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Module 1 · L3 of 21 30 min ⚡ +50 XP in Learn · +25 to complete

Separation Techniques, Physical Methods

Today's hook, Every glass of tap water you have ever drunk started out as muddy river water. The mud did not disappear. Someone separated it out.

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Warm up and recall

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Recall, your gut answer first

+5 XP warm-up

A student mixes sand and water in one beaker, and salt and water in another. They pour both mixtures through filter paper in a funnel. What do you expect to collect on the filter paper and in the filtrate for each mixture?

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What you'll master, and the words for it

What you'll master

Know

Key facts

The principle behind filtration and crystallisation
The key equipment used in each technique
Which mixture types each technique is suited for

Understand

Concepts

Why particle size determines whether filtration works
Why solubility changes with temperature drive crystallisation
How to choose between techniques based on mixture properties

Can do

Skills

Describe filtration and crystallisation procedures step-by-step
Compare the two techniques in a structured table
Apply the correct technique to a novel mixture scenario

Filtration

core concept

Principle

Filtration separates an insoluble solid from a liquid using a porous barrier (typically filter paper in a funnel). The liquid passes through the tiny pores; the solid particles are too large to pass through and are retained on the filter paper.

Separation basis: Particle size. The solid must be insoluble, if it dissolves in the liquid, it will pass through with the filtrate and cannot be collected this way.

Equipment and Procedure

Fold filter paper into cone shape
Place in glass funnel, moisten with solvent
Position funnel over conical flask (to collect filtrate)
Pour mixture gently down a glass rod into funnel
Allow liquid to drain through
Solid (residue) remains on filter paper
Liquid (filtrate) collects in flask below

What is separated

What you collect	Name	Location
Insoluble solid	Residue	On the filter paper
Liquid + dissolved substances	Filtrate	In the flask below

Limitations

Cannot separate dissolved solutes from solution (they pass through with the liquid)
Does not separate two dissolved substances from each other
Requires the solid to be insoluble in the solvent being used

Liquid passes through filter paper in a funnel; the insoluble solid stays behind as residue.

Filtration separates an insoluble solid from a liquid using a porous barrier (filter paper). The solid retained is the residue; the liquid collected is the filtrate. Separation basis: particle size, the solid must be insoluble. Cannot separate dissolved solutes from solution.

Pause, copy the highlighted definition into your book before moving on.

Two truths, one lie about filtration. Pick the lie.

Crystallisation

core concept

Principle

Crystallisation separates a dissolved solid (solute) from a solution by removing solvent until the solution becomes saturated, then cooling or continuing evaporation so that the excess solute comes out of solution as pure crystals. It exploits the fact that most solids become less soluble as temperature decreases.

Separation basis: Difference in solubility with temperature. As a hot saturated solution cools, it can no longer hold as much solute in solution, the excess crystallises out.

Procedure

Dissolve solid in minimum volume of hot solvent
Filter if any undissolved particles are present
Heat solution in evaporating basin until concentrated
Allow to cool slowly (slow cooling → larger, purer crystals)
Filter to collect crystals
Dry crystals (air-dry or low-temperature oven)

Safe heating. Wear safety glasses. Heat gently on an electric hotplate or over a water bath rather than a strong direct flame, and stop heating before the basin goes dry, so the hot solution cannot spit or the dry solid scorch. Heating liquids can "bump", so heat slowly and never aim the basin at yourself or others. Hot glassware looks the same as cold, so let it cool on a heatproof mat before handling.

Why Slow Cooling Gives Better Crystals

Slow cooling allows solute particles to arrange themselves into an ordered lattice, producing larger, purer crystals. Rapid cooling traps impurities and produces small, impure crystals. This is why the technique is used for purification, not just separation. Evaporation to dryness is different: it simply drives off all the solvent and leaves behind everything that was dissolved, including any soluble impurities, so it recovers more solid but at lower purity. Crystallisation trades some yield (some product stays in the cooled solution) for much purer crystals, so choose evaporation when you want maximum recovery and crystallisation when you want a pure product.

