Answers · Paper chromatography
Paper chromatography — final check key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. HK1 · Final check
Model answer. The sample spots are submerged and may wash into the reservoir; the ink line may dissolve and add spots; after drying the solvent front may no longer be visible. Use fresh paper with a pencil line and small dried spots, shallow suitable solvent below the baseline, lower and cover, develop below the top, lift, immediately mark the wet front, then dry.
A full answer includes
- I explained submerged spots, dissolving ink and the lost wet front.
- I described a fresh pencil setup with spots above solvent and marking before drying.
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. HK2 · Final check
Model answer. Z contains at least two separated substances. Its 18 mm spot matches P and supports P being present. Q doesn't match. One visible spot is consistent with purity under those conditions, but overlapping or invisible components could conceal a mixture.
A full answer includes
- I identified Z as a mixture and recognised its 18 mm match with P.
- I stated that Q does not match.
- I explained the limits of one visible spot as evidence of 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. HK3 · Final check
Model answer. Distances from baseline: 24 mm and 48 mm; front 60 mm. Rf = 0.40 and 0.80, no units. Two revealed spots show at least two separated substances, hence a mixture. Without a recorded front, the solvent travel distance and reliable Rf are unavailable after drying. A suitable locating agent is used on the removed dry chromatogram after development and immediate wet-front marking. The solvent carries/separates the substances; the agent reveals their positions.
A full answer includes
- I used travel distances 24 mm, 48 mm and 60 mm.
- I obtained Rf 0.40 and 0.80, without units.
- I used two separated spots to identify a mixture.
- I explained that the front mark preserves the denominator.
- I distinguished solvent separation from later locating on removed dry paper.
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.