CAIE A Level Chemistry 9701 · Analytical techniques
A plant extract is separated by chromatography on a plate coated with aluminium oxide. The solvent is a non-polar hydrocarbon. Two pigments are separated: pigment A has Rf = 0.85 and pigment B has Rf = 0.30. Explain, in terms of the stationary phase and the mobile phase, why pigment A has a higher Rf value than pigment B.
5 marks, so the mark scheme is looking for about 5 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.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.