A scientist wants to find out whether a food colouring is a pure substance or a mixture of dyes. She uses paper chromatography with two different solvents, Solvent A and Solvent B. In both solvents, the food colouring produces only a single spot. Explain what this result tells the scientist about the food colouring.
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. A pure compound produces a single spot in all solvents, whereas a mixture separates into different spots.
Model answer (4 marks)
A single spot in both solvents shows that the food colouring is a pure substance, containing only one compound.
If it were a mixture, the different dyes would have different solubilities in the solvents and would separate into multiple spots.
Chromatographic separation depends on the relative solubility of each component in the mobile phase.
Using two different solvents confirms the result, because a pure compound will give only one spot in any solvent, whereas a mixture would give more than one spot in at least one solvent.
If it were a mixture, the different dyes would have different solubilities in the solvents and would separate into multiple spots.
Chromatographic separation depends on the relative solubility of each component in the mobile phase.
Using two different solvents confirms the result, because a pure compound will give only one spot in any solvent, whereas a mixture would give more than one spot in at least one solvent.
Examiner tips
- State the result first (single spot in both solvents).
- Explain why a single spot means purity (no separation).
- Mention the role of solubility in chromatography.
- Note that two solvents strengthen the conclusion.
Common mistakes
- Saying the colour is pure without linking to the single spot.
- Confusing the solvent with the stationary phase.
Mark scheme (4 marks)
- A single spot in both solvents indicates the food colouring is a pure substance / contains only one compound
- If it were a mixture, different substances would separate and produce more than one spot
- Separation in chromatography depends on how soluble each substance is in the mobile phase / solvent
- Testing in two different solvents strengthens the conclusion / a pure compound produces a single spot in all solvents
Key terms in this question
paper chromatography · pure substance · mixture
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Rate of chemical change questions
- A student carries out crystallisation to obtain a sample of copper sulfate cryst…
- A student uses simple distillation to separate ethanol from a mixture of ethanol…
- A student has two white powders, substance A and substance B. She wants to find …
- A student carries out paper chromatography on a sample of black ink and on a pur…