How does chromatography separate and identify substances?
Paper chromatography; the mobile and stationary phases; calculating Rf values; and distinguishing pure substances from mixtures.
A focused answer to AQA GCSE Chemistry 4.8.1, covering how paper chromatography separates mixtures, the mobile and stationary phases, calculating Rf values, and using chromatograms to tell pure substances from mixtures.
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What this dot point is asking
AQA wants you to describe paper chromatography, explain the mobile and stationary phases, calculate values, and use a chromatogram to distinguish a pure substance from a mixture. This is a required practical, so the technique (pencil line, solvent level) and the calculation are both reliably examined.
How chromatography works
A spot of the mixture is placed on a pencil line near the bottom of the paper (pencil does not dissolve), and the bottom of the paper is dipped into solvent so the solvent level is below the spot. As the solvent rises by capillary action, it carries the dissolved substances different distances, separating them into individual spots.
Pure substances and mixtures
Calculating Rf values
For example, if a spot moves cm and the solvent moves cm, the value is . A substance with a higher is more attracted to the solvent and less to the paper.
Why different substances separate
Each substance in the mixture has its own balance of attraction between the stationary phase (the paper) and the mobile phase (the solvent). A substance that is strongly attracted to the solvent and weakly held by the paper is carried a long way up, giving a high ; a substance strongly held by the paper moves only a little, giving a low . Because every substance has a characteristic in a given solvent, the acts like a fingerprint: by running an unknown alongside known reference substances and comparing the values (or spot heights), an analyst can identify what is present. This is why chromatography is used to check the dyes in foods, the inks in forensic samples, and the amino acids in biological mixtures.
Why the conditions must be standardised
A given substance only gives the same if the conditions are kept the same, in particular the solvent and the type of paper. If the solvent is changed, the balance of attractions changes and the values shift, so comparisons are only valid when the unknown and the reference are run on the same chromatogram in the same solvent.
Try this
Q1. Name the stationary and mobile phases in paper chromatography. [2 marks]
- Cue. Stationary phase: the paper. Mobile phase: the solvent.
Q2. A spot travels cm and the solvent travels cm. Calculate the value. [1 mark]
- Cue. .
Q3. State how a chromatogram shows that a substance is pure. [1 mark]
- Cue. It produces a single spot.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA student uses paper chromatography to analyse the dyes in a food colouring. On the chromatogram, the solvent front travelled cm and one dye spot travelled cm from the start line. (a) Calculate the value of this dye. (b) Explain how the student could tell from the chromatogram whether the food colouring is a pure substance or a mixture.Show worked answer →
A 4-mark required-practical question with an calculation.
(a) (2 marks). (b) Count the spots produced by the food colouring (2 marks): a pure substance gives a single spot, while a mixture separates into two or more spots; if more than one spot appears, the colouring is a mixture of dyes.
Markers reward the correct formula and answer (no units, between 0 and 1) and the single-spot-versus-several-spots distinction.
AQA 20213 marksExplain why the start line in paper chromatography must be drawn in pencil and must be above the level of the solvent. Describe what the stationary phase and mobile phase are in this experiment.Show worked answer →
A 3-mark question on chromatography technique and phases.
Pencil line (1 mark): pencil (graphite) is insoluble in the solvent, so it does not move up the paper; ink would dissolve and run, ruining the result. Above the solvent (1 mark): if the start line were below the solvent level, the spots would dissolve directly into the solvent rather than being carried up the paper. Phases (1 mark): the stationary phase is the paper; the mobile phase is the solvent.
Markers want the insoluble-pencil reason and the spots-would-dissolve reason, plus correct identification of both phases.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)