Chromatography
Recap Chromatography for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The required practical uses paper chromatography; calculate Rf = distance moved by the substance ÷ distance moved by the solvent.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Chromatography separates the substances in a mixture based on how they distribute between two phases. This is a required practical.
The stationary phase is the chromatography paper, and the mobile phase is the solvent that moves through it. Substances that are more soluble in the solvent and less attracted to the paper travel further up.
The Rf value is calculated as distance moved by the substance divided by distance moved by the solvent. It is always between 0 and 1 and has no units. For a given solvent and paper, each substance has its own Rf value, so it can be used for identification.
Revision notes
The two phases
The stationary phase is the chromatography paper. The mobile phase is the solvent.
Substances more soluble in the solvent and less strongly attracted to the paper travel further. A pure substance produces a single spot in all solvents.
The method
Draw the start line in pencil, not pen, because pen ink would dissolve and separate too.
The solvent level must be below the start line, or the samples would dissolve straight into the solvent instead of travelling up the paper.
Rf values
Rf = distance moved by substance ÷ distance moved by solvent.
Measure from the pencil start line to the centre of the spot. Rf is always between 0 and 1 and has no units. Each substance has a characteristic Rf in a given solvent.
Key points
- The stationary phase is the paper.
- The mobile phase is the solvent.
- More soluble substances travel further.
- Rf = distance of substance ÷ distance of solvent.
- Rf is always between 0 and 1.
- A pure substance gives a single spot.
Worked examples
Example 1
Calculate the Rf value if a spot moves 3.6 cm and the solvent moves 8.0 cm. [2 marks]
Model answer
Rf = distance moved by substance ÷ distance moved by solvent = 3.6 ÷ 8.0 (1 mark) = 0.45 (1 mark).
Example 2
Explain why the start line is drawn in pencil rather than pen. [2 marks]
Model answer
Pencil is insoluble in the solvent so it does not move (1 mark), whereas pen ink would dissolve and separate, interfering with the results (1 mark).
Example 3
Explain why the solvent level must be below the start line. [2 marks]
Model answer
If the solvent were above the start line the samples would dissolve directly into the solvent (1 mark), so they would not travel up the paper and no separation would occur (1 mark).
Common mistakes
Drawing the start line in pen.
Pen ink dissolves and separates, ruining the result.
Starting with the solvent above the line.
The samples would dissolve into the solvent rather than travelling up.
Giving Rf a unit.
It is a ratio of two distances, so it has no units.
Measuring to the edge of the spot.
Measure to the centre of the spot.
Exam tips
- Always use a pencil start line.
- Keep the solvent below the start line.
- Give Rf with no units and between 0 and 1.
- Measure to the centre of each spot.
Key terms
- Stationary phase
- The chromatography paper.
- Mobile phase
- The solvent that moves through the paper.
- Rf value
- The ratio of substance distance to solvent distance.
- Chromatogram
- The finished paper showing the separated spots.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.