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Chemistry · Grades 9–12

Chromatography and evidence

Interpret a supplied chromatogram without claiming one spot proves identity.

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Learn the idea

Separation

Components distribute differently between a mobile phase and a stationary phase.

Rf

In a planar example, Rf = distance traveled by spot / distance traveled by solvent front from the same origin.

Comparison

Rf values are comparable only under matching conditions.

Limits

Co-migration is possible, so matching spots alone need not prove identical substances.

Worked examples

A spot travels 3.0 cm while the solvent front travels 6.0 cm from the same baseline. Find Rf.

Rf = 3.0/6.0 = 0.50.

0.5

Which baseline should be used for both distances in an Rf calculation?

Both distances must share the origin.

The same starting origin.

Check your understanding

  1. Does one visible spot prove a sample is pure?

    1. Only if the spot is round.
    2. Not necessarily; components can co-migrate or be undetected.
    3. Yes, always.

  2. A spot has Rf = 0.25 and the solvent front traveled 8 cm. How far did the spot travel?

  3. Why should solvent and stationary phase be controlled when comparing Rf values?

Answers and marking notes

Answers and marking notes
  1. Not necessarily; components can co-migrate or be undetected.

    Method resolution and detection impose limits.

  2. 2 cm

    Distance = 0.25 × 8 = 2 cm.

  3. Changing conditions changes the distribution and movement of components.

    Rf is condition-dependent.

Further reading and review references

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Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.