# A food scientist is investigating three sports drinks — X, Y and Z. She wants to find out whether each drink is a pure substance or a formulation, and to identify some of the colourings present. She carries out paper chromatography on all three drinks using the same solvent. Drink X produces a single spot. Drink Y produces three spots. Drink Z produces the same single spot as drink X, at the same position on the paper. Explain what the chromatography results tell the food scientist about drinks X, Y and Z, and explain how the Rf value could be used to help identify the colouring in drink X.

> WJEC A-Level Chemistry (Wales) — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks

## Mark scheme (5 marks)

1. Drink X is a pure substance / contains only one compound (as it produces a single spot)
2. Drink Y is a mixture / formulation (as it produces more than one spot / three spots, showing it contains more than one compound)
3. Drink Z contains the same colouring / compound as drink X (because the spot travels to the same position / has the same Rf value in the same solvent)
4. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
5. The Rf value of the spot from drink X can be compared with known/reference Rf values (for known colourings in the same solvent) to identify the colouring

## Key terms

- [pure substance](https://www.gradenine.co.uk/glossary/pure-substance)
- [formulation](https://www.gradenine.co.uk/glossary/formulation)
- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)
- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-food-scientist-is-investigating-three-10f62ae5) · Published by Druglandscape Ltd.