Separation and PurificationCambridge IGCSE Chemistry: Revision notes
Section 1
What are the main separation and purification methods?
The key separation and purification techniques you must know are:
- Filtration – separates solids from liquids using filter paper and a funnel
- Crystallisation – obtains pure solid crystals from a saturated solution by evaporation and cooling
- Simple distillation – separates a liquid from a solution by heating and condensing the vapour
- Fractional distillation – separates liquids with different boiling points using a fractionating column
- Using a suitable solvent – dissolves one substance while leaving others insoluble, allowing separation
Each method exploits different physical properties of substances (solubility, boiling point, density) rather than breaking chemical bonds.
Examiners expect you to name the specific method AND describe the key steps. Simply saying 'use distillation' without explaining heating, condensation, and collection is insufficient for full marks.
Section 2
How does crystallisation purify a solid?
Crystallisation is the best method for purifying solid solutes from solutions:
- Dissolve the impure solid in a hot solvent (usually water) – more solute dissolves at high temperature
- Filter the hot solution to remove any insoluble impurities
- Cool the solution slowly – the pure solute becomes insoluble and forms crystals, while impurities remain dissolved
- Collect the crystals by filtration and wash with cold solvent to remove any remaining impurities
- Dry the crystals (in an oven or by pressing between filter paper)
Why it works: This method relies on the fact that solubility changes with temperature. At room temperature, the solute is less soluble and precipitates as pure crystals while dissolved impurities stay in solution.
Key requirement: The chosen solvent must dissolve the impure substance at high temperature but allow crystallisation at low temperature.
To purify copper(II) sulphate contaminated with salt: dissolve both in hot water. Cool the solution – copper(II) sulphate crystallises (low solubility when cold) but salt remains dissolved (soluble at all temperatures). Filter to collect the crystals.
Students often forget that the solvent must be removed after cooling. Simply cooling the solution and collecting it by decanting leaves impure liquid-coated crystals. You must filter AND wash with cold solvent.
Section 3
How do simple and fractional distillation differ?
Both methods separate liquids by boiling point, but they suit different situations:
| Feature | Simple Distillation | Fractional Distillation |
|---|---|---|
| Purpose | Separates a liquid from a solution (large boiling point difference) | Separates liquids with similar boiling points |
| Apparatus | Thermometer, Liebig condenser, round-bottom flask | Fractionating column (glass beads/packing) plus condenser |
| How it works | Heat solution; vapour rises and condenses; collect pure liquid | Heat mixture; vapour rises through fractionating column; cooler at top, hotter at bottom creates temperature gradient |
| Separation mechanism | Boiling point difference exploited once | Multiple repeated condensation/vaporisation cycles |
| Example use | Separating water from salt solution | Separating petrol from diesel in crude oil; separating ethanol from water |
Simple distillation works when boiling points differ by >40°C (e.g., water 100°C, ethanol 78°C).
Fractional distillation is essential when boiling points are close (e.g., crude oil components differ by only 10–50°C). The fractionating column provides a large surface area for repeated vaporisation and condensation, progressively separating components.
Examiners test whether you can choose the right method. Always check the boiling point difference: if >40°C, simple distillation is adequate; if similar, fractional distillation is necessary. State this reasoning in your answer.
Simple distillation is like separating oil from vinegar by boiling – they have very different boiling points. Fractional distillation is like separating slightly different shades of water – you need multiple 'attempts' (the column) to purify each one.
Section 4
How do you choose a suitable separation method?
The correct method depends on the physical properties of the substances being separated:
Decision-making flowchart:
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Is the substance a solid mixed with a liquid?
- Yes → Filtration (if solid is insoluble) or Crystallisation (if solid is soluble and needs purifying)
- No → Go to step 2
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Are you separating liquids or a liquid from a solution?
- Check boiling point differences
- Difference >40°C → Simple distillation
- Difference <40°C → Fractional distillation
- Go to step 3
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Can you use differences in solubility?
- Yes → Use a suitable solvent – choose one that dissolves one substance but not others
- No → Use distillation method
Key considerations:
- Solubility data – Does the solute dissolve in the chosen solvent at high temperature but not at low temperature?
- Boiling point data – Are the boiling points significantly different (>40°C)?
- Purity requirements – Crystallisation and distillation produce purer products than filtration alone
- Cost and safety – Some solvents are expensive or hazardous; consider alternatives
Separating sand and salt: Sand is insoluble in water, salt is soluble. Add water to dissolve salt. Filter to collect sand (residue). Evaporate the filtrate to recover salt crystals. Method: filtration + crystallisation.
Section 5
How do you identify purity using melting and boiling points?
Melting point and boiling point are used to assess the purity of substances:
Pure substances have:
- Sharp, characteristic melting points – a narrow range (e.g., pure ice melts at exactly 0°C)
- Sharp, characteristic boiling points – a narrow range (e.g., pure water boils at exactly 100°C at 1 atmosphere pressure)
- Melting/boiling occurs over a range of only 1–2°C
Impure substances have:
- Lower melting points – impurities disrupt the crystal lattice
- Wider melting point range – melting begins at a lower temperature and finishes over a range of several degrees
- Higher boiling points – impurities raise the boiling point (colligative property)
- Wider boiling point range – boiling is not sharp
Why this happens:
Impurities interfere with the organised structure of pure crystals (during melting) or the ordered evaporation process (during boiling). This requires extra energy (higher temperature) and causes a gradual transition rather than a sharp one.
Practical assessment:
- Measure melting point – Use apparatus like a Thiele tube and thermometer; record the temperature at which melting starts and finishes
- Measure boiling point – Heat the liquid in a flask with a thermometer; record the steady temperature when vapour condenses on the thermometer bulb
- Compare to known values – If measured values match published data (within ±2°C) and range is narrow, the substance is pure
- Interpretation – A wide melting/boiling range or values far from expected indicates impurity
Examiners expect you to link purity to melting/boiling point data quantitatively. State the expected value, the measured value, and say 'the narrow range of X°C indicates high purity' or 'the wide range suggests impurities'.
Pure naphthalene melts at 80°C (sharp, 79–81°C range). A sample melts at 72–78°C. This wide range and lower starting point show it is impure – other substances lower the melting point and disrupt crystallisation.
Must Know
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Five main separation methods: Filtration (solid from liquid), Crystallisation (solid purification from solution), Simple Distillation (liquid from solution, >40°C boiling point difference), Fractional Distillation (liquids with similar boiling points), Solvent Choice (exploits solubility differences)
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Crystallisation steps: Dissolve in hot solvent → Filter hot solution → Cool to crystallise → Filter crystals → Wash with cold solvent → Dry. Works because solubility decreases with temperature.
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Simple vs. Fractional Distillation: Simple distillation suits large boiling point differences (>40°C); Fractional distillation separates similar boiling points using a fractionating column with multiple evaporation/condensation cycles.
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Choosing a method: Check if separation exploits solubility differences (use suitable solvent or crystallisation) or boiling point differences (use distillation). Base choice on physical properties of substances, not chemical reactions.
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Purity assessment: Pure substances have sharp melting/boiling points (narrow 1–2°C range). Impure substances have lower melting points, higher boiling points, and wider ranges. Compare measured values to known data within ±2°C tolerance.
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Key principle: All separation methods exploit physical properties (solubility, boiling point, density), not chemical bonds. Measure and compare to published data to verify purity.
That's the notes covered.
Carry on to the next subtopic.