Why is a separating funnel only useful for immiscible liquids?

Only immiscible liquids form two layers, the less dense one on top, which can be run off in turn; miscible liquids never separate and need distillation instead.

Why can purity and recovery not usually both be maximised?

Raising purity costs product, since repeated recrystallisation leaves some of the target in the mother liquor, while chasing recovery carries impurities along.

S4 Chemistry: Separation and Purification Paper 4 (solubility, recovery and everyday use)

2026-11-03

S · 4

This paper ties each separation technique back to experimental numbers: solubility, Rf, purity and recovery constrain one another, and one slip reverses the conclusion.

Quick answer　30 g per 100 cm3; immiscible liquids of different density; Rf 0.60; 80 per cent; 92.5 per cent and 81.1 per cent; 2.0, 4.5 and 8.0 cm, 6.0 cm apart; 0.20 g/cm3 or 200 g per dm3; 75.0 per cent with 11 g lost; 37.5 per cent; filtration cannot remove dissolved salt.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Chemistry: Separation and Purification
Level	S4

02　Questions (10 items)

FoundationAt room temperature 20 cm3 of water dissolves at most 6 g of a solid. What is the solubility per 100 cm3 of water?
FoundationWhat kind of mixture can a separating funnel separate, and on what property does it rely?
FoundationA spot lies 4.5 cm from the start line and the solvent front is 7.5 cm from it. What is the Rf?
FoundationDistillation of 250 cm3 of a mixture collects 200 cm3 of distillate. What is the recovery?
IntermediateAn 80 g crude product contains 6 g of impurity. What is its purity, and what is the recovery relative to the pure content if recrystallisation gives 60 g, both to one decimal place?
IntermediateOn one chromatogram the solvent front is 10.0 cm from the start and three spots have Rf 0.20, 0.45 and 0.80. How far is each from the start, and how far apart are the lowest and highest?
IntermediateThree hundred cm3 of brine contains 24 g of salt. After 180 cm3 of water is distilled off, 120 cm3 of solution remains. What is its concentration in g/cm3 and in g per dm3?
ChallengeFifty grams of crude product is 88 per cent pure and recrystallisation gives 33 g of crystals. What is the recovery relative to the pure content, and how many grams are lost, to one decimal place?
ChallengeFractional distillation of 400 cm3 gives 150 cm3 at 78 degrees Celsius and 210 cm3 at 100 degrees, leaving 40 cm3 in the flask. What percentage of the total is the first fraction, to one decimal place?
Error redoA student writes that filtering brine separates out the salt. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	30 g per 100 cm3	Six over twenty times 100 gives 30 g, since solubility is quoted per 100 cm3 of solvent.
2	Two immiscible liquids of different density, run off one layer after the other.	The less dense layer sits on top, so the lower layer is run off first and the upper layer into a second container; miscible liquids do not layer at all.
3	Rf 0.60	Four point five over 7.5 gives 0.60, and an Rf always lies between 0 and 1.
4	80 per cent	Two hundred over 250 times 100 gives 80 per cent, the rest left in the flask or on the condenser.
5	Purity 92.5 per cent; recovery 81.1 per cent	The pure content is 74 g, and 74 over 80 gives 92.5 per cent; 60 over 74 gives 81.1 per cent, the rest staying in the mother liquor.
6	2.0 cm, 4.5 cm and 8.0 cm, with 6.0 cm between the lowest and the highest	Ten times 0.20, 0.45 and 0.80 gives 2.0, 4.5 and 8.0 cm, and 8.0 minus 2.0 leaves 6.0 cm.
7	0.20 g/cm3, that is 200 g per dm3	The salt stays at 24 g, and 24 over 120 gives 0.20 g/cm3; one dm3 is 1,000 cm3, so 200 g per dm3.
8	Recovery 75.0 per cent with 11 g lost	The pure content is 44 g, and 33 over 44 gives 75.0 per cent, leaving 11 g in the mother liquor.
9	37.5 per cent	One hundred fifty over 400 gives 37.5 per cent, and the three parts add up to exactly 400 cm3.
10	The salt is dissolved and passes through the paper with the filtrate, so filtration removes only insoluble solids.	Recovering the salt needs evaporation or crystallisation, while filtration suits mixtures such as sand in brine that contain an insoluble solid.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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