A student uses paper chromatography to analyse a mixture of food colourings. The student calculates the Rf value for one of the food colourings and finds it is identical to the Rf value of a known pure dye. Explain what Rf values are, how they are calculated, and what the student can conclude from this result.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography is a technique used to separate and identify substances in a mixture. Each substance travels a characteristic distance up the chromatography paper relative to the solvent front.
Model answer (5 marks)
The Rf value is the ratio of the distance travelled by the substance to the distance travelled by the solvent front.
It is calculated by dividing the distance the spot has moved from the origin by the distance the solvent front has moved.
The Rf value is always between 0 and 1.
Each pure substance has a unique or characteristic Rf value in a given solvent system.
Because the Rf value of the food colouring matches that of the known pure dye, the student can conclude that the food colouring may be (or is likely to contain) the same substance as the known pure dye.
However, this result alone cannot conclusively prove the substances are identical; a different solvent or an additional test would be needed to confirm identity.
It is calculated by dividing the distance the spot has moved from the origin by the distance the solvent front has moved.
The Rf value is always between 0 and 1.
Each pure substance has a unique or characteristic Rf value in a given solvent system.
Because the Rf value of the food colouring matches that of the known pure dye, the student can conclude that the food colouring may be (or is likely to contain) the same substance as the known pure dye.
However, this result alone cannot conclusively prove the substances are identical; a different solvent or an additional test would be needed to confirm identity.
Examiner tips
- Define Rf and give the formula; state it is 0–1; mention uniqueness; explain conclusion and limitation; use correct terminology
Common mistakes
- Calculating Rf as distance of spot divided by distance from origin; forgetting the solvent front distance; claiming definitive identity without further tests
Mark scheme (5 marks)
- The Rf value is the ratio of the distance travelled by the substance to the distance travelled by the solvent front
- The Rf value is always between 0 and 1
- Each pure substance has a unique/characteristic Rf value (in a given solvent)
- Because the Rf value of the food colouring matches the Rf value of the known pure dye, the food colouring may be (or is likely to contain) the same substance as the known pure dye
- This result alone cannot conclusively prove the substances are identical; a different solvent or additional test would be needed to confirm identity
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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