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K324 / 2027
Experimental Chemistry

Lesson 5 of 6

Read the evidence in a chromatogram

Compare spots under the same conditions.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can describe separation methods.Syllabus

    Syllabus K324, 1.2(a). Describe separation methods

  • I can interpret chromatograms.Syllabus

    Syllabus K324, 1.2(c). Interpret chromatograms

  • I can explain locating agents.Syllabus

    Syllabus K324, 1.2(d). Explain locating agents

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.

A and B were run alongside U using the same paper and solvent.

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.

Pure only

Worked exampleCalculate an Rf value

A spot centre moves 3.6 cm from the baseline; the solvent front moves 6.0 cm.

Try it first. Then show the worked stepsHide the worked steps
  1. Rf = distance travelled by substance / distance travelled by solvent front.
  2. Both distances start at the baseline, not at the bottom of the paper.
  3. Rf = 3.6/6.0 = 0.60.
Answer

Rf = 0.60, with no units. Compare Rf values only under the same conditions.

Pure only

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.

Check your understanding

U gives two spots that match the spots of A and B. Can you be certain that U contains A and B and nothing else?

  1. No. U cannot contain A or B, because a mixture gives spots in new positions.
  2. Yes. Two spots means two substances, and both spots match a known reference.
  3. No. A and B are probably present, but another substance could hide under a spot.
Show the answer

No. A and B are probably present, but another substance could hide under a spot.

Matching spots support A and B being present. Another substance could have the same Rf in this solvent, or not show up under these conditions, so the conclusion is not certain.