Paper Chromatography Virtual Lab: Food Colors and Rf

This paper chromatography virtual lab follows the AQA GCSE Chemistry required practical on separating colored substances and calculating Rf values, and it extends to the A-level Biology practical on leaf pigments. Students spot two candy colorings and four reference food dyes on a pencil baseline, run water up the paper, mark the solvent front and measure every spot with an on-screen ruler. They calculate Rf, match the unknowns to the references and test why the baseline must be in pencil and above the solvent. One period gives a full Rf table and a clear identification.

Curriculum links

• AQA GCSE Chemistry 4.8.1.3 (chromatography) and its required practical: separate colored substances by paper chromatography and calculate Rf = distance moved by substance ÷ distance moved by solvent.
• AQA A-level Biology 3.5.1, required practical 7: separation of photosynthetic pigments by chromatography (see Extension).
• Cambridge IGCSE Chemistry: chromatography, Rf values and identifying substances against references.

Simulic is not affiliated with or endorsed by AQA, Cambridge or any exam board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"A drop of brown candy coloring is spotted on the baseline, and water soaks up the paper. What will you see?"

Many students expect one brown spot that moves up. Some expect nothing to happen, because "brown is one color".

Method in the simulation

The simulation opens on distillation, so students switch method first.

1. Under the simulation, press Paper chromatography, then Food colors and Water. Keep Draw the baseline in pencil and Solvent level below the baseline ticked.
2. Read the lane list: A green candy, B brown candy, C to F the references E102, E110, E129 and E133.
3. Press Run. Let the solvent rise until the paper is taken out (front at 110 mm), or press Pause earlier: Rf comes out the same.
4. Tap the center of each spot. The sim reads the distance with the ruler, from the baseline, and adds a row to the table: spot distance, front distance, Rf and a comment.
5. Match each spot in lanes A and B to the reference with the same Rf and color.
6. Untick Draw the baseline in pencil and run again. Then tick it again, untick Solvent level below the baseline and run once more.

Lane	Spot color	Spot distance (mm)	Front distance (mm)	Rf	Matches reference
A green candy
B brown candy
C to F references

Expected results

All values come from the simulation, with water as the solvent and the front at 110 mm:

Dye (color)	Spot distance (mm)	Rf
E133 brilliant blue (blue)	101	0.92
E102 tartrazine (yellow)	88	0.80
E110 sunset yellow (orange)	68	0.62
E129 allura red (red)	50	0.45

• Green candy (A): a yellow spot at Rf 0.80 and a blue spot at 0.92, so it contains E102 and E133.
• Brown candy (B): yellow 0.80, red 0.45 and blue 0.92, so it contains E102, E129 and E133. No candy contains E110.
• Ink baseline: the line dissolves and runs up the paper as a blue band that mixes with the results.
• Solvent above the baseline: the spots dissolve into the solvent in the tank and nothing is left to measure.

Questions for students

1. (Prediction, asked again after the lab) What do you see when brown candy coloring is run in water?
2. To identify the dyes by Rf, what must be the same for the candy samples and the references?
3. What is the Rf of the red spot in the brown candy?
4. Which food colorings does the brown candy contain?
5. Why must the baseline be drawn in pencil, and the solvent start below it?

Answers for teachers: (1) Three separate spots of different colors at different heights. (2) The same solvent and the same paper (run side by side). (3) 0.45 (accept 0.43 to 0.47). (4) E102, E129 and E133. (5) Graphite is insoluble, so a pencil line stays put; ink would dissolve and run as a band. If the solvent covers the baseline, the spots dissolve into the solvent instead of moving up the paper.

Common misconceptions

• "A pure substance gives no spot." It gives one spot. A mixture gives several.
• "Rf depends on how long you run it." Distances grow with time, but their ratio stays the same. Pausing early, with the front at 93 mm, gives the same Rf values.
• "The dye that moves farthest is the heaviest." It is the one most soluble in the solvent and least held by the water in the paper.
• "Rf is the same in every solvent." Switch to Ethanol with Pen inks: the yellow ink drops from Rf 0.92 to 0.30.

Extension

• Leaf pigments (A-level Biology): choose Leaf pigments and Pet. ether–propanone. The sim gives carotene Rf 0.95, xanthophyll 0.71, chlorophyll a 0.65 and chlorophyll b 0.45. Compare the sun and shade leaf lanes.
• Insoluble ink: with Pen inks in water, the black permanent marker stays on the baseline. In ethanol it moves to Rf 0.86. Ask students what this says about the solvent choice.

FAQ

Can the class link open straight in paper chromatography?

Yes. On your class link, set the starting values Method = Paper chromatography, Samples spotted on the paper = Food colorings and Paper chromatography solvent = Water. Students can still change them, so step 1 of the method stays useful.

Does this replace the required practical?

No. AQA required practicals must be done by hand. Use the simulation to plan, to practice Rf calculations, or for students who missed the lab. See virtual labs vs physical labs.

Are these real Rf values?

They are illustrative values in the same order as school experiments. Real values depend on the paper, solvent and temperature.

Related simulations and guides

Separating mixtures: decanting, filtration, evaporation, separating funnel, distillation Recrystallization and melting point – purifying and identifying organic solids

For another separation practical, see the water purification virtual lab. For more ideas, see interactive chemistry lesson ideas.