What you'll learn
Chemical analysis: chromatography and Rf values is the topic in AQA GCSE Chemistry concerned with separating mixtures and identifying what they contain. Chromatography is also one of the two required practicals in the analysis unit, so the method carries as many marks as the theory. The underlying principle is a single idea applied carefully: substances separate because they distribute themselves differently between two phases, and how far each travels is characteristic of that substance under those conditions. By the end of this guide you should be able to define pure and impure in the chemical sense, distinguish a pure substance from a formulation, describe the chromatography method including every detail examiners look for, explain separation in terms of the two phases, calculate and interpret Rf values, and identify unknown substances by comparison with references.
Key terms and definitions
Pure substance — a single element or compound not mixed with any other substance
Formulation — a mixture designed as a useful product, in which each component is present in a measured quantity for a purpose
Chromatography — a technique separating the components of a mixture according to how they distribute between a stationary and a mobile phase
Stationary phase — the phase that does not move, which in paper chromatography is the paper
Mobile phase — the phase that moves through the stationary phase, which in paper chromatography is the solvent
Solvent front — the furthest point reached by the solvent
Baseline — the pencil line on which samples are spotted at the start
Rf value — the distance moved by a substance divided by the distance moved by the solvent
Chromatogram — the finished paper showing the separated spots
Reference substance — a known pure substance run alongside an unknown for comparison
Core concepts
Purity in chemistry
In everyday speech, pure usually means natural or unadulterated. In chemistry the word is far narrower: a pure substance is a single element or compound containing nothing else at all.
By this definition, pure orange juice and pure spring water are not pure substances, because both are mixtures. Questions set this contrast deliberately, so read carefully for which sense is intended.
Pure substances melt and boil at specific sharp temperatures. Pure water melts at exactly 0 degrees Celsius and boils at exactly 100 degrees Celsius at atmospheric pressure.
Mixtures behave differently: they melt and boil over a range of temperatures, and an impurity lowers the melting point and raises the boiling point relative to the pure substance. This gives a practical purity test — a sample melting sharply at the expected value is pure, while one melting gradually over several degrees, or at a depressed temperature, is impure.
Formulations
A formulation is a mixture designed as a useful product, in which each component is present in a carefully measured quantity and contributes a particular property to the whole.
Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods are all formulations. A paint, for instance, contains a pigment for colour, a binder to fix the pigment to the surface, a solvent to make it spreadable, and additives that modify drying.
When explaining why something is a formulation, both halves are required: it is a mixture, and its components are present in measured amounts each serving a specific purpose. Saying only that it is a mixture does not earn the mark.
The principle of chromatography
Chromatography separates substances because they distribute themselves differently between two phases.
The stationary phase does not move. In paper chromatography this is the chromatography paper.
The mobile phase moves through the stationary phase, carrying dissolved substances with it. In paper chromatography this is the solvent.
A substance that is more strongly attracted to the mobile phase, and less strongly held by the stationary phase, travels further up the paper. A substance more strongly attracted to the paper travels only a short distance. Because different substances have different balances of attraction, they separate into distinct spots.
The full explanation therefore involves both phases. Answers referring only to solubility in the solvent are incomplete, since the attraction to the paper matters equally.
The method
Several practical details are examined, and each has a reason that must be stated rather than merely the instruction.
Draw the baseline in pencil, not ink. Pencil is insoluble in the solvent and will not move, whereas an ink line would itself separate into its components and contaminate the result.
Place the baseline above the level of the solvent in the container. If the spots were below the solvent level they would dissolve straight into the solvent and be washed away instead of travelling up the paper.
Keep the spots small and concentrated, applying several drops and allowing each to dry. Large spots spread sideways and overlap, making the chromatogram difficult to interpret.
Place a lid on the container. This reduces evaporation of the solvent, which would otherwise alter the conditions during the run.
Remove the paper before the solvent reaches the top, and mark the solvent front immediately in pencil. Once the solvent evaporates the front cannot be located, and without it no Rf value can be calculated.
Interpreting a chromatogram
A pure substance produces a single spot in all solvents. This qualification matters: a single spot in one solvent is suggestive but not conclusive, because two substances may happen to have the same Rf value in that solvent while separating in another.
A mixture separates into two or more spots, and the number of spots gives the minimum number of components present. It is a minimum rather than an exact count for the same reason — two components might travel together.
A substance that does not move from the baseline is insoluble in the solvent used.
Rf values
The Rf value measures how far a substance has travelled relative to the solvent. It is the distance moved by the substance divided by the distance moved by the solvent.
Both distances are measured from the baseline. The distance to the substance is measured to the centre of its spot; the distance to the solvent is measured to the solvent front.
