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IGCSE · Practice questions · Chromatography

Read the chromatogram.

Six original Cambridge-style questions on paper chromatography and Rf values.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Pencil line, low solvent, match the spots.

01
Recall
[3 marks]

Give three precautions when setting up paper chromatography, with a reason for each.

  • Draw the baseline in pencil, because ink would dissolve in the solvent. ✓
  • Keep the solvent level below the baseline, so the samples do not dissolve into the solvent in the beaker. ✓
  • Cover the beaker, so the solvent does not evaporate from the paper (or use small spots so they do not merge). ✓
02
Analysis
[2 marks]

A food colouring gives three spots. Two of them are at the same heights as spots from the permitted dyes E102 and E133. What can you conclude?

  • The food colouring is a mixture of at least three substances. ✓
  • It contains E102 and E133, plus one other substance that is not either of these. ✓
03
Calculation
[2 marks]

(Extended) On a chromatogram the solvent front is 8.5 cm from the baseline. Spot S is 3.4 cm from the baseline. Calculate the Rf value of S.

  • Rf = distance moved by substance ÷ distance moved by solvent front ✓
  • = 3.4 ÷ 8.5 = 0.40 ✓
04
Calculation
[2 marks]

(Extended) Amino acid T has an Rf value of 0.35 in a particular solvent. In a run with the same solvent, the solvent front moves 6.0 cm. How far from the baseline will the spot for T be?

  • Distance = Rf × solvent front distance ✓
  • = 0.35 × 6.0 = 2.1 cm ✓
05
Explain
[2 marks]

(Extended) Amino acids are colourless. Explain how their positions on a chromatogram are found.

  • The dried chromatogram is sprayed with a locating agent. ✓
  • The locating agent reacts with the amino acids to form coloured spots that can be seen and measured. ✓
06
Explain
[2 marks]

Explain why different substances in a mixture travel different distances up the paper.

  • Each substance has a different solubility in the solvent and a different attraction to the paper. ✓
  • More soluble substances (less attracted to the paper) are carried further by the solvent. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.