AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Chromatography involves a stationary phase and a mobile phase.
Full specification wording
Chromatography: Chromatography involves a stationary phase and a mobile phase.
Every chromatography method has two phases.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Confusing the stationary and mobile phases.
Put it right 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.
Put it right by stating that the solvent is the mobile phase because it moves.
Thinking the phases must be liquid and solid.
Put it right by recognising that a mobile phase can be a gas and a stationary phase can be a liquid or solid.
Exam tips
- 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.
Try the lesson
The start of the Chromatography: Chromatography involves a stationary phase and a mobile phase. lesson, made from checked teaching notes for this specification point.
Have a go first
In paper chromatography, which part of the set-up acts as the stationary phase?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Confusing the stationary and mobile phases. Instead, remembering the stationary phase (e.g. paper) does not move and the mobile phase (e.g. solvent) moves. Exam tip: Name both phases explicitly and state which one moves.
Key words
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Stationary phase
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Mobile phase
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Distribution between phases
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See every definition
- Stationary phase
- The phase that does not move, such as the chromatography paper or a thin layer of solid such as silica on a plate.
- Mobile phase
- The phase that moves, a solvent or gas that carries the substances through or over the stationary phase.
- Distribution between phases
- Each substance spends a different proportion of time in each phase, and this difference is what causes separation.
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