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Chemistry notebook Cambridge IGCSE 0620
Contains the answers

Answers · Paper chromatography

Paper chromatography — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. HR1 · Return

Model answer. Use a pencil baseline with small dry sample spots above shallow suitable solvent. Suspend paper with bottom edge in solvent, cover and develop. Lift before the front reaches the top, immediately mark the wet front, then dry. Pencil avoids soluble ink; high spots avoid washing off; front mark preserves the solvent distance.

A full answer includes

  • I described a pencil baseline and small dry spots above suitable solvent.
  • I included covering, development, lifting, immediate marking and drying.
  • I explained the pencil, spot-height and front-mark precautions.

Also accept

  • Equivalent scientifically correct reasoning; exact wording is not required.

Watch for

  • “The sample spots should start submerged.” — Only the lower paper edge touches the reservoir. Solvent rises to the spots; submerging them can wash sample off.

2. HR2 · Return

Model answer. Run unknown and known samples under the same conditions and compare colours and positions or Rf. Matches support identity; one spot can contain overlapping components or conceal invisible substances.

A full answer includes

  • I described comparison with known samples under the same conditions.
  • I explained why a match supports identity and one spot does not guarantee purity.

Also accept

  • Equivalent scientifically correct reasoning; exact wording is not required.

Watch for

  • “One visible spot guarantees purity.” — One spot may contain overlapping substances or conceal invisible components. Two separated spots establish a mixture.

3. HR3 · Return

Model answer. Rf=distance baseline to spot centre/distance same baseline to solvent front, with equal distance units and no unit for Rf. After development, wet-front marking, removal and drying, a suitable locating agent reveals positions of separated colourless substances.

A full answer includes

  • I wrote spot-centre travel divided by front travel, both from the baseline with equal units.
  • I gave Rf no unit and placed locating after development, marking and drying.

Also accept

  • Equivalent calculation with correct baseline distances, unit cancellation and decimal ratio.

Watch for

  • “Rf uses distances from the bottom paper edge, or front divided by spot.” — Subtract the baseline offset from both edge readings. Divide spot-centre travel by front travel with equal units; the ratio is unitless.