A student carries out paper chromatography to investigate the dyes present in three food colourings labelled P, Q and R. Food colouring P produces one spot, food colouring Q produces three spots and food colouring R produces two spots. The student wants to identify the individual dyes by comparing their Rf values with reference data. Explain what the results tell the student about the nature of each food colouring, and explain how Rf values can be used to identify the dyes present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Food colouring P produces one spot, food colouring Q produces three spots and food colouring R produces two spots during paper chromatography using the same solvent.
Model answer (5 marks)
Food colouring P gives only one spot, so it contains a single dye – it is a pure substance.
Food colouring Q gives three spots and food colouring R gives two spots, so each of these contains more than one dye – they are mixtures.
In paper chromatography the mobile phase (solvent) carries the dyes up the paper. A dye that is more soluble in the solvent moves further, while a dye that is less soluble moves less, so the different dyes separate.
The Rf value of a spot is calculated as the distance travelled by the dye divided by the distance travelled by the solvent front.
Because each dye has a characteristic Rf value in a given solvent, the student can compare the measured Rf values of the spots with reference Rf values to identify which dyes are present.
Food colouring Q gives three spots and food colouring R gives two spots, so each of these contains more than one dye – they are mixtures.
In paper chromatography the mobile phase (solvent) carries the dyes up the paper. A dye that is more soluble in the solvent moves further, while a dye that is less soluble moves less, so the different dyes separate.
The Rf value of a spot is calculated as the distance travelled by the dye divided by the distance travelled by the solvent front.
Because each dye has a characteristic Rf value in a given solvent, the student can compare the measured Rf values of the spots with reference Rf values to identify which dyes are present.
Examiner tips
- State that P is pure and Q/R are mixtures. Explain separation by solubility. Define Rf and how to calculate. Show that Rf values are characteristic and used for identification.
Common mistakes
- Confusing the number of spots with the number of dyes. Using the wrong formula for Rf (e.g., dividing by the distance of the spot instead of the solvent front). Assuming all dyes have the same Rf in every solvent.
Mark scheme (5 marks)
- Food colouring P is a pure substance (contains only one compound / single dye) because it produces only one spot
- Food colourings Q and R are mixtures because they produce more than one spot, showing they contain more than one compound / dye
- In paper chromatography, substances separate because they have different solubilities in the mobile phase (solvent), so they travel different distances up the paper
- The Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent
- Each dye has a characteristic / unique Rf value in a given solvent, so the Rf values of the spots can be compared to reference / known Rf values to identify the dyes
Key terms in this question
paper chromatography · Rf value
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