CAIE A Level Chemistry 9701 · Analytical techniques
A student uses thin-layer chromatography (TLC) to separate pigments extracted from a plant leaf. The stationary phase is aluminium oxide on a glass plate. The mobile phase is a mixture of ethanol and hexane. Two pigments, X and Y, are identified. Pigment X has an Rf value of 0.25; pigment Y has an Rf value of 0.75. Explain why pigment Y has a higher Rf value than pigment X.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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