Purity advantage: Crystallisation is a purification technique as well as a separation technique. Each crystallisation cycle removes impurities because the pure substance crystallises preferentially from the solution. Repeated crystallisation (recrystallisation) can achieve very high purity.

KNO₃ solubility rises steeply with temperature, so much less dissolves when the solution is cooled.

Crystallisation apparatus: solution in an evaporating basin on a tripod and wire gauze, heated by a Bunsen burner on a low flame, with crystals forming on slow cooling; process — heat gently, evaporate some solvent, cool slowly, crystals form as solubility decreases.

Crystallisation separates a dissolved solid (solute) by exploiting decreasing solubility on cooling: dissolve in hot solvent → evaporate to saturate → cool slowly → filter and dry. Slow cooling gives larger, purer crystals. It also purifies, each recrystallisation cycle raises purity.

Add the highlighted point to your notes before the check below.

Fill the blanks: drag each token into the matching gap.

decreases saturated slowly purer

Crystallisation works because the solubility of most solids ___ as temperature drops. The hot solution is concentrated until it becomes ___, then cooled ___ so that excess solute forms larger, ___ crystals.

Comparing Separation Techniques, Physical Methods

core concept

Feature	Filtration	Crystallisation
Separation basis	Particle size	Solubility change with temperature
What is separated	Insoluble solid from liquid	Dissolved solid from solution
Key equipment	Filter paper, funnel, conical flask	Evaporating basin, heat source, funnel (for final step)
Result	Residue + filtrate	Dry crystals + mother liquor
Purification?	Limited, removes insoluble impurities only	Yes, each cycle improves purity
Cannot separate	Dissolved solutes from each other	Insoluble solids from liquids

Key decision rule: Is the solid dissolved or undissolved? If undissolved → filtration. If dissolved → crystallisation. This is the first question to ask when choosing a technique.

We just saw how crystallisation recovers dissolved solids. That raises a question: with filtration and crystallisation both available, how do chemists choose the right technique? This card answers it → the decision depends on which property differs between the components.

Match technique to the property that differs: undissolved solid → filtration (particle size); dissolved solid → crystallisation (solubility–temperature change); liquids with large BP difference → simple distillation; close BPs → fractional distillation; differential phase affinity → chromatography. Multi-component mixtures often need a sequence.

Pause, write the highlighted decision rule into your book.

Quick check: A mixture contains sand stirred into salt water. Which technique sequence cleanly separates all three components (sand, salt, water)?

Short Answer Questions

core concept

We just saw the decision rules for choosing between separation techniques. That raises a question: how do you write full-mark exam answers for each technique? This card answers it → structure each answer around the technique name, its basis (property exploited), and the outcome.

6. Describe the principle of filtration and explain why it cannot be used to separate sodium chloride from a sodium chloride solution. 3 MARKS

✏️ Answer in your book

7. A student dissolves 80 g of potassium chloride in 100 mL of hot water, then allows the solution to cool to room temperature. They observe crystals forming. Explain why crystals form on cooling, referring to solubility and saturation. 3 MARKS

✏️ Answer in your book

8. A chemist has a sample of impure table salt (NaCl with some coloured pigment impurities that are also soluble in water). Evaluate whether a single crystallisation step will produce chemically pure NaCl, and suggest how the chemist could improve the purity of their final product. 4 MARKS

✏️ Answer in your book

For "describe filtration" answers: name equipment, state separation basis (particle size), give residue + filtrate outcome. For "explain crystallisation" answers: solubility decreases with temperature → solution becomes supersaturated → excess solute crystallises. Always justify the technique with the relevant property of the components. To choose a technique, state three things: the physical state of the mixture, which component you want to keep, and the property that differs between the components (particle size, solubility, or boiling point); then match the technique to that property, do not pick it from a single clue.

