Answers · Paper chromatography
What can the pattern tell you? — answer key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. HQ5 · Practice
Model answer. U is a mixture: two substances have separated. The matches support A and B being present. Solvent, paper and temperature affect travel, so compare under the same conditions. A match doesn't establish one unique identity.
A full answer includes
- I used two separated spots as evidence of a mixture.
- I required matching conditions and treated the A/B matches as supporting evidence.
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.
2. HQ6 · Practice
Model answer. Two separated spots from one lane demonstrate a mixture. One spot is consistent with purity under those conditions, but overlap or invisible components can conceal a mixture. A blank lane doesn’t establish absence.
A full answer includes
- I recognised that two separated spots show at least two substances.
- I explained how overlap or colourless substances can conceal a mixture.
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. HQ7 · Practice
Model answer. X has two separated spots, so it's a mixture. Its lower blue spot matches P on the same paper; Q matches neither spot. This supports P being present, but another substance could travel the same distance.
A full answer includes
- I identified X as a mixture from its two spots.
- I recognised the P match and absence of a Q match.
- I stated that a match is not unique proof of identity.
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.
4. HQ8 · Practice
Model answer. Colour alone isn't enough. M and N reach different heights under the same conditions, so these results don't support identifying N as M.
A full answer includes
- I rejected identification by colour alone.
- I used the different heights under the same conditions as evidence against a match.
Also accept
- Equivalent scientifically correct reasoning; exact wording is not required.
Watch for
- “The same colour identifies the same substance.” — Colour alone is insufficient. Different spot heights on the same paper under the same conditions don’t support a match.
5. HT2 · Apply
Model answer. X is a mixture: T separates at least two components that S didn't resolve. Equal heights measured in different solvents don't establish a match. Repeat the unknown and known samples under matching conditions.
A full answer includes
- I used the two-spot run to identify X as a mixture.
- I rejected a height match across different solvents.
- I suggested comparing unknown and known samples under matching conditions.
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.