Paper chromatography can be used to separate the pigments in a food colouring. A student runs paper chromatography on a green food colouring and on two pure dyes, dye X and dye Y, using the same solvent. The green food colouring produces two spots. One spot travels 4.8 cm and the other travels 2.4 cm. The solvent front travels 6.0 cm. Dye X has an Rf value of 0.80 and dye Y has an Rf value of 0.30. Explain what the results tell the student about the green food colouring and identify the dyes it contains.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student uses the same solvent for all three samples. The solvent front travels 6.0 cm in each case. The green food colouring produces two spots: one travelling 4.8 cm and one travelling 2.4 cm.
Model answer (4 marks)
The green food colouring is a mixture, not a pure substance, because it gives two spots on the chromatogram.
The spot that travels 4.8 cm has an Rf value of 0.80 (4.8 cm ÷ 6.0 cm = 0.80). This matches the Rf of dye X, so the green food colouring contains dye X.
The other spot travels 2.4 cm, giving an Rf of 0.40 (2.4 cm ÷ 6.0 cm = 0.40). This does not match the Rf of dye X (0.80) or dye Y (0.30), so the second pigment cannot be identified from the data provided.
The spot that travels 4.8 cm has an Rf value of 0.80 (4.8 cm ÷ 6.0 cm = 0.80). This matches the Rf of dye X, so the green food colouring contains dye X.
The other spot travels 2.4 cm, giving an Rf of 0.40 (2.4 cm ÷ 6.0 cm = 0.40). This does not match the Rf of dye X (0.80) or dye Y (0.30), so the second pigment cannot be identified from the data provided.
Examiner tips
- Show the calculation of Rf for each spot. Mention that a mixture gives multiple spots. Match the Rf of the 4.8 cm spot to dye X. State that the 2.4 cm spot’s Rf does not match either pure dye.
Common mistakes
- Calculating Rf incorrectly (e.g. using 4.8 cm ÷ 6.0 cm = 0.8 but writing 0.80 incorrectly). Forgetting to recognise that a single spot means a pure substance. Assuming the 2.4 cm spot is dye Y because it is a green colour, ignoring the Rf mismatch.
Mark scheme (4 marks)
- The green food colouring is a mixture (not a pure substance) because it produces more than one spot.
- Rf value of the spot travelling 4.8 cm is 0.80 (4.8 ÷ 6.0 = 0.80).
- The spot with Rf 0.80 matches dye X, so the food colouring contains dye X.
- The spot travelling 2.4 cm has an Rf value of 0.40, which does not match dye X or dye Y, so the second pigment is unknown / cannot be identified from the data given. OR: Rf of the second spot is 0.40 and does not match either pure dye.
Key terms in this question
Rf value · paper chromatography
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Water questions
- A student has a mixture of two liquids, ethanol and water. Explain how fractiona…
- Crystallisation can be used to obtain a pure salt from a solution. Describe the …
- A student tests two white solid substances, substance A and substance B, to find…
- A student wants to obtain a sample of pure water from a salt solution. Describe …