Chromatography — AQA GCSE Chemistry
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Chromatography explained
Chromatography covers 4 specification points, including Chromatography can be used to separate… and Chromatography involves a stationary phase and….
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Open a point below for its explanation and revision guidance.
Chromatography can be used to separate mixtures and can give information to help identify substances.
Chromatography is a separation technique in which a mixture is carried by a mobile phase through a stationary phase. Because different substances are distributed differently between the two phases, they travel different distances and separate. In paper chromatography, a spot of mixture is placed on chromatography paper and the bottom edge is dipped into a solvent below the spot. The solvent is the mobile phase and the water held in the paper is the stationary phase. As the solvent rises, substances move at different rates and form separate spots. The distance moved by a substance compared with the distance moved by the solvent front gives an Rf value. Rf values can be compared with known values to help identify substances, and a pure substance usually gives a single spot.
Chromatography involves a stationary phase and a mobile phase.
Every chromatography method has two phases. The stationary phase does not move; in paper chromatography it is the chromatography paper, and in thin-layer chromatography it is a thin layer of solid such as silica on a plate. The mobile phase moves; it is a solvent or gas that carries the substances through or over the stationary phase. A mixture is carried by the mobile phase, and each substance spends a different proportion of time in each phase. A substance that is more strongly attracted to the stationary phase moves slowly, while one that is more soluble in the mobile phase moves quickly. This difference in distribution is what causes separation. You should be able to name the phases in a described method and explain the role of each.
Separation depends on the distribution of substances between the phases.
In chromatography, each substance in a mixture distributes itself between the stationary phase and the mobile phase. Distribution means that some of the substance is held by the stationary phase while some is carried by the mobile phase. A substance that is more strongly attracted to, or more soluble in, the mobile phase spends more time moving and travels further. A substance that is more strongly attracted to the stationary phase spends more time held back and travels less far. Because different substances have different distributions, they move at different rates and separate into spots or bands. This explains why a mixture produces several spots while a pure substance usually produces one. The distribution also explains why changing the solvent can change the separation.
Required practical 6: investigate how paper chromatography can be used to separate and tell the difference between coloured substances. Students should calculate R values.
Paper chromatography separates coloured substances by allowing a solvent to move up paper, carrying dissolved dyes at different rates. A pencil baseline is drawn near the bottom, small spots of known dyes and an unknown mixture are applied, and the paper is lowered into solvent so the baseline stays above the liquid. As solvent rises, components partition between mobile and stationary phases and separate. The distance each spot travels relative to the solvent front gives its Rf value. Comparing Rf values and spot colours identifies components in an unknown mixture. For example, a purple ink may separate into red and blue spots matching known dyes.
Your focus
- Describe how paper chromatography separates the substances in a mixture.
- Calculate and interpret Rf values from measurements on a chromatogram.
- Use chromatography results to help identify substances and recognise a pure substance from a single spot.
Show all 12 objectives
- Identify the stationary and mobile phases in a given chromatography method.
- Explain the role of each phase in carrying and separating substances.
- Apply the terms stationary phase and mobile phase to unfamiliar chromatography contexts.
- Explain how the distribution of a substance between the phases determines how far it travels.
- Use distribution to explain why the substances in a mixture separate.
- Predict how changing the mobile phase can affect the separation of a mixture.
- Carry out paper chromatography to separate coloured substances in a mixture.
- Measure spot and solvent front distances accurately from a pencil baseline.
- Calculate Rf values and use them with spot colours to identify substances.
Chromatography exam tips
Marking Points
- State that chromatography separates substances in a mixture because they move at different rates through the stationary and mobile phases.
- Describe the practical method: place a spot of mixture on the baseline, dip the bottom of the paper in solvent below the spot, and allow the solvent to rise.
- Calculate Rf as the distance moved by the substance divided by the distance moved by the solvent front.
- Use Rf values or the number of spots to help identify substances, comparing results with known values.
- Explain that a pure substance typically produces a single spot in a given solvent.
