Collect the crystals. Follow the liquid. — answer key

1. CQ9 · Practice

Options as numbered on the pupil sheet: 1 — Residue on the paper; 2 — Filtrate in the beaker.

Model answer. Solid crystals→residue on paper; water in the remaining liquid solution→filtrate; dissolved salt→filtrate. Paper retains solid crystals, but dissolved salt passes with the water.

A full answer includes

• I kept solid crystals on paper as residue.
• I put water and still-dissolved salt together in the filtrate.
• I explained why paper retains crystals but not dissolved salt.

Watch for

• “Filter paper catches all salt.” — Paper retains crystals, not the dissolved salt in the liquid.

2. CQ10 · Practice

Options as numbered on the pupil sheet: 1 — Blot crystals between fresh filter papers; 2 — Filter the cooled mixture; 3 — Concentrate gently and test a cooled drop; 4 — Stop heating and cool/stand.

Model answer. 1 Concentrate gently and test a cooled drop; 2 stop heating and cool/stand; 3 filter the cooled mixture; 4 blot crystals between fresh filter papers. Filtering separates crystals from solution; blotting removes adhering liquid while preserving hydrated crystals.

A full answer includes

• I ordered concentration/test, cooling, filtration and blotting.
• I explained filtration as separating crystals from remaining solution.
• I explained blotting as removing surface liquid while preserving hydrated crystals.

Watch for

• “Filter the original clear solution to collect dissolved salt.” — First form solid crystals; dissolved solute passes through paper.

3. CQ11 · Practice

Model answer. The claim is wrong. Solid salt crystals are on the paper, while some salt remains dissolved in the water and passes into the filtrate. Clear blue filtrate is still a solution; clear doesn’t mean pure.

A full answer includes

• I rejected the pure-water label.
• I accounted for salt in solid crystals and dissolved in the filtrate.
• I explained why clear does not mean pure.

Also accept

• The claim is wrong. Solid salt crystals are on the paper, while some salt remains dissolved in the water and passes into the filtrate. Clear blue filtrate is still a solution; clear does not mean pure.

Watch for

• “No visible grains means no solute.” — Dissolved salt need not be visible as grains. The filtrate contains it.

4. CQ12 · Practice

Model answer. Gently evaporate some water, test a cooled drop, then stop heating when that drop forms crystals. Cool and stand the main solution so crystals grow while liquid remains. Filter the cooled mixture to retain solid crystals. Gently blot between fresh filter papers to remove surface solution, leaving water of crystallisation within the hydrated crystal structure. Avoid direct strong heating.

A full answer includes

• I concentrated gently, tested a cooled drop, then stopped heating with liquid remaining.
• I cooled and stood the solution before filtering crystals from it.
• I blotted surface liquid without removing water of crystallisation.

Watch for

• “Dry means every form of water has been removed.” — Surface drying and loss of water of crystallisation are different changes.

5. CT3 · Apply

Model answer. Filter the cooled mixture, then blot the collected crystals between fresh filter papers. Most dissolved impurity passes in the remaining solution/filtrate. Some impurity-containing liquid may cling to the crystals, so this observation doesn’t prove perfect purity.

A full answer includes

• I filtered the cooled mixture and blotted the collected crystals.
• I put most dissolved impurity in the filtrate.
• I explained that impurity-containing liquid can cling to crystals, limiting purity.

Also accept

• Filter the cooled mixture, then blot the collected crystals between fresh filter papers. Most dissolved impurity passes in the remaining solution/filtrate. Some impurity-containing liquid may cling to the crystals, so this observation does not prove perfect purity.

Watch for

• “Crystallisation automatically guarantees absolute purity.” — Separation can help purify, but adhering solution or co-crystallising impurities can limit purity.