A student carries out paper chromatography on a sample of food colouring to find out whether it is a pure substance or a mixture. The food colouring produces three separate spots on the paper. Explain what this result tells the student about the food colouring, and describe how the student could use Rf values to identify each of the substances present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography separates substances in a mixture based on how soluble each substance is in the mobile phase. The more soluble a substance is, the further up the paper it travels.
Model answer (5 marks)
The appearance of three distinct spots shows that the food colouring is a mixture, not a pure substance.
Each spot represents a different compound present in the mixture.
The Rf value of a spot is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent front.
The student calculates an Rf value for each of the three spots.
By comparing these Rf values with reference Rf values obtained under the same conditions, the student can identify each substance in the mixture.
Each spot represents a different compound present in the mixture.
The Rf value of a spot is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent front.
The student calculates an Rf value for each of the three spots.
By comparing these Rf values with reference Rf values obtained under the same conditions, the student can identify each substance in the mixture.
Examiner tips
- Use the word "shows" to link the observation to the conclusion. Explain the definition of Rf before using it. Show the Rf formula. Mention that reference values must be from the same solvent system.
Mark scheme (5 marks)
- Three separate spots show the food colouring is a mixture (not a pure substance)
- Each spot represents a different compound/substance in the mixture
- Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent
- An Rf value is calculated for each of the three spots
- The Rf values are compared to known/reference Rf values (in the same solvent) to identify each substance
Key terms in this question
paper chromatography · pure substance · mixture · Rf value
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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