Answers · Paper chromatography
Measure travel, then reveal what you cannot see — answer key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. HQ9 · Practice
Model answer. Rf = 24 ÷ 60 = 0.40. Both distances begin at the same baseline and use the same unit. The distance units cancel, so Rf has no unit.
A full answer includes
- I divided spot travel 24 mm by front travel 60 mm.
- I obtained Rf 0.40 and gave no unit.
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.
2. HQ10 · Practice
Model answer. Apply a suitable locating agent to the removed dry paper. It reveals positions of substances already separated during development; it is not the developing solvent or the front mark.
A full answer includes
- I used suitable locating on the removed, dry paper.
- I explained that it reveals positions separated earlier.
Also accept
- Equivalent scientifically correct reasoning; exact wording is not required.
Watch for
- “The locating agent is the solvent that separates the substances.” — The suitable solvent separates substances during development. A locating agent reveals their positions on the removed dry paper afterwards.
3. HQ11 · Practice
Model answer. Subtract the baseline offset: spot travel is 42 − 12 = 30 mm; solvent travel is 72 − 12 = 60 mm. Rf = 30 ÷ 60 = 0.50, with no unit.
A full answer includes
- I subtracted 12 mm from both edge readings, giving 30 mm and 60 mm.
- I divided 30 by 60 to obtain 0.50 with no unit.
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.
4. HQ12 · Practice
Model answer. Pencil baseline; small sample spot and dry; suitable shallow solvent below baseline; lower paper with bottom edge in solvent, cover and develop; lift before top and immediately mark wet front; remove/dry; use a suitable locating agent on the dry chromatogram to reveal separated spots.
A full answer includes
- I described the full method with a suitable solvent and dry starting spot above it.
- I marked the wet front before drying.
- I used locating on the removed dry chromatogram.
Also accept
- Equivalent scientifically correct reasoning; exact wording is not required.
Watch for
- “The locating agent is the solvent that separates the substances.” — The suitable solvent separates substances during development. A locating agent reveals their positions on the removed dry paper afterwards.
5. HT3 · Apply
Model answer. From the baseline, the lower spot travelled 1.8 cm, the upper spot 4.2 cm and solvent 6.0 cm. Rf values are 1.8 ÷ 6.0 = 0.30 and 4.2 ÷ 6.0 = 0.70. Colourless substances can separate without visible stains. Locating reveals their positions. Two separated spots show a mixture.
A full answer includes
- I used baseline distances 1.8 cm, 4.2 cm and 6.0 cm.
- I obtained Rf 0.30 and 0.70 without units.
- I explained that colourless substances separated before locating, and two spots show a mixture.
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.