Can one colour hide several substances? — answer key

1. HQ1 · Practice

Model answer. Keep the baseline and spots above the reservoir while the bottom paper edge touches solvent. Submerged spots may dissolve into the reservoir instead of travelling up the paper.

A full answer includes

• I kept the spots above solvent while the bottom paper edge touches it.
• I explained that submerged spots can dissolve into the reservoir.

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. HQ2 · Practice

Options as numbered on the pupil sheet: 1 — Preserve the solvent-front distance; 2 — Avoid adding soluble ink from the line; 3 — Reduce solvent evaporation.

Model answer. Pencil avoids a dissolving ink line. The cover reduces solvent evaporation. Marking the wet front immediately preserves its distance before the wet edge disappears.

A full answer includes

• I linked pencil to avoiding a dissolving ink line.
• I linked the cover to less solvent evaporation.
• I linked immediate marking to preserving the wet front position.

Also accept

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

Watch for

• “The cover keeps the pencil baseline in place.” — Pencil graphite stays in place in the chosen solvent. The cover instead reduces solvent evaporation.

3. HQ3 · Practice

Options as numbered on the pupil sheet: 1 — Immediately mark the wet front; 2 — Dry the removed paper; 3 — Develop until the front is below the top; 4 — Lift the paper.

Model answer. Develop before the front reaches the top. Lift the paper, mark the wet front immediately, then dry. Solvent evaporation can erase the wet boundary, so drying must come after marking.

A full answer includes

• I ordered develop, lift, mark the wet front, then dry.
• I explained that drying can erase the wet boundary.

Also accept

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

Watch for

• “Dry the paper before marking the solvent front.” — The wet boundary may disappear as solvent evaporates. Mark it immediately after lifting, before drying.

4. HQ4 · Practice

Model answer. No. It may be insoluble in water, or the solvent may not carry/separate it effectively. Check solubility and choose a suitable solvent; a stationary spot alone doesn’t establish purity.

A full answer includes

• I rejected a purity conclusion from a stationary spot.
• I checked whether the solvent dissolves and carries the sample.

Also accept

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

Watch for

• “No movement means the pigment is pure.” — A stationary spot may mean an unsuitable solvent or an insoluble pigment. It can’t establish purity or count the components.

5. HT1 · Apply

Model answer. Filter to retain the insoluble grit while dissolved dyes pass into the filtrate. Use paper chromatography on a small portion of the filtrate to separate and compare soluble dyes with known samples. Use a pencil baseline, small dried spots above shallow suitable solvent, bottom edge in solvent, cover and develop; lift before the top, immediately mark the wet front, then dry.

A full answer includes

• I used filtration for grit and kept dissolved dyes in the filtrate.
• I chose chromatography to separate and compare soluble dyes.
• I described small dry spots above suitable solvent, development, lifting, immediate front marking and 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.