- Identify the stationary phase as the phase that does not move, such as the paper or the solid coating on a chromatography plate.
- Identify the mobile phase as the phase that moves, such as the solvent in paper chromatography or a carrier gas in gas chromatography.
- Explain that the mobile phase carries the substances through or over the stationary phase.
- Explain that separation occurs because substances are distributed differently between the two phases.
- Apply the terms correctly to an unfamiliar chromatography method described in the question.
- State that each substance distributes between the stationary phase and the mobile phase.
- Explain that a substance spending more time in the mobile phase travels further up the chromatogram.
- Explain that a substance spending more time in the stationary phase travels less far.
- Link different distributions of different substances to the separation of a mixture into separate spots or bands.
- Use the idea of distribution to explain why changing the solvent or phase can change the separation.
- Draw the baseline in pencil, not ink, because ink would dissolve and contaminate the chromatogram.
- Keep the solvent level below the baseline so the applied spots do not wash directly into the solvent reservoir.
- Allow the solvent front to rise most of the paper length, then remove the paper and mark the solvent front immediately in pencil before it evaporates.
- Measure the distance travelled by each spot from the baseline to the centre of the spot, and measure the solvent front distance from the same baseline.
- Calculate Rf as spot distance divided by solvent front distance; Rf has no units and is always less than 1.
- Identify substances by matching both spot colour and Rf value with known reference spots run on the same chromatogram.
Examiner Tips
- 💡Show the Rf calculation clearly, including the measured distances and the division.
- 💡When identifying a substance, compare its Rf value with a known value obtained under the same conditions.
- 💡Describe the method in order, including where the spot is placed and why the solvent must stay below the baseline.
- 💡Name both phases explicitly and state which one moves.
- 💡Link each phase to its role: the mobile phase carries substances, and the stationary phase slows them to different extents.
- 💡Use the terms stationary and mobile consistently when explaining any chromatography method.
- 💡Use the phrase distribution between the phases when explaining why substances separate.
- 💡Link the distance travelled to the time spent in each phase rather than to a vague idea of attraction.
- 💡When a question gives a changed solvent, explain the effect by referring to the changed distribution of the substances.
- 💡State the formula Rf = distance moved by spot ÷ distance moved by solvent front before substituting values.
- 💡Give Rf values to two significant figures or as a decimal, and never attach units because it is a ratio.
- 💡When identifying an unknown, compare its Rf and colour with known spots run on the same paper under the same conditions.
Common Mistakes
- Placing the spot below the solvent level; correct this by ensuring the baseline is above the solvent so the mixture does not dissolve into the solvent reservoir.
- Measuring the distance moved by the substance from the top of the paper; correct this by measuring from the baseline to the centre of the spot.
- Treating Rf as a fixed value for a substance in every solvent; correct this by stating that Rf depends on the solvent and conditions used.
- Confusing the stationary and mobile phases; correct this by remembering the stationary phase (e.g. paper) does not move and the mobile phase (e.g. solvent) moves.
- Calling the solvent the stationary phase; correct this by stating that the solvent is the mobile phase because it moves.
- Thinking the phases must be liquid and solid; correct this by recognising that a mobile phase can be a gas and a stationary phase can be a liquid or solid.
- Saying that heavier substances always travel further; correct this by explaining that distance travelled depends on distribution between the phases, not mass alone.
- Thinking all substances travel the same distance because they are in the same solvent; correct this by stating that each substance has its own distribution and therefore its own rate of movement.
- Describing separation as substances being filtered by size; correct this by explaining that separation depends on how each substance distributes between the stationary and mobile phases.
- Using a pen to draw the baseline, which dissolves and adds extra spots; correction: use a pencil because graphite is insoluble in the solvent.
- Placing the baseline below the solvent surface, so spots dissolve into the solvent rather than separating; correction: keep the solvent level below the baseline.
- Measuring spot distance to the top edge of a spot rather than its centre, which skews Rf; correction: measure from the baseline to the centre of the spot.