Knowledge Organiser: Chromatography
Key Terms
- Mobile phase: solvent that moves
- Stationary phase: paper (stays fixed)
- Solvent front: furthest point of solvent
- Rf = substance distance ÷ solvent distance
Rf Value Rules
- Always between 0 and 1
- No units
- More soluble → higher Rf
- Same Rf in same solvent = probably same substance
Interpreting Results
- 1 spot = possibly pure substance
- Multiple spots = definitely a mixture
- Matching Rf values → likely same compound
- Spot at baseline = insoluble in solvent used
Practical Technique
- Use PENCIL (not pen) for baseline
- Start line must be above solvent
- Mark solvent front immediately
- Measure from start line to spot centre
Key Equations
- Rf = distance moved by substance ÷ distance moved by solvent
- Rf has no units and is always between 0 and 1
- Measure both distances from the BASELINE (pencil start line)
Common Mistakes
- Measuring Rf from the solvent level not the baseline: Both the substance distance AND solvent front distance must be measured from the PENCIL BASELINE — not from where the solvent started
- Drawing the baseline in pen: Always use PENCIL for the baseline — pen dissolves in the solvent and contaminates the chromatogram
- Saying one spot proves purity: One spot suggests a pure substance but does NOT prove it — a different solvent might separate components that appear as one spot
- Putting the baseline below the solvent surface: The baseline must be ABOVE the solvent level — if it is in the solvent, the sample washes off before separation
Memory Aid
Rf formula: "Rf = substance over Front" Rf = distance by Substance ÷ distance by solvent FrontThe capital S on top, capital F on bottom — "Substance over Front" For what moves where: "More Soluble = Sprints further up" For phases: "Mobile MOVES, Stationary STAYS" Quick Check: A student runs chromatography on a suspected food dye and gets three spots. What does this tell them? Show Answer The food dye is a mixture — it contains at least three different coloured compounds. It is not a pure substance.