A paint manufacturer produces a new green paint. A quality control technician uses paper chromatography to check whether the green pigment in the paint is a pure substance or a mixture of pigments. After running the chromatography, the technician observes that the green pigment separates into two spots on the paper. Explain what this result tells the technician about the green pigment, and describe how paper chromatography works to produce this result.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A paint manufacturer produces a new green paint. A quality control technician uses paper chromatography to check whether the green pigment in the paint is a pure substance or a mixture of pigments. After running the chromatography, the technician observes that the green pigment separates into two spots on the paper.
Model answer (5 marks)
The green pigment is a mixture of two different pigments, not a single pure substance, because a pure substance would give only one spot.
Paper chromatography works by using the paper as the stationary phase. The solvent, which is the mobile phase, is placed at the bottom of the paper and rises by capillary action. As the solvent moves up, the pigments are carried with it but are distributed between the stationary and mobile phases. The two pigments have different solubilities in the solvent, so they move at different rates and separate into two distinct spots.
The Rf value of each spot can be calculated and used to identify the individual pigments in the mixture.
Paper chromatography works by using the paper as the stationary phase. The solvent, which is the mobile phase, is placed at the bottom of the paper and rises by capillary action. As the solvent moves up, the pigments are carried with it but are distributed between the stationary and mobile phases. The two pigments have different solubilities in the solvent, so they move at different rates and separate into two distinct spots.
The Rf value of each spot can be calculated and used to identify the individual pigments in the mixture.
Examiner tips
- Use the word "mixture" to show you understand the result; mention a single spot for a pure substance. Explain the roles of stationary and mobile phases clearly. State why the pigments separate (different solubilities/distributions). Mention Rf values as a way to identify the components.
- common_mistakes
- :
- Saying the pigment is pure because two spots were seen. Confusing the paper with the solvent as the moving phase. Forgetting to explain that different solubilities cause separation.
Mark scheme (5 marks)
- The green pigment is a mixture (not a pure substance), because a pure substance produces only a single spot in all solvents.
- The paper acts as the stationary phase (the non-moving phase).
- The solvent acts as the mobile phase and carries the pigments up the paper.
- The two pigments separate because they have different solubilities in the mobile phase / they are distributed differently between the mobile and stationary phases.
- The Rf value of each spot can be used to identify the individual pigments / substances in the mixture.
Key terms in this question
paper chromatography · pure substance · mixture
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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