Because a substance can never travel further than the solvent that carries it, the Rf value always lies between 0 and 1. Any answer greater than 1 indicates the division has been inverted, which makes this a useful self-check.
Rf values have no units, because the two distances cancel.
The same substance in the same solvent always gives the same Rf value, which is what allows identification by comparison with published data. Change the solvent and the Rf value changes, which is why the solvent must always be stated alongside an Rf value.
Identifying unknown substances
The standard approach is to run the unknown alongside reference substances on the same paper, in the same solvent, at the same time.
If a spot from the unknown travels the same distance as a reference spot, the two have the same Rf value under identical conditions and the unknown probably contains that substance.
Running the references on the same paper rather than comparing with a separate chromatogram is important, because it guarantees identical solvent, temperature and paper.
This technique is used to check food colourings, to detect banned substances in sport, to analyse inks in forensic work, and to monitor the purity of pharmaceutical products.
Worked examples
Example 1: Calculating an Rf value (3 marks)
On a chromatogram the solvent front is 9.5 centimetres from the baseline and the centre of a spot is 6.1 centimetres from the baseline. Calculate the Rf value.
The Rf value is the distance moved by the substance divided by the distance moved by the solvent, both measured from the baseline. That is 6.1 divided by 9.5, which gives 0.64 to two significant figures. The answer has no units, and as a check it lies between 0 and 1, as every Rf value must.
Example 2: Interpreting a chromatogram (4 marks)
A food colouring is run alongside three known dyes, A, B and C. The colouring produces two spots. One has the same Rf value as dye B, the other matches none of the references. State what can be concluded.
The food colouring is a mixture, because it separated into more than one spot, and it contains at least two different substances. Because one of its spots travelled the same distance as dye B on the same paper in the same solvent, that spot has the same Rf value as B, so the colouring probably contains dye B.
The second spot does not match any reference, so it is a substance not among those tested and cannot be identified from this chromatogram alone. Dyes A, B and C are each pure, since each produced only a single spot.
Example 3: Explaining a method detail (3 marks)
Explain why the baseline must be drawn in pencil and must sit above the level of the solvent.
Pencil is used because it is insoluble in the solvent and therefore stays in place during the run. Ink would dissolve and separate into its own components, which would travel up the paper and contaminate the chromatogram, making the results impossible to interpret.
The baseline must sit above the solvent level so that the spots are not submerged. If the spots were below the surface of the solvent they would simply dissolve into the bulk solvent and be washed away, so no separation would take place.
Common mistakes and how to avoid them
The most frequently penalised error in this topic is explaining separation by solubility alone. Both phases matter: the attraction to the solvent and the attraction to the paper together determine how far a substance travels.
Students often measure to the top or bottom of a spot rather than its centre, which produces a systematically wrong Rf value.
Dividing the wrong way round gives an Rf value above 1, which is impossible. Check every answer against that limit.
Many candidates state that a single spot proves a substance is pure. It shows the substance is pure in that solvent; a different solvent is needed to be confident.
Another common slip is forgetting to mark the solvent front before the paper dries. Without it, no calculation is possible.
Finally, in formulation questions, answers frequently stop at calling the product a mixture, omitting the measured quantities and specific purposes.
Exam technique for "Chemical analysis: chromatography and Rf values"
Show the division in Rf calculations even though it is a single step, and give the answer to two significant figures with no unit.
When describing the method, give the reason alongside every instruction. Marks are awarded for explaining why pencil is used and why the baseline sits above the solvent, not for the instructions themselves.
For interpretation questions, count the spots first and state the minimum number of components, then look for matches with references.
When asked whether a substance is pure, mention the qualification about different solvents. It distinguishes a complete answer from a partial one.
If a question supplies melting point data instead of a chromatogram, quote the figures in your answer — the range and how it compares with the expected value — rather than simply concluding the sample is impure.
Quick revision summary
A pure substance in chemistry is a single element or compound, melting and boiling at sharp temperatures, while impurities lower the melting point and spread it over a range. A formulation is a mixture designed as a product, with each component in a measured quantity for a specific purpose. Chromatography separates substances by how they distribute between a stationary phase, the paper, and a mobile phase, the solvent, with substances more attracted to the solvent travelling further. The baseline is drawn in pencil because pencil is insoluble, and sits above the solvent level so the spots are not washed away; the container is covered to reduce evaporation, and the solvent front is marked before the paper dries. A pure substance gives one spot in all solvents, while a mixture gives several, the number being the minimum count of components. The Rf value is the distance moved by the substance divided by the distance moved by the solvent, measured from the baseline to the spot centre and to the solvent front, always between 0 and 1 and with no units. Unknowns are identified by running them alongside references on the same paper in the same solvent.