Worked example 1
A spot travels 3.6 cm while the solvent front travels 8.0 cm. Calculate Rf.
Try it first: Identify which distance is the denominator.
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Types of Solids and IMFSLab: Paper & Thin-Layer Chromatography of DyesContent level: Core 20 min
Chromatography and Intermolecular Forces
What you'll be able to do: Interpret a chromatogram, calculate Rf values and explain separation using intermolecular forces.
Best after: Justifying Properties with Intermolecular Forces
Introduction
Chromatography turns the whole unit into a measurement. A dye that clings to the paper barely moves; one that prefers the solvent races ahead, and the ratio tells you which forces dominate.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Identify the stationary and mobile phases in paper and thin layer chromatography
- Calculate Rf values from a chromatogram
- Explain differing Rf values using intermolecular forces
- Recognise common procedural errors and their effect on results
Lesson
Two phases in competition
In paper chromatography the stationary phase is the polar cellulose paper with its many O-H groups. In thin layer chromatography it is usually polar silica gel. The mobile phase is the solvent that rises by capillary action. Every component partitions between the two.
Calculating Rf
Measure from the pencil origin line to the centre of the spot, and from the same line to the solvent front. Divide. Rf is always between 0 and 1 and is reproducible for a given solvent and stationary phase.
Rf = dspot / dsolvent
Reading Rf as an IMF statement
A high Rf means the component was carried along, so its attraction to the mobile phase beat its attraction to the stationary phase. A low Rf means the opposite. With a polar stationary phase and a nonpolar solvent, polar dyes hydrogen bond to the paper and stay near the origin.
Rf is a ratio of attractions, not a measure of molecular size alone.
Procedure and error analysis
Draw the origin line in pencil, since ink separates too. Keep the spot above the solvent level, otherwise the sample dissolves into the reservoir and no separation occurs. Cover the chamber so the atmosphere stays saturated and the front rises evenly.
A spot below the solvent level is the single most common cause of a failed run.
Key ideas
Definition
Stationary phase
The fixed material, such as cellulose paper or silica gel, that components adhere to.
Definition
Mobile phase
The solvent that moves through the stationary phase carrying components with it.
Definition
Retention factor
Rf is the spot distance divided by the solvent front distance, always between 0 and 1.
Rule
Interpretation
High Rf means stronger attraction to the mobile phase; low Rf means stronger attraction to the stationary phase.
Equation
Retention factor
Rf = dspot / dsolvent
- Rf = retention factor, dimensionless
- dspot = distance from origin to the centre of the spot
- dsolvent = distance from origin to the solvent front
Worked examples
Worked example 2
On polar paper with a nonpolar solvent, dye X has Rf 0.85 and dye Y has Rf 0.12. Which dye is more polar?
Try it first: Decide which phase is polar before interpreting either number.
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Worked example 3
A student marks the origin line in pen and starts the run with the line below the solvent surface. Predict the outcome.
Try it first: Consider each error separately.
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Common mistakes
Dividing the solvent distance by the spot distance.
Why it's wrong: That inverts the ratio and gives values greater than 1.
Check instead: Confirm your Rf lies between 0 and 1.
Reporting Rf with units.
Why it's wrong: Both distances are lengths, so the units cancel.
Check instead: Write Rf as a bare decimal.
Assuming a high Rf always means a small molecule.
Why it's wrong: Rf reflects the balance of attractions, not size alone.
Check instead: Explain Rf by comparing attraction to each phase.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Chromatography separates components by competition between a stationary phase and a mobile phase. The retention factor Rf is the distance travelled by the spot divided by the distance travelled by the solvent front. Components with stronger attraction to the mobile phase have higher Rf values.
- Chromatography separates by competition between stationary and mobile phases
- Rf is the spot distance divided by the solvent front distance
- High Rf means stronger attraction to the mobile phase
- Pencil origin lines and a spot above the solvent level are essential
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.