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AQA GCSE · Question 04.3 · Chemical analysis
The student investigated a different orange dye (B). Figure 5 shows the results of Experiment 3 using orange dye B.
Compare the purity of the orange dyes A and B. Give reasons for your answer. Use Figure 4 and Figure 5.
The student investigated a different orange dye (B). Figure 5 shows the results of Experiment 3 using orange dye B.
Compare the purity of the orange dyes A and B. Give reasons for your answer. Use Figure 4 and Figure 5.
How to approach this question
The question asks you to compare the purity of two dyes based on their chromatograms.
1. Look at the chromatogram for dye A (in either experiment). How many spots does it separate into? What does this indicate about its purity?
2. Look at the chromatogram for dye B. How many spots does it produce? What does this indicate about its purity?
3. Formulate a comparison statement, clearly stating which is pure and which is a mixture, and use the number of spots as your evidence.
Full Answer
Orange dye B is pure, whereas orange dye A is a mixture. This is because dye B produces only one spot on the chromatogram, while dye A separates into two different spots (red and yellow).
Chromatography is a technique used to separate mixtures and test for purity.
- A **pure substance** will produce only a single spot on a chromatogram, as it consists of only one compound which will move a specific distance in a given solvent. In Figure 5, dye B produces only one orange spot, indicating it is a pure substance.
- A **mixture** contains two or more different substances. Each substance will have a different solubility in the solvent and attraction to the paper, so they will separate and travel different distances, forming multiple spots. In Figure 4, dye A separates into a yellow spot and a red spot, indicating that it is a mixture of at least two different dyes.
Common mistakes
✗ Stating that A is a mixture but not stating that B is pure (or vice versa).
✗ Not giving the reason based on the number of spots.
✗ Incorrectly identifying which dye is pure and which is the mixture.
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