Separation Techniques — Physical Methods
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Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Key facts
- The principle behind filtration and crystallisation
- The key equipment used in each technique
- Which mixture types each technique is suited for
Concepts
- Why particle size determines whether filtration works
- Why solubility changes with temperature drive crystallisation
- How to choose between techniques based on mixture properties
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
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
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.
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)
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.
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.
| 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 |
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)?
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
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
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
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.
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 · reveal as you go
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.
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.
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.
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.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Pick your answer, then rate your confidence — that tells the system what to drill next.
Q1. 6. Describe the principle of filtration and explain why it cannot be used to separate sodium chloride from a sodium chloride solution.
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.
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.
📖 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
1. B — Filtration separates insoluble solids from liquids; solubility is the key factor.
2. C — Filtrate = everything that passes through (water + dissolved substances). Insoluble mud is trapped as residue.
3. A — Slow cooling allows ordered lattice formation, excluding impurities. Rapid cooling traps them inside.
4. D — Filter first (remove sand), then crystallise the filtrate (obtain CuSO₄). Reversing the order would embed sand into crystals.
5. C — 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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