Pause, copy the highlighted exam strategy into your book before moving on.

Two truths, one lie about choosing a physical separation method. Pick the lie.

Worked examples

Worked examples · reveal as you go

Worked example +5 XP on full reveal

A student has a beaker of muddy water (water + sand + dissolved salt). Describe how to obtain: (a) pure dry sand, and (b) pure dry salt. Justify your choice of technique in each case.

Identify what is dissolved vs undissolved

Sand is insoluble in water, it does not dissolve. Salt (NaCl) is soluble in water, it is fully dissolved. These two properties determine which technique applies to each substance.

Part (a), Obtaining pure dry sand: Use filtration

Pour the muddy water through filter paper in a funnel. The sand (insoluble, large particles) is trapped as residue on the filter paper. The filtrate (water + dissolved salt) passes through. Rinse the residue with distilled water to remove any remaining salt, then dry.

Part (b), Obtaining pure dry salt: Use crystallisation

First filter to remove the sand, collecting the filtrate (salt water). Then heat the filtrate in an evaporating basin until saturated, cool slowly, filter off the crystals, and dry. The salt crystallises out of solution as temperature decreases.

Justify technique choices

Filtration is used for sand because it is insoluble (separation by particle size). Crystallisation is used for salt because it is dissolved (separation by solubility change with temperature). Neither technique alone can separate both components, the two techniques must be used in sequence.

Worked example +5 XP on full reveal

A student dissolves impure copper sulfate (CuSO₄) in hot water, then allows the solution to cool slowly. They collect large blue crystals and filter off the remaining solution. Explain why the crystals are purer than the original sample.

What happens when a hot solution cools?

As temperature decreases, solubility decreases. The solution can no longer hold all the dissolved CuSO₄, the excess comes out of solution as solid crystals.

Why does crystallisation select for the pure substance?

Crystal growth is a highly ordered process. CuSO₄ molecules (or ions) fit neatly into the crystal lattice; impurities have different shapes and sizes and are mostly excluded from the lattice. The crystal lattice preferentially incorporates pure CuSO₄.

Where do the impurities go?

Most impurities remain in the solution (the "mother liquor"). When the crystals are filtered off and washed with a small amount of cold solvent, the remaining impurities on the crystal surface are removed.

Misconception to fix

Common errors · the 3 traps that cost marks

Misconception to fix

Wrong: Filtration can separate dissolved solids from a solution.

Misconception to fix

Right: Filtration separates insoluble solids from liquids. Dissolved solids pass through the filter paper with the solvent because they are at the molecular level. Crystallisation or evaporation is needed to recover dissolved solids.

Cooling crystals quickly to "speed up" the practical

Students often plunge the hot saturated solution into an ice bath thinking it will improve yield. Rapid cooling produces many tiny crystals that trap impurities and the mother liquor, purity actually drops.

Fix: Cool the saturated solution slowly to room temperature (then optionally chill) so large, pure crystals form.

Drill, then revisit

Quick-fire practice · 5 reps +2 XP per reveal

Name the residue and the filtrate when you filter sand stirred into water.

Why can't filtration separate dissolved NaCl from water?

Explain in one sentence why slow cooling gives larger, purer crystals.

A solution of KNO₃ has 80 g dissolved per 100 g water at 60 °C. From the solubility curve, what mass will crystallise out on cooling to 20 °C?

Design a procedure to recover all three components (sand, salt, water) from a mixture of sand + salt water.

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Revisit your thinking

Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?

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Interactive Tool, Separation Techniques Open fullscreen ↗

The Separation Techniques tool shows that filtration is best used to separate…

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Multiple choice

+5 XP per correct · +25 XP all-correct

Pick your answer, then rate your confidence. That tells the system what to drill next.

Short answer

ApplyApply3 MARKS

Q1. 6. Describe the principle of filtration and explain why it cannot be used to separate sodium chloride from a sodium chloride solution.

