Which method fits this mixture? — answer key

1. DQ9 · Practice

Options as numbered on the pupil sheet: 1 — Filtration; 2 — Simple distillation; 3 — Fractional distillation.

Model answer. Sand/water with sand wanted → filtration. Salt solution with water wanted → simple distillation. Miscible volatile liquids with close boiling points → fractional distillation.

A full answer includes

• I chose filtration for sand, simple distillation for water from salt solution, and fractional distillation for close-boiling miscible liquids.

Watch for

• “Filtration removes any solid from water.” — Ordinary filter paper retains insoluble solids, not dissolved solute.

2. DQ10 · Practice

Model answer. Fractional distillation: a packed column provides repeated condensation/vaporisation to improve separation of these close-boiling miscible liquids. The early fraction is expected to be P-rich, not guaranteed pure.

A full answer includes

• I chose fractional distillation and explained repeated phase changes in the packed column.
• I expected a P-rich early fraction without claiming guaranteed purity.

Watch for

• “The lower boiling liquid is collected perfectly pure.” — A column improves enrichment; both liquids can still be present.

3. DQ11 · Practice

Model answer. No. A transparent salt solution can pass through ordinary filter paper with the salt still dissolved. Clear appearance and filtration don’t establish purity.

A full answer includes

• I explained that dissolved salt can pass with water through ordinary paper.
• I rejected purity claims based only on clarity or filtration.

Also accept

• No. A transparent salt solution can pass through ordinary filter paper with the salt still dissolved. Clear appearance and filtration do not establish purity.

Watch for

• “Clear means pure.” — A dissolved-salt solution may be transparent and pass through filter paper.

4. DQ12 · Practice

Model answer. The rise from near 78°C to a higher head-vapour temperature suggests a change in vapour composition around sample 3. This can guide receiver change but doesn’t alone prove identity or purity.

A full answer includes

• I used the temperature rise around sample 3 as a possible change in vapour composition.
• I said the readings alone cannot prove identity or purity.

Also accept

• The rise from near 78°C to a higher head-vapour temperature suggests a change in vapour composition around sample 3. This can guide receiver change but does not alone prove identity or purity.

Watch for

• “A steady temperature proves purity.” — A trend can help indicate changing vapour composition; it doesn’t prove identity or purity alone.

5. DT3 · Apply

Model answer. First filter out the insoluble grit. Then use simple distillation on the filtrate: water vaporises and condenses into a receiver while nonvolatile salt remains in the flask. Stop heating while liquid remains.

A full answer includes

• I filtered out grit before using simple distillation on the filtrate.
• I collected condensed water, kept salt in the flask and stopped before dryness.

Also accept

• First filter out the insoluble grit. Then simply distil the filtrate: water vaporises and condenses into a receiver while nonvolatile salt remains in the flask. Stop heating while liquid remains.

Watch for

• “Filtering grit also removes dissolved salt.” — Dissolved salt passes with water; a later distillation uses its nonvolatility.