Lesson focus: Paper chromatographyA method of separating soluble substances in a mixture: a solvent (the mobile phase) carries them different distances through paper (the stationary phase). separates dissolved components by their relative attraction to the mobile and stationary phaseThe part of chromatography that stays still, such as the paper..
Carry out and interpret paper chromatography, including calculating and qualifying Rf valueA number found by dividing how far a spot moved by how far the solvent moved, used to identify the substance..
Build on earlier learning
ChromatographyYear 7 separated dyes. This lesson adds phase reasoning, reference comparison and Rf calculations.
Practical
Chromatography and Rf values
- Run a chromatogram of the inks with a pencil start line.
- Measure the distance moved by each spot and by the solvent front.
- Rf = distance moved by spot ÷ distance moved by solvent.
Record: Rf values for each spot and identification of the unknown.
You will be successful when you can…
- Describe a valid method.
- Explain separation using phases.
- Calculate Rf.
- Evaluate identification evidence.
Words that carry the science
- chromatography
- A method separating mixture components through different distributions between phases.
- stationary phase
- The phase that remains fixed; paper is treated as the stationary phase in this model.
- mobile phase
- The solvent moving through the stationary phase.
- solvent front
- The furthest point reached by the solvent.
- chromatogram
- The separated pattern produced.
- Rf value
- Distance moved by substance divided by distance moved by solvent front.
A valid method protects the evidence
01A valid method protects the evidence
Draw a pencil baseline above the solvent level, place small concentrated sample spots on it and stand the paper in solvent without submerging the spots. Cover the vessel and allow solvent to rise.
Remove the paper before solvent reaches the top and mark the solvent frontThe highest point the solvent reaches as it soaks up the chromatography paper. immediately. Pencil is used because graphite does not dissolve like many inks.
02Components distribute differently between phases
The solvent is the mobile phaseThe liquid in chromatography that moves up the paper and carries the substances with it.; paper acts as the stationary phaseThe part of chromatography that stays still, such as the paper. in this school model. Components that are more soluble in the solvent or less strongly retained travel farther.
A pure substance can give one spot under a particular set of conditions, while a mixture can give several. A colourless substance needs a locating method.
03Rf is conditional identification evidence
Calculate Rf = distance moved by substance ÷ distance moved by solvent front. Values lie between 0 and 1 when measured from the same baseline.
Compare unknown and reference spots only on the same chromatogramThe pattern of spots left on the paper after chromatography. or under identical stationary phaseThe part of chromatography that stays still, such as the paper., solvent and temperature conditions. Matching Rf supports an identification but does not prove it alone.
Paper chromatography
The solvent soaks up the paper and carries each dye with it. Dyes that dissolve better and stick less to the paper travel further.
Controls
Dyes and Rf values are illustrative. X matches A, B and C wherever spots are at the same height.
Calculate an Rf value
A dye spot moves 4.8 cm while the solvent frontThe highest point the solvent reaches as it soaks up the chromatography paper. moves 8.0 cm.
- Measure both distances from the pencil baseline.
- Use Rf = substance distance ÷ solvent-front distance.
- Substitute 4.8 ÷ 8.0.
- Calculate Rf = 0.60 with no unit.
How to make the reasoning visible: The ratio is conditional on the chromatographyA method of separating soluble substances in a mixture: a solvent (the mobile phase) carries them different distances through paper (the stationary phase). conditions and should be compared with references run alongside.
Chromatography investigation
Analyse unknown U against three reference dyes run on the same paper; the solvent front travelled 10.0 cm.
| Sample spot | Distance / cm | Rf | Interpretation |
|---|---|---|---|
| reference A | 2.0 | 0.20 | low travel |
| reference B | 5.5 | 0.55 | middle travel |
| reference C | 8.0 | 0.80 | high travel |
| unknown U: spot 1 | 2.0 | 0.20 | matches A |
| unknown U: spot 2 | 8.1 | 0.81 | supports C |
What does U probably contain?
Why must the solvent start below the baseline?
Check your understanding
Answer from memory, then check against the suggested answer.
State the Rf equation.
Why use pencil?
Does matching Rf prove identity?
A spot remains on the baseline. Give two possible explanations.
How could a second solvent strengthen an identification?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student makes a chromatogram. The ink baseline is below the solvent level and the solvent front is not marked.
Evaluate the method. Identify the errors and their effects.
(4)Suggest improvements and state what a matching spot shows.
(2)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., connect each flaw to the evidence it damages and give a matched correction.