Rf Value – IGCSE Chemistry Definition
IGCSE Chemistry definition of Rf value: the ratio of distance moved by a substance to distance moved by the solvent in chromatography. Covers calculation and 0620 tips.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Rf value calculations appear on Paper 6 and Paper 4. The calculation itself is simple division, but candidates often lose marks by measuring from the wrong point or by not showing their working. Understanding what the Rf value means — and its limitations — is just as important as the calculation.
The 0620 definition
The Rf value (retention factor) is the ratio of the distance moved by a substance to the distance moved by the solvent front during chromatography.
The formula
Rf = distance moved by substance / distance moved by solvent front
Both distances are measured from the baseline (start line) to the centre of the spot (for the substance) or to the solvent front line.
How to measure
- Measure from the pencil baseline to the centre of the spot (substance distance)
- Measure from the pencil baseline to the solvent front (solvent distance)
- Divide: Rf = substance distance / solvent distance
Key properties of Rf values
| Property | Detail |
|---|---|
| Range | Always between 0 and 1 |
| Units | None (it is a ratio) |
| Rf = 0 | Substance did not move from the baseline |
| Rf = 1 | Substance moved with the solvent front |
| Specific to conditions | Same substance has different Rf values in different solvents |
Using Rf values to identify substances
If an unknown substance has the same Rf value as a known reference substance (when run on the same chromatogram, in the same solvent):
- The unknown is likely the same substance
- This is not absolute proof — different substances can occasionally have the same Rf value
Worked exam question
In a chromatography experiment, the solvent front moves 12.0 cm from the baseline. A spot of dye moves 7.2 cm. (a) Calculate the Rf value. [1] (b) A reference table shows that dye X has an Rf value of 0.60 in this solvent. Identify the dye. [1]
Mark scheme
(a) Rf = 7.2 / 12.0 = 0.60 [1]
(b) The dye is X [1] (the Rf values match)
Common exam mistakes
- Dividing the solvent distance by the substance distance (getting Rf > 1) — the substance distance goes on top.
- Measuring from the bottom of the paper instead of from the baseline — all measurements start from the pencil line.
- Giving the Rf value as a percentage — it is a decimal ratio, not a percentage.
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