AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Chromatography: interpret chromatograms and determine R_f values from chromatograms
A chromatogram shows spots produced by substances carried different distances by a solvent.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A chromatogram shows spots produced by substances carried different distances by a solvent. To interpret it, identify the solvent front, the start line and each spot. The R_f value of a spot is calculated using R_f = distance moved by substance ÷ distance moved by solvent front. Both distances are measured from the start line to the centre of the spot and to the solvent front, in the same units, so R_f has no units and is always between 0 and 1. For example, if a spot moves 3.0 cm and the solvent front moves 6.0 cm, R_f = 3.0 ÷ 6.0 = 0.50. Compare R_f values with known values or reference spots to identify substances, and count spots to judge purity.
What earns the marks
- Identify the start line and the solvent front on the chromatogram before taking any measurements.
- Measure the distance moved by the substance from the start line to the centre of the spot.
- Measure the distance moved by the solvent front from the start line to the solvent front.
- Calculate R_f using R_f = distance moved by substance ÷ distance moved by solvent front.
- Give R_f as a value between 0 and 1 with no units, since it is a ratio of two lengths.
- Use R_f values and spot positions to identify substances by comparison with known or reference values obtained under the same conditions.
- Interpret the number of spots to deduce the minimum number of substances present in a mixture.
Common mistakes and how to put them right
Measuring the distance moved by the substance from the bottom of the paper rather than from the start line.
Put it right: Measure from the start line to the centre of the spot.
Dividing the distance moved by the solvent front by the distance moved by the substance.
Put it right: Use R_f = distance moved by substance ÷ distance moved by solvent front.
Giving R_f a unit such as cm.
Put it right: R_f is a ratio of two lengths, so it has no units.
Measuring to the top edge of a spot instead of its centre.
Put it right: Measure to the centre of the spot for a consistent and accurate value.
Exam tips
- Show the measurements and the division clearly so that the method is visible.
- Check that the R_f value is between 0 and 1; if it is greater than 1, the calculation has been inverted.
- Use the same unit for both distances, or convert them before dividing.
- When identifying a substance, compare its R_f value with known values obtained using the same solvent and conditions.
Try the lesson
The start of the Chromatography: interpret chromatograms and determine R_f values from chromatograms lesson, made from checked teaching notes for this specification point.
Have a go first
On a chromatogram, a spot of dye has moved 2.0 cm from the start line. The solvent front has moved 8.0 cm from the start line. What is the R_f value of the dye?
Listen
A chromatogram shows spots produced by substances carried different distances by a solvent. To interpret it, identify the solvent front, the start line and each spot. The R_f value of a spot is calculated using R_f = distance moved by substance ÷ distance moved by solvent front. Both distances are measured from the start line to the centre of the spot and to the solvent front, in the same units, so R_f has no units and is always between 0 and 1. For example, if a spot moves 3.0 cm and the solvent front moves 6.0 cm, R_f = 3.0 ÷ 6.0 = 0.50. Compare R_f values with known values or reference spots to identify substances, and count spots to judge purity. A strong answer earns marks for these points. Identify the start line and the solvent front on the chromatogram before taking any measurements. Measure the distance moved by the substance from the start line to the centre of the spot. Measure the distance moved by the solvent front from the start line to the solvent front. Calculate R_f using R_f = distance moved by substance ÷ distance moved by solvent front. A common slip: measuring the distance moved by the substance from the bottom of the paper rather than from the start line. Instead, measure from the start line to the centre of the spot. Exam tip: Show the measurements and the division clearly so that the method is visible.
Key words
Say what each word means before you turn the card.
Chromatogram
Try answering aloud or in your head before turning the card.
R_f value
Try answering aloud or in your head before turning the card.
Separation and identification
Try answering aloud or in your head before turning the card.
See every definition
- Chromatogram
- Shows spots produced by substances carried different distances by a solvent, and is interpreted by identifying the solvent front, the start line and each spot.
- R_f value
- Calculated using distance moved by substance divided by distance moved by solvent front, measured from the start line, with no units and always between 0 and 1.
- Separation and identification
- Comparing R_f values with known values or reference spots identifies substances, and counting spots judges purity.
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: interpret chromatograms and determine R_f values from chromatograms
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