AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Chromatography: explain how paper chromatography separates mixtures
Paper chromatography separates mixtures because each component distributes differently between two phases.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Paper chromatography separates mixtures because each component distributes differently between two phases. The stationary phase is the chromatography paper; the mobile phase is the solvent that moves up the paper. A component that is very soluble in the solvent and only weakly attracted to the paper travels far with the solvent front. A component that is strongly attracted to the paper and less soluble in the solvent moves only a short distance. Because each substance has its own balance of solubility and attraction, the components separate into distinct spots at different heights.
For example: A mixture of coloured inks may produce several spots, and comparing these with spots from known dyes can identify the components.
What earns the marks
- State that the stationary phase is the paper and the mobile phase is the solvent.
- Explain that components dissolve in the mobile phase and are carried up the paper at different rates.
- Explain that components more strongly attracted to the stationary phase move more slowly and less far.
- Explain that components more soluble in the mobile phase move faster and further up the paper.
- Use the different distances moved to identify components by comparison with known substances or by Rf values.
Common mistakes and how to put them right
Confusing the mobile and stationary phases.
Put it right: The solvent is the mobile phase and the paper is the stationary phase.
Claiming that larger molecules always travel further.
Put it right: Travel depends on solubility in the mobile phase and attraction to the stationary phase.
Thinking that the solvent front and the component spot are the same thing.
Put it right: The solvent front is the furthest point reached by the solvent, while spots are the separated components.
Exam tips
- Use the phrases mobile phase and stationary phase correctly and link each to its role in the separation.
- When explaining a chromatogram, compare the position of spots from the mixture with spots from known reference substances.
- If asked why the solvent must not cover the baseline, state that the mixture would dissolve into the solvent instead of travelling up the paper.
Try the lesson
The start of the Chromatography: explain how paper chromatography separates mixtures lesson, made from checked teaching notes for this specification point.
Have a go first
In paper chromatography, what is the name of the liquid that moves up the paper and carries the substances in the mixture with it?
Listen
Paper chromatography separates mixtures because each component distributes differently between two phases. The stationary phase is the chromatography paper; the mobile phase is the solvent that moves up the paper. A component that is very soluble in the solvent and only weakly attracted to the paper travels far with the solvent front. A component that is strongly attracted to the paper and less soluble in the solvent moves only a short distance. Because each substance has its own balance of solubility and attraction, the components separate into distinct spots at different heights. For example, a mixture of coloured inks may produce several spots, and comparing these with spots from known dyes can identify the components. A strong answer earns marks for these points. State that the stationary phase is the paper and the mobile phase is the solvent. Explain that components dissolve in the mobile phase and are carried up the paper at different rates. Explain that components more strongly attracted to the stationary phase move more slowly and less far. Explain that components more soluble in the mobile phase move faster and further up the paper. A common slip: Confusing the mobile and stationary phases: the error is stating the paper moves or the solvent is stationary. Instead, the solvent is the mobile phase and the paper is the stationary phase. Exam tip: Use the phrases mobile phase and stationary phase correctly and link each to its role in the separation.
Key words
Say what each word means before you turn the card.
Separation of mixtures
Try answering aloud or in your head before turning the card.
Distribution between phases
Try answering aloud or in your head before turning the card.
Solubility and attraction
Try answering aloud or in your head before turning the card.
Identification of substances
Try answering aloud or in your head before turning the card.
See every definition
- Separation of mixtures
- Paper chromatography separates mixtures because each component distributes differently between two phases, so the components separate into distinct spots at different heights.
- Distribution between phases
- Each component distributes differently between the stationary phase, which is the chromatography paper, and the mobile phase, which is the solvent that moves up the paper.
- Solubility and attraction
- Each substance has its own balance of solubility and attraction, so a component very soluble in the solvent and only weakly attracted to the paper travels far with the solvent front.
- Identification of substances
- Comparing spots from a mixture with spots from known dyes can identify the components, using the different distances moved.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Chromatography: explain how paper chromatography separates mixtures
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge