Mixtures

Five separation techniques and how to choose the right one. The crystallisation method here is the foundation for RP1 — get it precise and you secure marks across multiple later questions.

SPEC 4.1.1.2 • ATOMIC STRUCTURE

Why This Lesson

Mixtures appear constantly in this course — in RP1 (preparing salts), RP6 (chromatography), water purification, and the analysis of fuels. The principle is always the same: separate the components without changing them chemically.

The Core Idea

A mixture's components are not chemically combined. Each one keeps its own properties, so you can separate them by exploiting physical differences — solubility, particle size, boiling point, or affinity for a surface.

Examiner Pattern

This topic doesn't get a direct one-marker, but it underpins multiple 4–5 mark method questions in RP1 and RP6. The most repeatedly-flagged mistake in examiner reports: writing "evaporate to dryness with a Bunsen burner". Get this right and you score marks most candidates leave on the table.

Definition: Mixture

The mark-scheme wording is short and exact. Memorise it.

Mixture

Two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged.

The Iron + Sulfur Test Case

Mix iron filings with sulfur powder → it's a mixture. The iron keeps its grey magnetic appearance; the sulfur keeps its yellow colour — you can tell them apart in the bowl, and you can pull the iron out with a magnet because it still behaves like iron.

Heat that same mixture strongly → it reacts to form iron sulfide (FeS) — a compound. The product is a single dull-grey solid; the magnet does nothing now: the iron has chemically combined and lost its individual properties.

The Decisive Question

Are the substances chemically combined? No → mixture | Yes → compound.

The Five Techniques

The spec names exactly five physical separation methods. Each one is suited to one specific type of mixture. Learn the matching, not just the names.

TechniqueUse it for…Example

FiltrationInsoluble solid + liquidRemoving sand from water

CrystallisationSoluble solid from solutionGetting copper sulfate crystals back from solution

Simple distillationSolvent from solute (very different boiling points)Pure water from seawater

Fractional distillationTwo or more liquids with similar/different boiling pointsEthanol from water; crude oil fractions

ChromatographyDissolved substances in a mixture (e.g. coloured dyes)Separating dyes in food colouring (RP6)

Why no chemistry happens

None of these techniques involve a chemical reaction. No new substances are made. We're just exploiting physical differences — particle size for filtration, solubility for crystallisation, boiling points for distillation, surface affinity for chromatography.

Filtration

Used when you have an insoluble solid mixed with a liquid — the solid won't dissolve, so it can be physically blocked by filter paper.

Two Words to Use

Residue = the solid left on the filter paper. Filtrate = the liquid that passes through. Mark schemes accept these specific words; "the leftover stuff" loses the mark.

Crystallisation

Used when you have a soluble solid dissolved in a solvent (e.g. salt in water). The technique is more nuanced than students realise — and the wrong wording costs marks every year.

The Method (mark-scheme)

Heat the solution gently — using a water bath or electric heater, NOT a Bunsen burner directly

Continue until the solution is saturated — small crystals appear at the edge of the dish (the "point of crystallisation")

Remove from heat. Leave to cool slowly — larger, purer crystals form as the solution cools

Filter or pour off remaining liquid; pat the crystals dry between filter paper

The trap, in one line

Never evaporate to dryness. Heating until all the liquid has gone destroys the crystal structure — the salt decomposes or scorches. Stop at saturation, then cool.

What's evaporating?

Be precise: it's the water (solvent) that evaporates. The dissolved solid stays in solution until saturation, then crystallises out on cooling. Saying "the acid evaporates" or "the salt evaporates" is wrong — and is a common examiner-flagged error.

Examiner Alert: Crystallisation

P1 Jun 2022 Q04.4 + P1 Jun 2023 Q03.1

Across two consecutive years, examiners flagged the same two errors on the crystallisation method:

Error A: "Evaporate to dryness with a Bunsen burner" — destroys the crystals. Mark lost.

Error B: "The acid evaporates from the solution" — wrong. Water (the solvent) evaporates; the salt stays. Mark lost.

Weak student

"Heat the solution with a Bunsen burner until all the water has evaporated. Then the acid will be left behind as crystals."

Two errors: Bunsen + evaporate to dryness; "acid" instead of "salt".

Strong student

"Heat the solution gently using a water bath until small crystals appear at the edge of the dish. Then remove from heat and leave to cool — the salt crystallises as the solution cools."

Hits all marks: water bath, saturation point, cool to crystallise.

Simple Distillation

Used to separate a solvent from a solute when their boiling points are very different — most commonly to recover pure water from a salt solution.

How it works

1. Heat the solution. Water boils at 100 °C; the dissolved salts have boiling points far higher than 100 °C, so they don't evaporate with the water. 2. Water vapour rises, enters the condenser, where cold water flowing through the outer jacket (counter-current — in at the far end, out at the flask end) cools it back to liquid. 3. Pure water (the distillate) collects in the second flask. The salts remain in the original flask.

Two examined details

Anti-bumping granules are added to the flask to promote even boiling and prevent the liquid from "bumping" violently. The thermometer sits at the side-arm exit (where vapour leaves the flask) to confirm the vapour temperature matches the boiling point of what you're collecting — confirming purity.

Fractional Distillation

Used when you need to separate two or more liquids with different (but possibly close) boiling points. Simple distillation isn't precise enough for this — you need a fractionating column.

How a fractionating column works

The column sits between the flask and the condenser. It creates a temperature gradient — hotter at the bottom (near the flask), cooler at the top.

Vapours of the lower-boiling-point liquid travel further up the column before cooling; higher-boiling-point liquids condense and fall back into the flask. So the lower-BP liquid is collected first.

What does the separating? The temperature gradient itself. Glass beads or packing inside some columns just provide more surface area for repeated evaporation/condensation — they aid the gradient, but they aren't the cause.

Lab example

Ethanol from water: ethanol BP = 78 °C, water BP = 100 °C. Ethanol vaporises first and is collected; water mostly stays behind.

Industrial example

Crude oil refining: different hydrocarbon fractions condense at different heights of a tall industrial column. Covered in detail in Lesson 7.02.

Memory hook

"Simple" = one liquid coming off (solvent recovery). "Fractional" = multiple fractions, each at a different temperature.

Simple vs Fractional

The most-mixed-up pair on this topic. Use this comparison to decide which one a question wants:

FeatureSimple distillationFractional distillation

Use it forSeparating a solvent from a solute (e.g. water from salt)Separating two or more liquids from each other

Boiling point gapVery different (one is non-volatile)Different but possibly close

ApparatusFlask + condenserFlask + fractionating column + condenser

ExamplePure water from seawaterEthanol from water; crude oil fractions

Quick decision

One of the components is a dissolved solid? → simple. Both components are liquids? → fractional (you need the column).

Chromatography

Separates dissolved substances in a mixture — most commonly coloured dyes, inks, or food colourings. Full method and Rf calculation appear in Lesson 8.02 (RP6); the principle belongs here.

Stationary phase

The phase that doesn't move — usually the chromatography paper.

Mobile phase

The solvent that travels up the paper, carrying the dissolved substances with it.

The principle

Each substance has a different balance of attractions between the two phases. Substances that prefer the mobile phase (solvent) travel further up the paper. Substances that prefer the stationary phase (paper) travel less far. The result is a separated chromatogram.

For Lesson 1.02

You don't need to do Rf calculations here — just know that chromatography is the technique for separating dissolved substances, and that it uses a stationary and a mobile phase. The full RP6 method is in Lesson 8.02.

Choosing the Right Technique

The spec explicitly requires you to "suggest suitable separation and purification techniques for mixtures when given appropriate information". There's a logic to it — answer two questions and the choice is forced.

The two questions

Q1: What's in the mixture?

Solid (won't dissolve) + liquid → filtration

Solid (dissolved) + liquid → crystallisation (recover solid) OR simple distillation (recover solvent)

Two or more liquids → fractional distillation

Coloured dissolved substances → chromatography

Q2: Which component do I want?

Want the solid back? → crystallisation

Want the solvent back pure? → simple distillation

Worked decisions

"Sand from sea water" = solid (insoluble) + liquid → filtration (then distillation if you also want pure water).

"Ethanol from a wine sample" = two liquids with close BPs → fractional distillation.

"Sugar crystals from sugar solution" = dissolved solid, want the solid → crystallisation.

PEE Framework — Worked Chain

Q: A student wants to obtain pure water from seawater. Describe a method and explain why it works. 3 MARKS

P — PointUse simple distillation — boil the seawater, then condense the vapour to collect pure water.

E — EvidenceSeawater is a mixture of water and dissolved salts. Water boils at 100 °C; salts don't evaporate at this temperature.

E — ExplainThe water vapour passes into the condenser, cools, and collects as pure water. The salts stay in the original flask — separation by physical difference, no chemical change.

How marks are scored

Mark 1: name the technique (simple distillation). Mark 2: state that water evaporates and salt stays. Mark 3: explain the condenser step / link to physical separation. Three clean marks for three clean sentences.

Common Errors

Students often writeCorrect versionWhy it matters

"Heat with a Bunsen burner"" Water bath or electric heater"Direct Bunsen heat → evaporates to dryness → destroys crystals

"The acid evaporates""The water evaporates; the salt stays in solution"Solvent evaporates, not solute

"Evaporate to dryness""Heat to saturation , then leave to cool"Cooling, not over-heating, is what grows the crystals

"Fractional distillation for sand and water"Filtration — sand is insoluble, no boiling neededMatch the technique to the type of mixture

"Mixtures involve a chemical reaction""Mixtures are separated by physical processes only"Foundational distinction from compounds

Practice Question 1

For each mixture below, name the most suitable separation technique.4 MARKS

1

Sand mixed with sea water (you want the sand only)

2

A solution of copper sulfate (you want the dry crystals)

3

A mixture of ethanol and water

4

The different coloured dyes in a sample of black ink

1

Filtration. Sand is an insoluble solid; filter paper traps it as the residue.

2

Crystallisation. You want the solid back from solution. Heat gently to saturation, then cool.

3

Fractional distillation. Two liquids with different boiling points (ethanol 78 °C, water 100 °C) — the column gives a temperature gradient.

4

Chromatography. Different dyes have different attractions to the paper vs the solvent — they travel different distances.

Practice Question 2

A student has prepared a solution of copper sulfate. Describe how they would obtain a pure, dry sample of copper sulfate crystals from the solution.4 MARKS

(Pattern from AQA 8462 RP1 questions, P1 Jun 2024 / Jun 2023)

1

Heat the solution gently using a water bath (or electric heater). [1 mark]

2

Continue until the solution is saturated — small crystals begin to appear at the edge of the dish (the point of crystallisation). [1 mark]

3

Remove from heat and leave to cool slowly — larger, purer crystals form as the solution cools. [1 mark]

4

Pat the crystals dry between two pieces of filter paper. [1 mark]

Examiner note: Saying "Bunsen burner" or "evaporate to dryness" loses Mark 1. This is the most-missed mark on this topic — fewer than half of candidates scored it in P1 Jun 2024.

Practice Question 3

A scientist needs to obtain pure water from a sample of mineral water. The mineral water contains dissolved minerals that are not chemically combined with the water.

(a) Explain why the mineral water is described as a mixture, not a compound. 2 MARKS

(b) Name the technique they should use to obtain pure water, and explain how it works. 3 MARKS

(a)

1

The minerals and water are not chemically combined; the components retain their original chemical properties. [1 mark]

2

Because they're not bonded together by chemistry, they can be separated by physical means (no chemical reaction needed) — unlike a compound, which can only be split by a chemical reaction. [1 mark]

(b)

P

Simple distillation.[1 mark]

E

Heat the mineral water. The water boils at 100 °C and evaporates; the dissolved minerals stay behind because they don't evaporate at this temperature. [1 mark]

E

The water vapour passes through the condenser, cools, and collects as pure liquid water in the second flask. [1 mark]

Summary

Mark-Scheme Definitions

Mixture: two or more elements or compounds not chemically combined . Properties of each component unchanged.

Filtration: separation of an insoluble solid from a liquid using filter paper

Crystallisation: separation of a soluble solid from solution by evaporating the solvent

Simple distillation: separation of a solvent from a solute by boiling and condensing

Fractional distillation: separation of liquids with different boiling points using a fractionating column

Chromatography: separation of dissolved substances by their different affinities for stationary vs mobile phase

Choosing the Technique

Insoluble solid + liquid → filtration

Soluble solid (want solid back) → crystallisation

Soluble solid (want solvent back) → simple distillation

Two or more liquids → fractional distillation

Dissolved coloured substances → chromatography

Exam Tip — the most-missed mark

For crystallisation: heat gently with a water bath until the solution is saturated, then cool to crystallise. Never "evaporate to dryness with a Bunsen burner" — that single phrase loses up to two marks every year in RP1 questions.

Up Next — Lesson 1.03: The Development of the Model of the Atom

From Dalton's solid sphere through the plum pudding model to the nuclear and Bohr models. Why each model was replaced — and the alpha scattering experiment that overturned plum pudding.