A forensic scientist is investigating an ink sample found at a crime scene. The scientist uses paper chromatography to analyse the ink. Explain how paper chromatography separates the different substances in the ink sample, and describe how the scientist could use Rf values to identify the substances present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A forensic scientist uses paper chromatography to separate the components of an unknown ink sample. Several spots are produced on the chromatogram.
Model answer (4 marks)
The solvent (mobile phase) moves up the paper (stationary phase) by capillary action, carrying the ink mixture with it.
Substances in the ink are separated because they have different solubilities in the solvent and therefore travel different distances up the paper.
The Rf value of each spot is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
The scientist can compare the Rf values obtained for the spots with reference Rf values for known substances; matching Rf values allow identification of the components in the ink.
Substances in the ink are separated because they have different solubilities in the solvent and therefore travel different distances up the paper.
The Rf value of each spot is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
The scientist can compare the Rf values obtained for the spots with reference Rf values for known substances; matching Rf values allow identification of the components in the ink.
Examiner tips
- Use the term ‘mobile phase’ and ‘stationary phase’. Explain that separation is due to differing solubilities. Show the Rf formula clearly. State that Rf comparison is used for identification.
Common mistakes
- Confusing the solvent front with the spot distance. Using the wrong order of terms in the Rf formula. Failing to mention that Rf values are compared with reference values.
Mark scheme (4 marks)
- The solvent (mobile phase) moves through/up the paper (stationary phase), carrying the ink mixture with it
- Substances in the ink separate because they have different solubilities in the mobile phase / travel different distances up the paper
- The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
- The Rf values obtained can be compared to known/reference Rf values to identify the substances present in the ink
Key terms in this question
paper chromatography · Rf value
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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