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ApplyApply3 MARKS

Q2. 7. A student dissolves 80 g of potassium chloride in 100 mL of hot water, then allows the solution to cool to room temperature. They observe crystals forming. Explain why crystals form on cooling, referring to solubility and saturation.

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ApplyApply4 MARKS

Q3. 8. A chemist has a sample of impure table salt (NaCl with some coloured pigment impurities that are also soluble in water). Evaluate whether a single crystallisation step will produce chemically pure NaCl, and suggest how the chemist could improve the purity of their final product.

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📖 Comprehensive answers (click to reveal)

️ Activity 1, Compare

A: Filtration. Steps: fold filter paper, place in funnel over conical flask, pour chalk mixture down a glass rod into funnel, allow water (filtrate) to drain through, collect chalk (residue) on filter paper, rinse with distilled water, dry residue.

B: Crystallisation. Steps: heat KNO₃ solution in evaporating basin to concentrate it, cool slowly to allow crystals to form, filter off crystals, dry on filter paper.

C: Filtration exploits particle size, the insoluble solid is too large to pass through the filter paper, and you collect the solid as residue and liquid as filtrate. Crystallisation exploits the decrease in solubility with temperature, the dissolved solid comes out of solution as crystals when cooled, and you collect dry crystals as the product.

Activity 2, Apply to Novel Context

Novel Context 1: Step 1, Filtration: pour the ore/water mixture through filter paper to collect gold particles as residue (gold is insoluble). The filtrate contains the NaCl solution. Step 2, Crystallisation: heat the filtrate in an evaporating basin to concentrate it, cool slowly, filter off NaCl crystals, dry them.

Novel Context 2: The method is wrong because salt (NaCl) is soluble in water, it dissolves and passes straight through the filter paper with the liquid. There would be no salt residue to collect. The student should instead use crystallisation: heat the salt solution in an evaporating basin to evaporate the water, allow to cool slowly so salt crystallises out, then filter and dry the crystals.

❓ Multiple Choice

Filtration separates insoluble solids from liquids; solubility is the key factor.

Filtrate = everything that passes through (water + dissolved substances). Insoluble mud is trapped as residue.

Slow cooling allows ordered lattice formation, excluding impurities. Rapid cooling traps them inside.

Filter first (remove sand), then crystallise the filtrate (obtain CuSO₄). Reversing the order would embed sand into crystals.

At 80°C, 100 g dissolves in 100 mL. At 20°C, only 31 g can remain dissolved. So 100 − 31 = 69 g crystallises out.

Short Answer Model Answers

Q6 (3 marks): Filtration separates mixtures based on particle size, insoluble solid particles are too large to pass through filter paper, while the liquid and any dissolved substances pass through (1 mark). In a sodium chloride solution, the NaCl is fully dissolved, it exists as individual Na⁺ and Cl⁻ ions dispersed throughout the water (1 mark). These ions are far too small to be trapped by filter paper; they simply pass through with the water, so filtration cannot separate them from the solution (1 mark).

Q7 (3 marks): As the solution cools, the solubility of KCl decreases, less KCl can remain dissolved at lower temperatures (1 mark). The solution becomes saturated and then supersaturated as it cools, it contains more dissolved KCl than can be held in solution at that temperature (1 mark). The excess KCl can no longer remain dissolved and comes out of solution as solid crystals (precipitates) (1 mark).

Q8 (4 marks): A single crystallisation step will improve purity but will not produce chemically pure NaCl (1 mark). Because the impurities are also soluble in water, some will remain in solution during the first crystallisation and some may be incorporated into the crystal surface (1 mark). To improve purity: the chemist should perform recrystallisation, dissolve the crystals again in minimum hot water, then allow to cool and crystallise again (1 mark). Each recrystallisation cycle further reduces the impurity level because the NaCl crystallises preferentially; after 2–3 cycles, purity will be significantly higher (1 mark).

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