AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Chromatography: distaₙce moved by solveₙt
In chromatography, the R_f value compares how far a dissolved substance travels with how far the solvent front travels.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In chromatography, the R_f value compares how far a dissolved substance travels with how far the solvent front travels. You measure the distance moved by the solvent front from the pencil baseline to the highest point the solvent reached, and the distance moved by the solute from the pencil baseline to the centre of its spot. Both distances must be measured in the same units, usually millimetres or centimetres, and the baseline must be drawn in pencil so it does not dissolve. For example, if a dye spot moves 3.0 cm and the solvent front moves 6.0 cm, R_f = 3.0 ÷ 6.0 = 0.50. R_f has no units and is always between 0 and 1. This measurement underpins identification because a compound's R_f under fixed conditions is characteristic.
What earns the marks
- State that the distance moved by the solvent is measured from the pencil baseline to the solvent front, not from the bottom of the paper.
- State that the distance moved by the solute is measured from the pencil baseline to the centre of the spot, not to its top or bottom edge.
- Explain that both distances must be measured in the same units so that the ratio is dimensionless.
- Calculate R_f using R_f = distance moved by solute ÷ distance moved by solvent, giving a value between 0 and 1.
- Explain that the baseline is drawn in pencil because pencil is insoluble and will not travel with the solvent.
- Describe how the solvent front is marked immediately before the solvent reaches the top of the paper, because it becomes difficult to locate once the paper dries.
Common mistakes and how to put them right
Measuring the solute distance to the top edge of the spot rather than its centre.
Put it right by marking and measuring to the centre of the spot.
Measuring the solvent distance from the bottom edge of the paper rather than the pencil baseline.
Put it right by measuring from the baseline.
Recording R_f with units such as cm.
Put it right by cancelling the units because it is a ratio of two lengths.
Exam tips
- Write the equation R_f = distance moved by solute ÷ distance moved by solvent before substituting values.
- Check that the calculated R_f lies between 0 and 1; a value outside this range indicates a measurement error.
- When comparing spots, use the same baseline and solvent front for every spot on that chromatogram.
Try the lesson
The start of the Chromatography: distaₙce moved by solveₙt lesson, made from checked teaching notes for this specification point.
Have a go first
In paper chromatography, what name is given to the furthest position or line reached by the solvent as it moves up the paper?
Listen
In chromatography, the R_f value compares how far a dissolved substance travels with how far the solvent front travels. You measure the distance moved by the solvent front from the pencil baseline to the highest point the solvent reached, and the distance moved by the solute from the pencil baseline to the centre of its spot. Both distances must be measured in the same units, usually millimetres or centimetres, and the baseline must be drawn in pencil so it does not dissolve. For example, if a dye spot moves 3.0 cm and the solvent front moves 6.0 cm, R_f = 3.0 ÷ 6.0 = 0.50. R_f has no units and is always between 0 and 1. This measurement underpins identification because a compound's R_f under fixed conditions is characteristic. A strong answer earns marks for these points. State that the distance moved by the solvent is measured from the pencil baseline to the solvent front, not from the bottom of the paper. State that the distance moved by the solute is measured from the pencil baseline to the centre of the spot, not to its top or bottom edge. Explain that both distances must be measured in the same units so that the ratio is dimensionless. Calculate R_f using R_f = distance moved by solute ÷ distance moved by solvent, giving a value between 0 and 1. A common slip: Measuring the solute distance to the top edge of the spot rather than its centre. Instead, marking and measuring to the centre of the spot. Exam tip: Write the equation R_f = distance moved by solute ÷ distance moved by solvent before substituting values.
Key words
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Measurement from a baseline
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R_f as a ratio
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Identification of compounds
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See every definition
- Measurement from a baseline
- Both the distance moved by the solvent front and the distance moved by the solute are measured from the pencil baseline.
- R_f as a ratio
- A value comparing how far a dissolved substance travels with how far the solvent front travels, having no units and always between 0 and 1.
- Identification of compounds
- A compound's R_f under fixed conditions is characteristic, so this measurement underpins identification.
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