Mixtures and separation — final check key

1. K01 · Final check

Model answer. Propanone is the solvent because Z dissolves in it. Z is the solute because it is the substance that becomes dissolved. The solution includes both propanone and dissolved Z.

A full answer includes

• Propanone is the solvent because Z dissolves in it.
• Z is the solute because it is the substance that becomes dissolved.
• Solution: the homogeneous mixture of propanone and dissolved Z.

Also accept

• The uniform liquid mixture, including both components.

Watch for

• “A solution contains only the dissolved solid.” — A solution includes the solvent as well as its dissolved solute.
• “The unfamiliar name prevents identification.” — The stated dissolving relationship supplies all the information needed.

2. K02 · Final check

Model answer. The label is incorrect because dissolved salt remains present. The liquid contains two substances even though crystals are not visible. It is a salt solution, not pure water.

A full answer includes

• I recognised that dissolved salt is still present with water.
• I explained that absence of visible solid does not establish purity.
• I described the sample as a salt solution/homogeneous mixture of salt and water.

Also accept

• A homogeneous salt-water mixture; clarity does not make it one substance.

Watch for

• “Any liquid with no visible solid is pure.” — Dissolved solute need not be visible as a solid. Use the stated components to judge purity.

3. K03 · Final check

Model answer. M is the residue on the paper because its insoluble grains are retained. The filtrate is the solution of L in water; solvent and dissolved solute pass together.

A full answer includes

• M is the retained filtration residue.
• Filtrate is water and dissolved L.
• I explained retention of insoluble grains versus passage of solution.

Also accept

• Retained M; collected solution containing water and L.

Watch for

• “Only water is collected below the paper.” — L is stated to be fully dissolved and not retained, so it passes with water.

4. K04 · Final check

Model answer. Choose B because it dissolves W and leaves Z undissolved. Z is retained as residue and the solution of W in B is the filtrate. The supplied facts don’t justify a heating procedure.

A full answer includes

• I chose B and explains selective dissolution of W while Z remains insoluble.
• Z is residue; B and dissolved W form the filtrate.
• Does not assume safe heating/recovery from missing data.

Also accept

• B provides the required contrast; no recovery claim is made beyond the data.

Watch for

• “Choose A, then ordinary paper separates the two dissolved solids.” — Both would be dissolved in A and pass together. Selective dissolution in B is the relevant difference.

5. K05 · Final check

Options as numbered on the pupil sheet: 1 — Solid is collected as residue; 2 — Solution passes; the wanted solid is not collected.

Model answer. B can yield a solid residue because its insoluble grains are retained. A passes as a solution; the dissolved solid is not retained by this ordinary paper. The same apparatus therefore doesn’t achieve the stated goal for A.

A full answer includes

• B yields retained insoluble solid and liquid filtrate.
• A passes as solution and does not yield the dissolved solid on ordinary paper.
• I explained the distinction between a separate insoluble solid and dissolved solute.

Also accept

• The method is suitable for B's stated grains, not A's dissolved solute.

Watch for

• “Both feeds contain a named solid, so both will leave solid on the paper.” — The word solid doesn’t establish undissolved grains in feed A. Follow the supplied physical state.

6. K06 · Final check

Options as numbered on the pupil sheet: 1 — Remove water from the separate filtrate by the approved procedure.; 2 — Filter the existing wet sample..

Model answer. Filter the wet sample to retain U and collect water plus dissolved T. Remove water from this separate filtrate using the approved procedure to recover T. Evaporating the whole starting sample first leaves T mixed with U, so it doesn’t achieve collection of separate solids.

A full answer includes

• Filter first: U residue, receiver contains water and dissolved T.
• Remove water from separate filtrate by the approved procedure to leave T residue.
• I explained that evaporating the unfiltered sample first leaves T and U together and fails the intended separation.

Also accept

• A justified filter-then-recover sequence with complete filtrate composition and assessment of the proposed error.

Watch for

• “The first receiver already contains pure T.” — Before water removal it contains a solution: water and dissolved T. Name both components.