In the 9th century, Arab chemist Jabir ibn Hayyan devised systematic methods for purifying substances — including filtration and crystallisation. His techniques, refined over 1200 years, are still used in every chemistry laboratory in the world today.
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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?
Before reading on, write your best answer. What is the key difference between sand and salt that causes the two mixtures to behave differently during filtration?
Core Content
Wrong: Filtration can separate dissolved solids from a solution.
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.
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.
| What you collect | Name | Location |
|---|---|---|
| Insoluble solid | Residue | On the filter paper |
| Liquid + dissolved substances | Filtrate | In the flask below |
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.
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.
| 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 |
Worked Examples
Activities
A student has a mixture of chalk powder (CaCO₃, insoluble in water) in water. They want to collect the dry chalk. Which technique should they use? Describe the steps.
A student has a solution of potassium nitrate (KNO₃) in water. They want to collect pure KNO₃ crystals. Which technique should they use? Describe the steps.
In two to three sentences, explain the key difference between filtration and crystallisation in terms of (i) what property of the mixture each technique exploits, and (ii) what you collect at the end of each process.
A mining company extracts gold ore from the ground. The ore is a mixture of gold particles (insoluble in water) and sodium chloride (dissolved in the water trapped in the ore). Outline a two-step separation procedure to obtain both pure gold particles and pure NaCl crystals from this ore sample.
A student attempts to purify salt by filtering a salt solution through filter paper, then collecting the residue on the filter paper as their "pure salt". Explain what is wrong with this method and what they should do instead.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Multiple Choice
5 random questions from a replayable lesson bank — feedback shown immediately
Short Answer
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
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.
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.
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.
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).
Return to your Think First response. You should now be able to explain the filtration outcomes using the particle-level distinction between soluble and insoluble substances:
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