. See forensic chemistry.
• Checking food colourings are permitted.
• Drug testing in sport. See how drug tests work.
• Separating plant pigments (chlorophylls and carotenoids) from spinach leaves: a beautiful extension practical using a nonpolar solvent.

Differentiation ideas

• Support: provide a pre-drawn chromatogram with a ruler printed on it for Rf practice; give a writing frame for explaining separation.
• Stretch: ask students to predict the order of separation of compounds of different polarity on silica; compare results in two solvents; interpret a GC trace.

A three-lesson plan

1. Lesson 1: mystery hook, the idea of two phases, first paper chromatogram of inks, observations only.
2. Lesson 2: food dye investigation with reference spots, Rf calculations, identifying components.
3. Lesson 3: purity and second solvents, links to TLC, GC and HPLC, exam-style questions.

Assessment questions

1. Explain why the baseline is drawn in pencil and above the solvent level.
2. A spot moves 4.5 cm when the solvent front moves 7.5 cm. Calculate Rf.
3. Describe how you would use chromatography to show that a food colouring contains a banned dye.
4. Explain why a single spot doesn’t prove a substance is pure.
5. (Advanced) Explain, in terms of intermolecular forces, why a nonpolar compound has a higher Rf than a polar one on silica with a nonpolar solvent.

Key takeaways

• Hook students with a mystery before defining terms.
• Explain separation as distribution between two phases, using analogies with discussed limits.
• Trial the practical and teach the key technique points explicitly.
• Tackle misconceptions about mass, colour, purity and Rf directly.
• Connect paper chromatography to TLC, GC and HPLC and to real forensic and food uses.