ExpertMedium2 marksMultiple Choice
AQA GCSE · Question 05.2 · Chemical analysis
Figure 2 shows the apparatus used. What two mistakes did the student make in setting up the apparatus? Select two options.
Figure 2 shows the apparatus used. What two mistakes did the student make in setting up the apparatus? Select two options.
Answer options:
A.
The paper does not touch the beaker
B.
The start line is drawn in ink
C.
The water level is below the start line
D.
Uses a lid on the beaker
E.
Uses water as the solvent
How to approach this question
Analyse the setup in Figure 2 and compare it with the correct procedure for chromatography and the information given in the question.
1. **Start Line:** What should the start line be drawn with and why? Is ink appropriate?
2. **Solvent:** The question states the colours are soluble in ethanol but insoluble in water. What solvent is being used in the diagram? Is this appropriate?
3. **Solvent Level:** Where should the solvent level be in relation to the start line?
4. **Lid:** Why is a lid used? Is it a mistake to use one?
Full Answer
There are two major mistakes in the student's setup:
1. **The start line is drawn in ink:** The baseline on a chromatogram must always be drawn in pencil. Pencil is graphite (carbon), which is insoluble in the solvents used and will not move. Ink contains dyes that are soluble and will separate along with the samples, ruining the chromatogram.
2. **Uses water as the solvent:** The introduction to the question explicitly states, "The colours are soluble in ethanol but are insoluble in water." For chromatography to work, the substances being separated must be soluble in the solvent. Since the flower colours are insoluble in water, they will not move from the baseline, and no separation will occur. The student should have used ethanol.
Common mistakes
✗ Thinking the lid is a mistake - it's necessary to prevent evaporation.
✗ Thinking the solvent level being below the start line is a mistake - this is correct procedure.
✗ Not referring back to the information that the dyes are insoluble in water.
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