A medicine manufacturer produces paracetamol tablets. A quality control chemist uses paper chromatography to check a batch of tablets for purity. The chemist spots a sample of the tablet contents onto chromatography paper alongside a sample of pure paracetamol and allows the solvent to travel up the paper. Explain how the chemist can use the results of this chromatography to determine whether the tablet sample contains only paracetamol or whether other substances are 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. A pure compound will produce a single spot in all solvents, whereas a mixture will separate into different spots.
Model answer (5 marks)
The solvent (mobile phase) moves up the paper carrying the components of the tablet sample.
1. Substances that are more soluble in the solvent travel further up the paper.
2. Because of this differential solubility, different substances separate into distinct spots.
3. If the tablet sample contains only paracetamol, it will produce a single spot.
4. The spot from the tablet sample must be at the same height (same Rf value) as the spot from the pure paracetamol reference.
5. If the tablet sample produces more than one spot, or a spot at a different Rf, the tablet contains other substances and is not pure.
1. Substances that are more soluble in the solvent travel further up the paper.
2. Because of this differential solubility, different substances separate into distinct spots.
3. If the tablet sample contains only paracetamol, it will produce a single spot.
4. The spot from the tablet sample must be at the same height (same Rf value) as the spot from the pure paracetamol reference.
5. If the tablet sample produces more than one spot, or a spot at a different Rf, the tablet contains other substances and is not pure.
Examiner tips
- Use the command word "Explain" – give a clear, step‑by‑step description of the process. Include the key idea that Rf values must match for purity.
- Show the logical link: solvent carries components → solubility determines distance → matching Rf indicates purity, extra spots indicate impurities.
Common mistakes
- Failing to mention that the tablet spot must match the reference Rf. Confusing the stationary phase with the mobile phase. Assuming any spot means impurity without checking Rf equality.
Mark scheme (5 marks)
- The solvent (mobile phase) carries the substances up the paper (stationary phase)
- Substances that are more soluble in the solvent travel further up the paper, so different substances separate into different spots
- If the tablet sample is pure paracetamol, it will produce only one spot
- The single spot from the tablet sample must be at the same height / same Rf value as the spot from the pure paracetamol reference sample
- If the tablet sample produces more than one spot, it contains other substances / is not pure paracetamol
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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