Why do crystals appear as it cools? — answer key

1. CQ5 · Practice

Options as numbered on the pupil sheet: 1 — Some leaves as vapour; 2 — Stays in the basin.

Model answer. Water→some leaves as vapour; copper(II) sulfate→stays in the basin. The same salt remains in less water, so the solution is more concentrated.

A full answer includes

• I sent some water into the air as vapour and kept salt in the basin.
• I explained that the same salt remains in less water.

Watch for

• “Heating creates more salt.” — The salt was already present. Removing some water changes its relative amount in the solution.

2. CQ6 · Practice

Options as numbered on the pupil sheet: 1 — Let the main solution cool and stand; 2 — Stop heating: test drop has crystals; 3 — Gently evaporate some water; 4 — Test a cooled drop.

Model answer. 1 Gently evaporate some water; 2 test a cooled drop; 3 stop heating when the drop forms crystals; 4 let the main solution cool and stand. Cooling then leaves less of this salt dissolved and forms crystals. Keep solution as the medium from which crystals grow; heating to dryness skips that stage and can change hydrated crystals.

A full answer includes

• I ordered partial evaporation, cooled-drop test, stopping heat, then cooling and standing.
• I explained why liquid must remain for crystals to grow.
• I recognised that strong heating can change hydrated crystals.

Watch for

• “Heat until every drop of water is gone.” — Keep liquid so crystals can grow from solution; strong drying can damage hydrated crystals.

3. CQ7 · Practice

Model answer. There is still enough water to keep the salt dissolved at the lower temperature. Gently concentrate the solution by evaporating some water, test a cooled drop, stop heating and cool/stand again. Keep liquid instead of heat to dryness.

A full answer includes

• I explained that the available water may still keep salt dissolved when cool.
• I concentrated gently, tested a cooled drop and tried cooling again with liquid remaining.

Also accept

• There is still enough water to keep the salt dissolved at the lower temperature. Gently concentrate the solution by evaporating some water, test a cooled drop, stop heating and cool/stand again. Keep liquid rather than heat to dryness.

Watch for

• “No crystals proves no salt remains.” — Dissolved salt can be invisible. Concentrate the same solution and try cooling again.

4. CQ8 · Practice

Model answer. Choose the first and third statements: Some salt forms solid crystals; water remains liquid with some salt dissolved. It is crystallisation, not freezing, and cooling doesn’t recover every bit of solute.

A full answer includes

• I selected the first and third statements.
• I distinguished solid salt crystals from liquid water containing remaining dissolved salt.
• I explained that crystallisation is not freezing and does not recover every bit of solute.

Also accept

• Choose the first and third statements: Some salt forms solid crystals; water remains liquid with some salt dissolved. It is crystallisation, not freezing, and cooling does not recover every bit of solute.

Watch for

• “The blue crystals are frozen water.” — Cooling forms solid solute crystals; liquid water with dissolved salt remains.

5. CT2 · Apply

Model answer. Choose A for this cooling method: much less can remain dissolved when cool. Partial evaporation removes some solvent while retaining the solute, making the solution more concentrated before cooling. B shows little change, so cooling alone may recover little; its method needs different evidence.

A full answer includes

• I chose A because less can remain dissolved when cool.
• I explained how partial evaporation concentrates the solution.
• I used B’s different temperature behaviour to limit the cooling claim.

Watch for

• “Cooling always produces the same result for every soluble solid.” — The temperature dependence belongs to the selected solute. Read the supplied observations.