Chemistry Lab · Free lesson · 22 minutes
A chromatogram is evidence, not a barcode
Calculate dimensionless Rf values from a common baseline and judge what a chromatogram can and cannot establish.
SS3 · practical data analysis01 · Think first
Start with a question.
Two coloured spots move the same distance on separate pieces of paper. Are they definitely the same substance? A comparison only makes sense when the conditions are known.
What you already know
- Measure distances from a common origin
- Divide decimal numbers
- Distinguish a mixture from a pure substance
02 · See it
Measure from the same baseline
Initial example: Use the centre of the spot and the solvent front, measured from the baseline. The diagram displays a 3.6 cm spot distance and 6.0 cm solvent-front distance.
3.6 cm / 6.0 cm = 0.60. The ratio has no unit. These are supplied measurements, not identification proof.
- Baseline
- 0 cm
- Current spot centre
- 3.6 cm
- Solvent front
- 6.0 cm
- Current Rf
- 3.6/6.0 = 0.60
Use the justified worked steps below to explore this idea.
03 · Show steps · explain why
One justified step at a time.
- 1
Identify the baseline, spot centre and solvent front
Using different origins changes the ratio. Marking the front promptly prevents losing its position when solvent evaporates.
04 · Mistake clinic
Find the first wrong idea.
The spot is 2 cm above the baseline and the front 5 cm above it. Rf = 5/2 = 2.5 cm.
Compare the correction and its reason
Rf = 2/5 = 0.40, with no unit.
The numerator and denominator were reversed, and a cancelled unit was retained. The component has moved only 40% of the front's distance.
Check: Check the origin, order and unit cancellation. A ratio above one signals a likely measurement or interpretation problem in this model.
A mistake is evidence about the next idea to repair. It is not a label for the learner.
05 · Your turn
Try first. Then use a hint.
06 · Check & reflect → next step
Understanding includes explaining.
Separate an observation, a calculation and an inference. State one limitation on your conclusion.
Apply this evidence-first reasoning to another practical dataset; revisit the particle lesson if separation mechanisms are unclear.
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JEM-CH-C16-002 · Phase 5D Organic Metals Environment Analysis Practical Exam Overlays · Lab 16 · printed pp. 38–39
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