Can we get the water back? — answer key

1. DQ1 · Practice

Model answer. Liquid water. Water vaporises and condenses; the nonvolatile salt stays in the flask.

A full answer includes

• I identified liquid water as the distillate.
• I explained vaporisation, condensation and nonvolatile salt staying behind.

Watch for

• “The distillate is cooling water.” — Coolant and sample remain in separate tubes. The receiver collects condensed sample.

2. DQ2 · Practice

Model answer. Water vaporises in the flask and condenses in the inner tube. Cooling water remains in the jacket.

A full answer includes

• I put vaporisation in the flask and condensation in the inner tube.
• I kept cooling water in the separate jacket.

Watch for

• “Water vapour is visible white smoke.” — Water vapour is invisible. Visible outlet drops are condensed liquid.

3. DQ3 · Practice

Model answer. Select coolant low→high, open receiver, and bulb in outgoing head vapour. Accept any explained reason: low→high fills the jacket; openness prevents pressure build-up; bulb placement measures outgoing vapour temperature.

A full answer includes

• I selected low-to-high coolant flow, an open receiver and the bulb in outgoing vapour.
• I explained one choice using jacket filling, pressure prevention or head-vapour measurement.

Also accept

• Any one of the three correct reasons, with the correct selections.

Watch for

• “A sealed receiver prevents water escaping safely.” — Sealing a heated apparatus risks pressure build-up. Keep a route to the atmosphere.

4. DQ4 · Practice

Model answer. Initially 114 g water; after collection 94 g water and 6 g salt remain. The same salt is dissolved in less water, so concentration increases.

A full answer includes

• I found 114 g water initially, then 94 g water and 6 g salt remaining.
• I explained that the same salt in less water makes the solution more concentrated.

Watch for

• “All 120 g of solution is water.” — Solution mass includes solvent and solute. Find the initial water before subtracting collected water.

5. DT1 · Apply

Model answer. Simple distillation: volatile R vaporises and condenses in the inner tube, then collects as liquid in the receiver; nonvolatile S stays in the flask. Coolant enters the lower jacket inlet and exits the upper outlet, filling the jacket while staying separate from the sample.

A full answer includes

• I chose simple distillation: R vaporises, condenses in the inner tube and collects as liquid.
• I kept nonvolatile S in the flask.
• I described separate coolant flow from lower jacket inlet to upper outlet.

Watch for

• “The collected liquid comes from the coolant.” — The sample vaporises in the flask; the cooling jacket removes heat without mixing.