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This lesson covers the principles, method and interpretation of paper chromatography, as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). Paper chromatography is used to separate and identify dissolved substances. It is one of the Edexcel core practicals, so you need to know the method in detail and be able to calculate and interpret Rf values.
Chromatography is a technique used to separate a mixture of dissolved substances. It works because different substances have different solubilities in a given solvent, and therefore travel different distances through the chromatography paper.
The separation depends on two things:
A substance that is very soluble in the solvent and not strongly attracted to the paper will travel a long way. A substance that is not very soluble in the solvent and strongly attracted to the paper will travel only a short distance.
| Term | Meaning |
|---|---|
| Stationary phase | The chromatography paper (does not move) |
| Mobile phase | The solvent that moves through the paper |
| Solvent front | The furthest point reached by the solvent |
Exam Tip: The two key terms are "stationary phase" (the paper) and "mobile phase" (the solvent). If a question asks you to explain how chromatography works, you must refer to both phases and explain that substances move different distances because of their different attractions to each phase.
Draw a pencil line near the bottom of the chromatography paper. This is the baseline (also called the origin line). Use a pencil, not a pen, because pencil is insoluble and will not dissolve in the solvent.
Place small spots of each substance to be tested on the baseline, spaced evenly apart. Use a capillary tube to apply small, concentrated spots. Label each spot in pencil below the baseline.
Place the chromatography paper into a beaker containing a shallow layer of solvent. The solvent level must be below the baseline — if the solvent touches the spots, the substances will dissolve directly into the solvent rather than separating.
Cover the beaker with a watch glass or lid. This prevents the solvent from evaporating, which would affect the results.
Wait for the solvent to travel up the paper by capillary action. As it moves, it carries the dissolved substances with it. Different substances travel at different rates.
When the solvent has nearly reached the top of the paper, remove the paper from the beaker.
Immediately mark the position of the solvent front with a pencil line. This is the furthest point the solvent reached.
Leave the paper to dry and observe the positions of the spots.
Exam Tip: Two critical details in the method: (1) the baseline must be drawn in PENCIL (not pen), and (2) the solvent level must be BELOW the baseline. These are frequently tested and losing either point is a common mistake.
In this Edexcel core practical, you may be asked to separate and identify the dyes in a food colouring or an ink.
The Rf value (retention factor) is a numerical value that allows you to identify substances from a chromatogram. It is calculated using the following formula:
Rf = distance moved by substance \u00f7 distance moved by solvent front
Both distances are measured from the baseline (origin).
Exam Tip: Always show your working when calculating Rf values. State the formula, substitute the values, and give the answer to 2 decimal places. Rf values have NO units because they are a ratio of two distances.
A substance travels 4.2 cm from the baseline. The solvent front travels 6.0 cm from the baseline.
Rf = 4.2 \u00f7 6.0 = 0.70
A chromatogram shows three spots from an unknown mixture:
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