CAIE A Level Chemistry 9701 · Analytical techniques

Analytical techniques: suggest question

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A student separates the pigments from a leaf extract using thin-layer chromatography. The stationary phase is a thin layer of aluminium oxide on a glass plate. The mobile phase is a mixture of cyclohexane and ethyl ethanoate, which is non-polar. The extract is applied as a small spot on the baseline. After the solvent has run, the plate is removed and dried. Two coloured spots are seen: spot A near the baseline and spot B near the solvent front.

Suggest why the two pigments travel different distances up the plate.

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7 marks, so the mark scheme is looking for about 7 separate creditworthy points. Making the same point twice in different words earns it once.

Suggest means: Apply what you know to an unfamiliar situation. There may be more than one acceptable answer. Reason from the principle you have learned to the new context, and say which principle you are using.

Analytical techniques: suggest question

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