lesson
A separation gives evidence about a mixture
1 · Learn
Chromatography separates components because they interact differently with mobile and stationary phases. In a suitable paper model, Rf is distance travelled by a spot divided by distance travelled by the solvent front from the same baseline.
An Rf comparison is meaningful only under comparable conditions. One spot does not prove absolute purity under every possible method, because components may not separate or be visible.
2 · Worked example
A spot travels 3 cm while the front travels 6 cm, giving Rf = 0.5.
3 · Your turn
Find Rf for a spot travelling 4 cm and solvent front 8 cm.
Check your answer
0.5.
4 · Apply your learning
Interpret a supplied chromatogram and explain one limitation of identifying a substance by a single spot position.
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Check a calculation · Year 5 onward
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Powers, roots and trigonometry · Year 7 onward
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