A pharmacy technician is making a painkiller tablet. The tablet contains four different substances, each present in a carefully measured quantity to ensure the tablet has the required properties. The technician uses paper chromatography to check that one of the substances, a white pain-relieving compound, is pure before it is added to the tablet. The chromatogram produced shows a single spot. Explain what the single spot tells the technician about the compound, and describe how paper chromatography separates the substances in a mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A pharmacy technician is making a painkiller tablet. The tablet contains four different substances, each present in a carefully measured quantity to ensure the tablet has the required properties. The technician uses paper chromatography to check that one of the substances, a white pain-relieving compound, is pure before it is added to the tablet. The chromatogram produced shows a single spot.
Model answer (5 marks)
A single spot shows that the white pain‑relieving compound is pure, i.e. it contains only one substance and is not a mixture of different compounds. If the compound were impure it would give more than one spot.
In paper chromatography the solvent (mobile phase) moves up the paper by capillary action. The paper itself is the stationary phase. As the solvent rises, the different substances in the mixture are carried with it but they move at different rates because they have different affinities for the stationary phase and different solubilities in the mobile phase. Substances that are more soluble in the solvent and less attracted to the paper travel further up the paper, whereas substances that are less soluble or more strongly adsorbed onto the paper travel less far. This differential movement separates the components of the mixture.
In paper chromatography the solvent (mobile phase) moves up the paper by capillary action. The paper itself is the stationary phase. As the solvent rises, the different substances in the mixture are carried with it but they move at different rates because they have different affinities for the stationary phase and different solubilities in the mobile phase. Substances that are more soluble in the solvent and less attracted to the paper travel further up the paper, whereas substances that are less soluble or more strongly adsorbed onto the paper travel less far. This differential movement separates the components of the mixture.
Examiner tips
- Use the word ‘pure’ and explain it means only one substance is present.
- Mention that a single spot indicates no other components are present.
- Explain the roles of mobile and stationary phases.
- Describe separation in terms of solubility and attraction to the paper.”]
Mark scheme (5 marks)
- The painkiller tablet is a formulation (a mixture designed for a particular use / made up of exact quantities of pure substances).
- A single spot shows the compound is pure (contains only one substance / is not a mixture).
- If the compound were impure / a mixture, it would produce more than one spot.
- In paper chromatography, the solvent (mobile phase) carries the substances up the paper (stationary phase).
- Substances separate because they differ in how soluble they are in the mobile phase / how strongly they are attracted to the stationary phase — the more soluble a substance, the further up the paper it travels.
Key terms in this question
mixture · paper chromatography
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