This topic summary covers Knowledge Organiser: Chromatography within Chromatography for GCSE Chemistry. Revise Chromatography in Chemical Analysis for GCSE Chemistry with 23 exam-style questions and 15 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 13 of 13 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
Practice questions for Chromatography
What is the purpose of chromatography?
Explain how a chromatogram can be used to determine whether a substance is pure or a mixture.
Quick recall flashcards
What is chromatography?
A separation technique used to separate mixtures into their individual components based on different affinities for mobile and stationary phases
What does Rf stand for and what does it measure?
Rf = Retention factor. It measures how far a substance travels compared to the solvent (distance moved by substance ÷ distance moved by solvent)