GCSE paper chromatography and Rf values

Paper chromatography separates substances in a mixture because they distribute differently between a stationary phase and a moving solvent. The result is a chromatogram: a pattern of separated spots.

Rf = distance travelled by the substance ÷ distance travelled by the solvent front. Measure both from the pencil baseline, using the centre of the spot and the same units. Rf has no units. A matching Rf can support an identification, but cannot prove it on its own.

This guide is for students studying GCSE Chemistry in England. In AQA GCSE Chemistry 8462, section 4.8.1.3 links chromatography and Rf calculations to required practical 6, separating and distinguishing coloured substances. That number belongs to this qualification: do not transfer it to Combined Science or another board. Use your teacher’s instructions for your course and apparatus.

At GCSE, the paper is described as the stationary phase: it stays in place. The solvent is the mobile phase: it travels up the paper and carries dissolved substances. Different substances distribute differently between the two phases, so they travel different distances.

The baseline, also called the origin, is the pencil line where the samples start. The solvent front is the furthest point the solvent reaches. It is not the highest coloured spot and not the top edge of the paper. Picture the baseline near the bottom, separated spots above it and a marked solvent front above those spots. All calculation distances start at that same baseline.

Use this to understand a teacher-supervised school practical, not as instructions for experimenting with household solvents. Follow the school’s risk assessment, wear the specified eye protection and let the teacher choose the solvent and apparatus. Take care with capillary tubes: broken glass can be sharp.

1. Draw a pencil baseline and label the samples. Pencil marks stay in place in the water-based method; ink could dissolve and add extra coloured spots.
2. Apply small spots of each sample to the baseline. Keep them separate so their resulting patterns can be compared rather than merging at the start.
3. Put the bottom of the paper into the solvent, with the baseline above the liquid level. The solvent must rise through the spots; submerged spots can dissolve into the liquid in the beaker instead. Keep the paper away from the beaker sides.
4. Leave the apparatus undisturbed while the solvent rises. Let the separated pattern develop and remove the paper before the solvent reaches its top edge.
5. Mark the solvent front immediately in pencil, then let the paper dry. The wet boundary becomes difficult to locate as the solvent evaporates. Measure from the origin to each spot centre and to the front.

These choices follow the water-based classroom method in AQA’s practical handbook. They do not establish that other solvents are safe to use unsupervised.

These are invented teaching measurements, not results from a real experiment. A sample produces two separated spots. Measured from the baseline, the solvent front is 80 mm away, spot A’s centre is 24 mm away and spot B’s centre is 56 mm away.

• Spot A: Rf = 24 mm ÷ 80 mm = 0.30.
• Spot B: Rf = 56 mm ÷ 80 mm = 0.70.

The units cancel. Do not write “0.30 mm”. Both calculations use the same 80 mm solvent-front distance, not the distance between the two spots. Spot B has travelled a larger fraction of the solvent distance.

A reversed calculation, 80 ÷ 24 = 3.33, is a warning that the fraction is upside down. The spot in this example has travelled less far than the solvent front, so its Rf must be less than 1. Measuring from the bottom of the paper instead of the baseline would also give the wrong ratio.

Two separated spots from the original sample show at least two detectable components: the sample is a mixture. Now imagine known reference X gives Rf 0.30 and reference Y gives Rf 0.70 under the same conditions. The results are consistent with the sample containing X and Y.

A careful conclusion is: “The sample gives two spots matching the reference Rf values, supporting the presence of X and Y under these conditions.” Avoid “This proves the sample contains only X and Y.” Different substances can share an Rf in a particular solvent, and overlapping spots can hide components.

Similarly, one spot is consistent with a pure substance in that test, but does not prove purity. For GCSE questions, distinguish the straightforward pattern interpretation from the stronger claim that a single run identifies everything present. CCEA’s chromatography factfile explains this identification limit; it is an A-level source used here for the scientific caveat, not for England GCSE assessment requirements.

Try this original practice calculation without looking back: a spot centre is 36 mm from the baseline and the solvent front is 60 mm from it. What is the Rf? A reference substance has the same Rf under the same conditions: what can you conclude?

Answer: 36 ÷ 60 = 0.60, with no units. The match supports a possible identity; it does not prove it. If your answer is 1.67, you divided the solvent distance by the spot distance.

Before finishing a chromatography answer, check that you have named the starting line, measured to the spot centre, marked the correct solvent front and written a conclusion no stronger than the evidence.

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