Paper Chromatography and the Rf Value · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Split the ink, read the spots

Science · Matter & Materials · ages 14-15
Name ______________________   Date ____________

  1. A pen baseline is suitable because its ink cannot move in the solvent.

    Circle one:   True   False

  2. Why can dyes separate as solvent moves through chromatography paper?

    • Dyes interact differently with solvent and stationary phase.
    • The solvent sorts every dye only by the mass of its molecules.
    • The paper changes each dye into a new substance as it moves up.

  3. Measured from the same baseline, a spot centre is 3 cm away and the solvent front is 10 cm away. What is the spot's Rf value?

    Answer: ______________

  4. Why should the starting sample spots be above the solvent level in the container?

    • To stop any solvent from rising up through the chromatography paper
    • To make the pencil line dissolve before it contaminates the sample
    • To keep sample out of the solvent pool at the start

  5. A sample produces two well-separated dye spots in a clean chromatography run. What does this show?

    • A pure substance whose dye always separates into two colours
    • A mixture with at least two detected components
    • A sample with exactly two components and no undetected ones

  6. Which observation supports, but does not prove, a match between an unknown ink and a known ink?

    • Matching spot colours and Rf values under shared conditions
    • Matching pen prices and barrel colours, even when ink patterns differ
    • Matching strip lengths, even when paper types and solvents differ

  7. The same unchanged dye has different Rf values with two solvents. Paper and temperature are unchanged. What explains the difference?

    • Its identity changed because each dye has one fixed Rf in all solvents.
    • Its molecules became heavier because farther-moving dyes have more mass.
    • The balance between moving and stationary phases changed.

  8. A spot travels 2 cm and the solvent front travels 8 cm, both from the baseline. Tom calculates Rf = 8 / 2 = 4. Which correction should he make?

    • Keep 4, because the solvent-front distance belongs on top.
    • Use spot distance / front distance: 2 / 8 = 0.25.
    • Add both distances, because their combined travel gives Rf = 10.
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Split the ink, read the spots W1-mt_BAcCcz11ls-s1

  1. False · False. Pen ink may dissolve and travel with the sample, adding unwanted spots. Use pencil for the baseline.
  2. Dyes interact differently with solvent and stationary phase. · Different interactions with the mobile and stationary phases can give dyes different travel distances without changing their identities.
  3. 0.3 · Rf = spot distance / solvent-front distance = 3 / 10 = 0.3. The centimetres cancel, so Rf has no units.
  4. To keep sample out of the solvent pool at the start · Submerged sample spots may dissolve directly into the liquid in the container instead of travelling from a clear starting point on the paper.
  5. A mixture with at least two detected components · Two separated sample spots show at least two detected components. Other components could overlap or remain undetected.
  6. Matching spot colours and Rf values under shared conditions · A matching pattern under shared conditions supports a possible match. It does not uniquely identify the ink or pen.
  7. The balance between moving and stationary phases changed. · A solvent change can alter the dye's distribution between mobile and stationary phases, changing Rf without changing the dye's identity.
  8. Use spot distance / front distance: 2 / 8 = 0.25. · Tom reversed the ratio. Rf = 2 / 8 = 0.25. His arithmetic gives 4, but that result is outside the valid range.
Worksheet · LightMySky