A student carries out paper chromatography to investigate an ink sample. The solvent travels 8.0 cm up the paper. One spot produced by the ink travels 6.0 cm and a second spot travels 2.0 cm. The student also runs a known pure dye alongside the ink and it produces only one spot that travels 6.0 cm. Explain what the results tell the student about the ink sample and about whether the known dye is present in the ink.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography separates substances based on how soluble they are in the mobile phase. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.
Model answer (4 marks)
The ink is a mixture, not a pure substance, because it gives two spots.
The Rf value of the first spot is 6.0 cm ÷ 8.0 cm = 0.75.
The known pure dye also has an Rf of 0.75 (it travels 6.0 cm).
Because the Rf values are identical, the dye is present in the ink as one of its components.
The Rf value of the first spot is 6.0 cm ÷ 8.0 cm = 0.75.
The known pure dye also has an Rf of 0.75 (it travels 6.0 cm).
Because the Rf values are identical, the dye is present in the ink as one of its components.
Examiner tips
- Show the Rf calculation explicitly; use the same units. State that identical Rf values mean the same substance. Mention that two spots prove a mixture.
Common mistakes
- Calculating Rf incorrectly (e.g. 8.0÷6.0). Forgetting to explain that two spots mean a mixture. Assuming the dye is absent because it only gives one spot, without comparing Rf values.
Mark scheme (4 marks)
- The ink is a mixture (not a pure substance) because it produces more than one spot
- The Rf value of the first spot is 6.0 ÷ 8.0 = 0.75 (or equivalent correct ratio stated)
- The known pure dye has the same Rf value as the first spot in the ink (both 0.75 / both travel 6.0 cm)
- This suggests the known pure dye is present in the ink (as one of its components)
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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