How does paper chromatography separate a mixture, and how do we read a chromatogram?
Carry out paper chromatography, interpret chromatograms, and calculate and use Rf values to identify substances in a mixture.
A focused answer to WJEC GCSE Chemistry topic 1.1, covering the method for paper chromatography, why components separate, how to interpret a chromatogram, and how to calculate Rf values to identify substances.
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What this topic is asking
WJEC topic 1.1 includes paper chromatography as a method for separating and identifying the substances in a mixture, such as the dyes in an ink or food colouring. You must know the method, be able to interpret a chromatogram, and calculate an Rf value to compare and identify substances.
How chromatography separates a mixture
Chromatography works because the different substances in a mixture are carried up the paper at different speeds. There are two "phases":
- the stationary phase is the chromatography paper, which holds the substances back, and
- the mobile phase is the solvent (such as water or ethanol), which moves up the paper and carries the substances with it.
A substance that is more soluble in the solvent and less strongly attracted to the paper travels further up. A substance that is less soluble or more attracted to the paper stays nearer the bottom. Because each substance has its own balance of these properties, the components of a mixture spread out into separate spots.
The method
The finished paper is called a chromatogram.
Two practical points are often credited in the exam. First, you should use a lid or cover on the container so the solvent does not evaporate, which keeps the conditions steady. Second, you should remove the paper and mark the solvent front before the solvent reaches the very top, because once the solvent runs off the end you can no longer measure how far it has travelled and the Rf values become impossible to calculate.
Interpreting a chromatogram
You read a chromatogram by counting and comparing the spots:
- A pure substance produces a single spot in all solvents.
- A mixture produces two or more spots, one for each component.
- To identify which substances are in an unknown mixture, you can run known reference substances alongside it. Any spot in the mixture that travels the same distance (the same height) as a reference spot is likely to be that substance.
Rf values
To compare results between experiments you calculate the retention factor, or Rf value.
Both distances are measured from the baseline; the substance distance is measured to the centre of the spot. The Rf value is always between 0 and 1 and has no units. In a given solvent, a particular substance always has the same Rf value, so it can be used to identify it.
Rf values let you compare results fairly even when the chromatograms are different sizes, because the value is a ratio rather than a raw distance. The same substance run on a larger or smaller piece of paper, or with the solvent allowed to rise further, still gives the same Rf as long as the solvent and conditions are the same. If you change the solvent, the Rf values change, so you must always state which solvent was used. To identify an unknown substance, you compare its Rf value with the Rf values of known reference substances measured in the same solvent.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample3 marksDescribe how you would use paper chromatography to find out whether a green food colouring is a single substance or a mixture.Show worked answer →
A Unit 1.1 method question. Reward: draw a pencil baseline near the bottom of the chromatography paper, place a small spot of the green colouring on the line, and stand the paper in a shallow depth of solvent so the solvent level is below the baseline. Let the solvent rise up the paper, then remove and dry it. If the green colouring is a single substance it leaves one spot; if it is a mixture it separates into two or more spots of different colours. Markers credit the pencil baseline, the solvent below the line, and the interpretation that more than one spot means a mixture. A common error is to draw the baseline in ink, which would itself run.
WJEC sample2 marksOn a chromatogram, a dye moves 4.5 cm while the solvent front moves 6.0 cm. Calculate the Rf value of the dye.Show worked answer →
A Unit 1.1 calculation. Reward: the Rf value is the distance moved by the substance divided by the distance moved by the solvent front: . Markers credit the correct formula (substance distance over solvent distance) and the answer 0.75. Rf values have no units because they are a ratio of two lengths. A common error is to divide the wrong way round, giving a value greater than 1, which is impossible.
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)