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AQA GCSE · Question 04.2 · Chemical analysis
The student used the same type of chromatography paper in Experiment 1 and in Experiment 2.
Explain why the yellow dye is in different positions in Experiment 1 and in Experiment 2. Use Figure 4.
The student used the same type of chromatography paper in Experiment 1 and in Experiment 2.
Explain why the yellow dye is in different positions in Experiment 1 and in Experiment 2. Use Figure 4.
How to approach this question
1. Look at Figure 4 and identify the difference between Experiment 1 and Experiment 2. The paper is the same, but the solvent is different.
2. State what the solvents are in each experiment.
3. Compare the distance travelled by the yellow dye in both experiments.
4. Relate the distance travelled to the solubility of the yellow dye in each specific solvent.
Full Answer
The solvent is different in each experiment (water in 1, ethanol in 2). The yellow dye is more soluble in ethanol than in water. Because it is more soluble in the mobile phase in Experiment 2, it travels further up the paper compared to Experiment 1.
The distance a substance travels in chromatography depends on its solubility in the mobile phase (the solvent) and its attraction to the stationary phase (the paper). In this question, the stationary phase (paper) is kept the same.
1. **Identify the variable:** The mobile phase is changed. In Experiment 1, the solvent is water. In Experiment 2, the solvent is ethanol.
2. **Observe the result:** The yellow dye spot travels further up the paper in Experiment 2 (with ethanol) than in Experiment 1 (with water).
3. **Explain the observation:** Since the dye travelled further in ethanol, it must be more soluble in ethanol than it is in water. Higher solubility in the mobile phase means the substance spends more time moving with the solvent and less time stuck to the paper, thus travelling a greater distance.
Common mistakes
✗ Failing to identify that the solvent is the key difference.
✗ Stating that the dye is more soluble in ethanol but not linking this to why it travels further.
✗ Just saying "the solvents are different" without explaining the consequence in terms of solubility.
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