Topic 5 of 6
Read the evidence in a chromatogram
Compare spots under the same conditions.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Read the evidence in a chromatogram
Compare spots under the same conditions.
Place small sample spots on a pencil baseline above the solvent level. Ink would dissolve and add unwanted spots. As solvent rises through the paper, substances repeatedly distribute between the solvent and paper; different substances move different distances. Keep the vessel covered and mark the solvent front before it evaporates.
Compare an unknown with standards
Unknown U has two spots at the same heights as one spot in A and one in B.
Several spots indicate a mixture of separated components. A single spot suggests purity under these conditions, but two substances might move together. A spot matching a known sample supports identification; comparison is meaningful only with the same solvent and conditions.
Worked example
Calculate an Rf value
A spot centre moves 3.6 cm from the baseline; the solvent front moves 6.0 cm.
- Rf = distance travelled by substance / distance travelled by solvent front.
- Both distances start at the baseline, not at the bottom of the paper.
- Rf = 3.6/6.0 = 0.60.
Rf = 0.60, with no units. Compare Rf values only under the same conditions.
A colourless substance may need a locating agent, which makes its spot visible, usually by forming a coloured product. The agent reveals where it travelled; it does not make it travel. Specific locating agents are not required.