A student carries out paper chromatography to investigate the pigments present in a sample of grass. After the experiment, the chromatogram shows three separate spots. Explain what can be concluded from these results, and state how the student could use the chromatogram to identify whether any of the pigments are the same as a known reference pigment.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student crushes fresh grass leaves and dissolves the pigments in a suitable solvent. This solution is spotted onto chromatography paper and the paper is placed in a beaker containing a small depth of solvent. The solvent is allowed to rise up the paper and the paper is then removed and allowed to dry.
Model answer (4 marks)
Three separate spots indicate that the grass sample contains a mixture of three different pigments.
Each pigment travels a different distance up the paper because each has a different solubility in the solvent and a different affinity for the paper.
The student can calculate the Rf value for each spot (distance travelled by the spot ÷ distance travelled by the solvent front). If the Rf value of a spot on the chromatogram matches the Rf value of a reference pigment that has been run under the same conditions, then that pigment is the same as the reference pigment.
Each pigment travels a different distance up the paper because each has a different solubility in the solvent and a different affinity for the paper.
The student can calculate the Rf value for each spot (distance travelled by the spot ÷ distance travelled by the solvent front). If the Rf value of a spot on the chromatogram matches the Rf value of a reference pigment that has been run under the same conditions, then that pigment is the same as the reference pigment.
Examiner tips
- Use the word ‘indicates’ to show inference; mention mixture of three pigments. Explain the reason for different distances (solubility/affinity). Show the Rf formula and state that it must be compared under identical conditions. Use ‘matches’ to link Rf values to identification.
Common mistakes
- Failing to state that the pigments are different substances. Not explaining why the pigments travel different distances. Calculating Rf incorrectly or not comparing to a reference run under the same conditions.
Mark scheme (4 marks)
- Three separate spots show the grass sample contains a mixture of three different pigments / substances
- Each pigment travels a different distance up the paper because each has a different solubility in the solvent / different affinity for the paper
- Calculate / compare the Rf value for each spot (distance travelled by spot ÷ distance travelled by solvent front)
- If the Rf value of a spot on the chromatogram matches the Rf value of the reference pigment (run under the same conditions), then that pigment is the same as the reference
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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