A forensic scientist is testing an unknown white powder found at a crime scene. She dissolves a small amount of the powder in water and carries out paper chromatography using two different solvents. In solvent A, the sample produces a single spot. In solvent B, the sample also produces a single spot, but the spot appears at a different position on the paper. She also measures the melting point of the dry powder and finds it melts sharply at 134 °C. A reference database lists compound X as having a melting point of 134 °C. Explain what the results of both tests tell the scientist about the identity and purity of the unknown powder.

WJEC A-Level Chemistry (Wales) — 1.3 Chemical bonding · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The single spot in both solvents shows the powder is a pure substance, not a mixture. The spot moves to a different position in solvent B, giving a different Rf value – this is normal as Rf depends on the solvent system and can be used to compare with reference Rf values. The sharp melting point at 134 °C indicates the powder is pure. The melting point matches the database value for compound X, so the unknown is most likely compound X.

Examiner tips

  • State that a single spot in each solvent proves purity; note the Rf change is expected and useful for identification.
  • Mention the sharp melting point as evidence of purity and the match to the database value to identify the compound.

Common mistakes

  • Confusing a single spot with a mixture; forgetting that Rf changes with solvent.
  • Claiming the compound is unknown because Rf differs, ignoring the database comparison.

Mark scheme (5 marks)

  1. A pure substance produces a single spot in chromatography (in all solvents), so the single spots in both solvents indicate the powder is a pure substance / not a mixture.
  2. The Rf value (position of the spot) differs between the two solvents, which is expected because different compounds have different Rf values in different solvents.
  3. The Rf value(s) can be compared to reference values / the database to help identify the substance.
  4. A pure substance has a sharp melting point, so the sharp melting point at 134 °C indicates the powder is pure.
  5. The melting point of 134 °C matches the database value for compound X, so the powder is likely to be compound X.

Key terms in this question

paper chromatography

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