How does paper chromatography separate a mixture, and what does an Rf value tell us?
Paper chromatography and Rf values: how chromatography separates a mixture, the core practical investigating inks, calculating Rf values, and identifying substances from a chromatogram.
A focused answer to Edexcel GCSE Chemistry topic 2, covering how paper chromatography separates a mixture using a stationary and mobile phase, the core practical investigating the composition of inks, calculating and interpreting Rf values, and using chromatograms to identify substances and judge purity.
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What this dot point is asking
Edexcel wants you to explain how paper chromatography separates the substances in a mixture, describe the core practical investigating the composition of inks (using simple distillation and chromatography), calculate Rf values, and interpret a chromatogram to identify substances and judge purity. The Rf calculation is a common short-answer mark.
How chromatography separates a mixture
Chromatography separates substances because they move at different speeds through the paper. There are two phases:
A substance that is more soluble in the solvent and less attracted to the paper travels further up the paper. A substance that is less soluble and more attracted to the paper stays nearer the start line. Because the substances in a mixture have different solubilities and attractions, they separate into different spots.
The core practical: investigating inks
The Edexcel core practical investigates the composition of inks using simple distillation and paper chromatography.
- Draw a start line in pencil near the bottom of the paper (pencil does not dissolve and move).
- Put a small spot of each ink on the line and let it dry.
- Stand the paper in a beaker with the solvent below the start line, so the ink does not dissolve straight into the solvent.
- Let the solvent rise up the paper, then remove the paper before the solvent reaches the top and mark the solvent front in pencil.
- Use simple distillation to recover the solvent from an ink and identify it.
Calculating Rf values
Each substance has a characteristic Rf value in a given solvent:
Because the spot never travels further than the solvent, the Rf value is always between 0 and 1. A higher Rf means the substance travelled further (more soluble, less attracted to the paper). Distances are measured from the start line to the centre of the spot.
Interpreting a chromatogram
A chromatogram is read like this:
- Number of spots tells you how many substances are present. A pure substance gives one spot; a mixture gives more than one.
- Matching substances. Two substances are the same if they have the same Rf value in the same solvent under the same conditions.
- Identifying components. Run the unknown alongside known reference substances; any spot in the unknown that lines up with a reference is identified.
Try this
Q1. Name the stationary phase and the mobile phase in paper chromatography. [2 marks]
- Cue. Stationary phase is the paper; mobile phase is the solvent.
Q2. A spot moves cm and the solvent front moves cm. Calculate the Rf value. [2 marks]
- Cue. .
Q3. Explain how a chromatogram shows that a substance is pure. [1 mark]
- Cue. A pure substance produces only one spot on the chromatogram.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20193 marksIn a chromatography experiment, a spot of dye travels cm while the solvent front travels cm. Calculate the Rf value of the dye and explain what an Rf value of less than 1 tells you about the dye.Show worked answer →
A 3-mark Rf calculation and interpretation.
(1 mark for the formula, 1 mark for the value). An less than 1 means the dye did not travel as far as the solvent, because it is more attracted to the paper (the stationary phase) and less soluble in the solvent (the mobile phase) (1 mark).
Markers reward the correct ratio (spot over solvent, always less than 1) and the link to attraction to the paper versus solubility.
Edexcel 20224 marksA food colouring is analysed by paper chromatography alongside four known dyes, A, B, C and D. Explain how the chromatogram can be used to identify which known dyes are present in the food colouring, and how it shows whether the food colouring is a pure substance.Show worked answer →
A 4-mark interpretation question.
The food colouring is run on the same paper in the same solvent as the known dyes (1 mark). A dye in the food colouring is identified if its spot travels the same distance and has the same value as one of the known dyes (1 mark). If the food colouring produces more than one spot, it is a mixture, not a pure substance (1 mark). A single spot would mean it is pure (1 mark).
Markers reward matching values under identical conditions, and using the number of spots to judge purity.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)