S2 Science: Separating Mixtures (Paper 3 (challenge))

2026-11-14

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Tutor Prep Guide · Filtration, distillation, fractional distillation and chromatography with Rf calculations

This paper raises the level: choosing between fractional and simple distillation, stationary and mobile phases, both directions of Rf calculation, identifying mixtures and apparatus safety.

Direct answers　In reverse questions write the formula before substituting, with the spot distance equal to Rf times the solvent front distance. When both components dissolve in water, filtration is useless, so use chromatography, crystallisation or distillation.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S2 Science: Separating Mixtures
Level	S2

02　Questions (10 items)

FoundationWhich type of mixture is fractional distillation mainly used for?
FoundationIn paper chromatography, what role does the filter paper with its adsorbed water play?
FoundationTwo salts are both soluble in water. Can filtration separate them?
IntermediateThe solvent front moves 8.0 cm and a pigment has an Rf value of 0.35. How far does the spot move?
IntermediateTo purify wet ethanol, should filtration or fractional distillation be used?
IntermediateWhat is the purpose of adding boiling chips in distillation?
Challenge250 g of sea water contains three and a half percent salt. How much salt remains after evaporation?
ChallengeTwo pigments on one sheet have Rf values of 0.65 and 0.35. Which is nearer the solvent front and by how much do they differ?
ChallengeChromatography is used to test whether an ink contains extra pigments. What should be observed?
Error redoA student says distillation only needs heating and no condensation. What must the apparatus include?

03　Answers and one-line notes

#	Answer	One-line note
1	Liquid mixtures with close boiling points, such as crude oil	Repeated vaporisation and condensation in the column separates components whose boiling points differ by only a few degrees.
2	It is the stationary phase	The stationary phase does not move while the solvent is the mobile phase, and the different interactions with the two phases cause separation.
3	No, both stay in the filtrate	Filtration only holds back undissolved solids, so dissolved components need chromatography or crystallisation.
4	2.8 cm	The distance moved is Rf times the solvent front distance, so 8.0 times 0.35 gives 2.8 cm.
5	Fractional distillation	Both are liquids that mix completely, so only a difference in boiling point can separate them.
6	To prevent bumping of the liquid	Boiling chips provide nucleation sites so boiling stays smooth instead of liquid surging out.
7	8.75 g	Two hundred and fifty times zero point zero three five equals 8.75, so 8.75 g of salt is left.
8	The 0.65 one, differing by 0.30	Subtracting 0.35 from 0.65 leaves 0.30, and a larger Rf means it travelled further.
9	Whether one spot splits into several spots after the run	A pure substance gives only one spot, while several spots show that the ink is a mixture.
10	A condenser to cool and collect the vapour	Without condensation the vaporised component escapes into the air and no distillate is collected.

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.

For separation questions ask two things: whether each component dissolves in water, and whether their boiling points or adsorption differ.Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.