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AQA GCSE · Question 04.1 · Chemical analysis
A student investigated an orange dye (A) using paper chromatography. Figure 4 shows the results of Experiment 1 and Experiment 2 using orange dye A.
Explain why the yellow dye and red dye travel different distances in Experiment 1. Refer to forces of attraction between the dyes and the chromatography paper in your answer.
A student investigated an orange dye (A) using paper chromatography. Figure 4 shows the results of Experiment 1 and Experiment 2 using orange dye A.
Explain why the yellow dye and red dye travel different distances in Experiment 1. Refer to forces of attraction between the dyes and the chromatography paper in your answer.
How to approach this question
This question is about the principles of chromatography. Separation occurs due to differences in solubility in the mobile phase (the solvent) and attraction to the stationary phase (the paper).
1. Identify which dye travelled further (yellow).
2. Explain what this means in terms of its solubility in the solvent (water) and its attraction to the paper.
3. Identify which dye travelled less far (red).
4. Explain what this means in terms of its solubility and attraction to the paper.
Full Answer
The yellow dye travels further up the paper because it is more soluble in the solvent (water) and has weaker forces of attraction to the stationary phase (the paper). The red dye travels a shorter distance because it is less soluble in the solvent and has stronger forces of attraction to the paper.
Paper chromatography separates substances based on their differential distribution between a stationary phase (the chromatography paper) and a mobile phase (the solvent, which is water in Experiment 1).
- The **yellow dye** travels further up the paper. This indicates that it is more soluble in the mobile phase (water) and has weaker forces of attraction to the stationary phase (paper). It spends more time moving with the solvent.
- The **red dye** travels a shorter distance. This indicates that it is less soluble in the mobile phase and has stronger forces of attraction to the stationary phase. It spends more time adsorbed onto the paper and less time moving with the solvent.
The balance between solubility in the mobile phase and attraction to the stationary phase determines how far each component travels.
Common mistakes
✗ Just stating that one is more soluble without mentioning the attraction to the paper (or vice versa).
✗ Getting the properties the wrong way round (e.g., saying the yellow dye is less soluble).
✗ Not referring to both dyes in the explanation.
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