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Rf = distance moved by substance ÷ distance moved by solvent, both measured from the origin. Free GCSE AQA notes with key terms and 9 questions answered.
Chromatogram Analysis and Rf Values is part of Purity, formulations and chromatography in GCSE AQA Higher Chemistry.
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Rf = distance moved by substance ÷ distance moved by solvent, both measured from the origin. Free GCSE AQA notes with key terms and 9 questions answered.
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Use this lesson to stay inside Purity, formulations and chromatography while connecting the detail back to the wider Topic 8: Chemical analysis topic.
Read the core explanations from Chromatogram Analysis and Rf Values before testing yourself from memory.
When forensic scientists analyse ink from a ransom note, they use chromatography. The result? A chromatogram - a piece of paper covered in coloured spots that reveals exactly which dyes were mixed to make that ink.
Each spot on a chromatogram has travelled a specific distance from where it started. This distance isn't random - it's like a fingerprint for that substance. The clever bit is turning these distances into numbers we can compare.
Think of a chromatogram like a race track. All the substances start at the same line (the origin), but they don't all finish at the same place. Some travel further up the paper than others, creating a unique pattern of spots.
Here's where chromatography gets mathematical. We need a way to compare results from different experiments, even when the papers are different sizes or the solvent travels different distances.
The Rf value is always a number between 0 and 1. Why? Because no substance can travel further than the solvent that's carrying it. If the solvent travels 10 cm, the maximum any substance can travel is also 10 cm, giving Rf = 10/10 = 1.
Most substances have Rf values between 0.1 and 0.9. An Rf of 0.1 means the substance barely moved (sticky to the paper), while 0.9 means it nearly kept up with the solvent front.
Getting accurate Rf values depends entirely on measuring correctly. One small error in measurement can completely change your calculated Rf value and lead to wrong conclusions about what substances are present.
For the substance distance, always measure to the centre of the spot, not the edge. Spots aren't perfect circles - they're often smudged or have fuzzy edges. The centre gives the most reliable measurement.
The solvent front is usually marked with a pencil line as soon as you remove the paper from the solvent. This line shows exactly how far the solvent travelled before you stopped the experiment.
Let's work through a typical exam question. Remember, examiners award marks for showing your method clearly, not just getting the right answer.
Worked example: Calculating Rf Values from a Chromatogram
A chromatogram shows a blue spot 4.2 cm from the origin. The solvent front is 7.0 cm from the origin. Calculate the Rf value. [3 marks]
Answer: Rf = 0.60
Notice how we gave the answer to 2 significant figures. This matches the precision of our measurements (4.2 cm and 7.0 cm both have 2 significant figures). Your final answer should never be more precise than your starting data.
Here's where Rf values become powerful tools for identification. Each substance has a characteristic Rf value under specific conditions (same solvent, same temperature, same type of paper).
If you know that caffeine has an Rf value of 0.42 in a particular solvent system, and you find a spot with Rf = 0.42 in your unknown sample, you've got strong evidence that caffeine is present.
Worked example: Identifying Unknown Substances
A food colouring sample produces spots with Rf values of 0.23, 0.45, and 0.67. Using the reference table, identify which dyes are present. Reference data: Red dye = 0.23, Blue dye = 0.45, Yellow dye = 0.67, Green dye = 0.31
Answer: The food colouring contains red dye, blue dye, and yellow dye
Rf calculations are straightforward, but students lose marks through careless errors. Here's how to avoid the most common traps and secure full marks.
The biggest mistake? Measuring from the wrong starting point. Always measure from the origin (where you spotted the sample), not from the bottom of the paper or from another spot.
In identification questions, examiners want you to be specific. Don't just say 'the substances are the same' - state exactly which substance is present based on the Rf match.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A chromatogram is the pattern of separated substances produced by chromatography, showing spots at different distances from the origin. Each spot's distance acts like a fingerprint for that substance, which is turned into an Rf value to compare.
Rf = distance moved by substance ÷ distance moved by solvent, both measured from the origin. For a spot 4.2 cm from the origin with the solvent front at 7.0 cm, Rf = 4.2 ÷ 7.0 = 0.60.
The solvent front is the furthest point reached by the solvent as it moves up the chromatography paper. It is marked with a pencil line as soon as the paper is removed from the solvent, showing how far the solvent travelled.
Forensic scientists use chromatography to analyse ink, such as from a ransom note. The chromatogram reveals which dyes were mixed to make the ink, and comparing their Rf values with known substances identifies them.
An Rf value is always between 0 and 1 because no substance can travel further than the solvent that carries it. An Rf of 0.1 means the substance barely moved, while 0.9 means it nearly kept up with the solvent front.
A substance is identified by matching its Rf value with reference values measured under the same conditions. For example, spots at Rf 0.23, 0.45 and 0.67 that match the reference values for red, blue and yellow dyes show those dyes are present.
On a chromatogram, measure from the origin, where the sample was spotted, to the centre of each spot, and from the origin to the solvent front. Measuring from the bottom of the paper or to the edge of a spot gives wrong Rf values.
An Rf value should be given to the same number of significant figures as the measurements. For example, 4.2 cm and 7.0 cm both have 2 significant figures, so Rf = 0.60.
A low Rf value, such as 0.1, means the substance barely moved because it is 'sticky' to the paper. A high Rf value, such as 0.9, means it nearly kept up with the solvent front; most substances have Rf values between 0.1 and 0.9.