What if both liquids can escape? — answer key

1. DQ5 · Practice

Model answer. Repeated condensation and vaporisation enrich rising vapour in the more volatile A. Both A and B can enter vapour.

A full answer includes

• I explained repeated condensation and vaporisation on the packing.
• I said rising vapour becomes enriched in A while both liquids can vaporise.

Watch for

• “The column is a filter.” — Both liquids can vaporise. Packing supports repeated condensation and vaporisation.

2. DQ6 · Practice

Options as numbered on the pupil sheet: 1 — Collect the early fraction; 2 — Switch off and let final drops drain; 3 — Exchange receiver after flow stops; 4 — Resume heat and collect later; 5 — Switch off, drain and cool before dryness.

Model answer. Collect early → switch off and let final drops drain → exchange receiver after flow stops → resume heat and collect later → switch off, drain and cool before dryness.

A full answer includes

• I collected early, switched off and drained, changed receiver after flow stopped, then collected later.
• I finished by switching off, draining and cooling before dryness.

Watch for

• “Exchange receivers while heated vapour still flows.” — After heat is off, residual vapour can still condense. Let it drain before exchange.

3. DQ7 · Practice

Model answer. Early has a greater proportion of A (8/10 versus 6/10). Later has a greater proportion of B (4/10 versus 2/10). Neither is pure because both contain A and B.

A full answer includes

• I compared early 8/10 A with later 6/10 A, and later 4/10 B with early 2/10 B.
• I said neither fraction is pure because both contain A and B.

Also accept

• Early has a greater proportion of A (8/10 versus6/10). Later has a greater proportion of B (4/10 versus2/10). Neither is pure because both contain A and B.

Watch for

• “A-rich means pure A.” — The early fraction still contains B. Richness compares proportions, not purity.

4. DQ8 · Practice

Model answer. Vapour must pass through all active packing before entering the condenser. The bulb must meet outgoing vapour to measure head-vapour temperature, instead of a stagnant space or the flask.

A full answer includes

• I required vapour to pass through all active packing before leaving by the sidearm.
• I put the bulb in outgoing vapour to measure head-vapour temperature.

Also accept

• Vapour must pass through all active packing before entering the condenser. The bulb must meet outgoing vapour to measure head-vapour temperature, rather than a stagnant space or the flask.

Watch for

• “A bulb anywhere above the flask measures the same temperature.” — The bulb must meet the outgoing head vapour; other positions can measure something different.

5. DT2 · Apply

Model answer. No. Both fractions can contain A and B, and mixing them combines their compositions. Keeping separate receivers preserves the different fractions; the column doesn’t guarantee pure A.

A full answer includes

• I rejected the purity guarantee.
• I explained that combining fractions mixes their components and loses the separate compositions.

Also accept

• No. Both fractions can contain A and B, and mixing them combines their compositions. Keeping separate receivers preserves the different fractions; the column does not guarantee pure A.

Watch for

• “Mixing the fractions makes pure A.” — Combining fractions mixes their components and loses the separate compositions.