# 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.

> WJEC GCSE Chemistry (Wales) — 1.2 Atomic structure and the periodic table · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. The ink is a mixture (not a pure substance) because it produces more than one spot
2. The Rf value of the first spot is 6.0 ÷ 8.0 = 0.75 (or equivalent correct ratio stated)
3. The known pure dye has the same Rf value as the first spot in the ink (both 0.75 / both travel 6.0 cm)
4. This suggests the known pure dye is present in the ink (as one of its components)

## Related

- [Revision notes for WJEC GCSE Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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