A student carries out paper chromatography on two ink samples, Ink A and Ink B, using two different solvents. In solvent 1, both inks produce a single spot at the same position. In solvent 2, Ink A still produces a single spot, but Ink B produces three separate spots. Explain what these results tell the student about the composition of each ink.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Ink A is a pure substance – it contains only one compound. A pure compound gives a single spot in all solvents, so the single spot in both solvent 1 and solvent 2 shows that Ink A is pure.
Ink B is a mixture – it contains more than one compound. In solvent 1 the components of Ink B have the same Rf value, giving one spot. In solvent 2 the components have different Rf values, so they travel different distances and give three separate spots, showing that Ink B contains at least three different compounds.
Ink B is a mixture – it contains more than one compound. In solvent 1 the components of Ink B have the same Rf value, giving one spot. In solvent 2 the components have different Rf values, so they travel different distances and give three separate spots, showing that Ink B contains at least three different compounds.
Examiner tips
- Use the word ‘pure’ for Ink A and ‘mixture’ for Ink B. Explain that a single spot in all solvents means a single compound. Show that different Rf values in solvent 2 mean different compounds in Ink B.
Common mistakes
- Saying Ink A is ‘pure’ but not explaining the single spot in all solvents. Claiming Ink B is pure because it has one spot in solvent 1. Forgetting to mention the different Rf values in solvent 2.
Mark scheme (4 marks)
- Ink A is a pure substance / contains only one compound
- A pure compound produces a single spot in all solvents
- Ink B is a mixture / contains more than one compound
- The three spots in solvent 2 show that the components of Ink B have different Rf values / travel different distances because they have different solubilities in that solvent
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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