What are the common methods of purification covered in the Cambridge IGCSE Coordinated Science syllabus?

The Cambridge IGCSE Coordinated Science syllabus covers several methods of purification, including filtration, crystallization, distillation, and chromatography. Each method is used to separate and purify different types of mixtures based on their physical properties.

How does filtration work as a method of purification?

Filtration is a method of purification used to separate insoluble solids from liquids. It involves passing a mixture through a filter paper or other porous material, which allows the liquid to pass through while retaining the solid particles. This technique is commonly used in laboratory settings to purify mixtures.

Can you explain the process of distillation and its applications in purification?

Distillation is a purification technique that separates components of a mixture based on differences in their boiling points. The mixture is heated until one component vaporizes, then the vapor is condensed back into a liquid and collected separately. This method is widely used to purify liquids, such as separating alcohol from water or obtaining pure water from saltwater.

What role does chromatography play in the purification of substances?

Chromatography is a method of purification that separates components of a mixture based on their movement through a stationary phase. Different substances move at different rates, allowing them to be separated and identified. This technique is particularly useful for purifying and analyzing complex mixtures, such as inks or plant pigments.

Why is crystallization considered an effective method for purifying solids?

Crystallization is an effective purification method for solids because it relies on the principle that pure substances form crystals more readily than impure ones. By dissolving a solid in a solvent and then allowing it to slowly crystallize, impurities remain in the solution while pure crystals form. This method is often used to purify salts and other crystalline substances.

Why is "Saying 'boil to dryness' to obtain crystals" a common mistake in Methods of Purification ?

Why it happens: Students think all water must be removed; they do not realise that boiling to dryness gives a powder or impure residue, not pure crystals. How to avoid it: Evaporate until saturated (test with glass rod), then cool slowly; this gives proper crystals. Source: 0654 Examiner Report 2022

Why is "Suggesting filtration to remove dissolved substances" a common mistake in Methods of Purification ?

Why it happens: Students confuse dissolved (invisible in solution) with suspended (cloudy). Filtration cannot remove dissolved solutes. How to avoid it: Filtration: insoluble solids only. Distillation/crystallisation: dissolved substances.

Why is "Using simple distillation to separate liquids with similar boiling points" a common mistake in Methods of Purification ?

Why it happens: Students apply simple distillation to all liquid separations without considering boiling point differences. How to avoid it: If boiling points differ by less than ~25 °C, use fractional distillation with a fractionating column.

Experimental TechniquesCambridge IGCSE Coordinated Sciences 0654

Methods of Purification

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Methods of Purification — Cambridge IGCSE Coordinated Science 0654

Choosing the right purification method depends on the nature of the mixture. Cambridge tests: filtration (insoluble solid from liquid), crystallisation (dissolved solid), distillation (liquids), and fractional distillation (mixtures of liquids).

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C2.4 — Describe and explain the methods of purification: filtration, crystallisation, simple and fractional distillation.
C2.5 — Choose the appropriate method for a given separation problem.

Filtration and Crystallisation

Filtration removes insoluble solids. Crystallisation purifies soluble solids from solution.

Filtration:

Used for: separating an insoluble solid from a liquid (or solution)
Method: Pour mixture through filter paper in a funnel. Insoluble solid remains as residue; liquid (filtrate) passes through.
Example: Separating sand from salt water. (The sand is filtered; the filtrate is salt water.)
Limitation: Cannot separate dissolved substances — they pass through filter paper.

Evaporation to dryness:

Used for: obtaining a dissolved solid from its solution when the solid is NOT damaged by heat
Method: Heat the solution; solvent evaporates; solid left behind
Example: Getting salt from salt water (fast, but crystals are small and may contain impurities)

Crystallisation:

Used for: obtaining PURE crystals of a dissolved solid
Method:
1. Heat solution to dissolve maximum solid (saturated solution)
2. Allow to cool slowly — large, pure crystals form as solubility decreases
3. Filter off the crystals
4. Dry gently (not too hot — to avoid melting crystals)
Why slow cooling? Slow cooling gives atoms time to arrange into a regular crystal lattice — larger, purer crystals.
Example: Purifying copper sulfate — blue crystals (CuSO₄·5H₂O) contain water of crystallisation.

Filtration: insoluble solid + liquid → solid on filter paper, filtrate passes through.
Crystallisation: slow cooling of hot saturated solution → large, pure crystals.
Evaporation to dryness: faster but produces smaller, less pure crystals.

Distillation — Separating Liquids

Simple distillation separates one liquid from a non-volatile solute. Fractional distillation separates miscible liquids with different boiling points.

Simple distillation:

Used for: separating a liquid from dissolved solids OR two liquids with very different boiling points (>25°C difference)
Method:
1. Heat mixture → liquid with lower BP vaporises first
2. Vapour travels through condenser (cooled by water)
3. Vapour condenses → pure liquid collected as distillate
Example: Getting pure water from salt water; getting water from an ink solution.

Fractional distillation:

Used for: separating a mixture of miscible liquids with similar but different boiling points
Extra component: a fractionating column (filled with glass beads or rings) between flask and condenser
How it works: Vapour rises; condenses on cooler glass beads, re-vaporises, rises further. Components with lower BP reach top first and are collected. Components with higher BP condense lower and fall back.
Example 1: Separating ethanol (BP 78°C) from water (BP 100°C) in beer/wine
Example 2: Fractional distillation of liquid air (N₂, O₂, Ar) and crude oil (petrol, diesel, kerosene, etc.)

Key apparatus: round-bottom flask, fractionating column, Liebig condenser, conical flask as receiver, thermometer at top of fractionating column.

The volatile liquid vaporises, condenses in the cooled Liebig condenser and drips into the receiver as pure distillate.

Simple distillation: liquid from solid, or liquids with very different boiling points.
Fractional distillation: mixtures of liquids with similar boiling points — fractionating column needed.
Component with LOWEST boiling point collects first in distillate.

How it’s examined

Paper 6: 'Describe how you would obtain pure salt crystals from a rock salt sample' (4 marks — dissolve in water, filter, heat filtrate, cool slowly, filter crystals, dry). 'Name the process used to separate ethanol from water' (1 mark — fractional distillation). 'Explain why a fractionating column is used' (2 marks — different boiling points, repeated condensation and vaporisation). Paper 4 also asks which technique is best for specific separations.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C2); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

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Step-by-step worked examples — Methods of Purification

Step-by-step solutions to past-paper-style questions on methods of purification , written exactly the way a tutor would explain them at the board.

1Obtaining copper sulfate crystals
Extended
Question
Describe how you would obtain pure, dry crystals of copper(II) sulfate from a solution of copper(II) sulfate.
Step-by-step solution
1. Step 1
  Heat the solution gently to evaporate water until the solution is saturated (test: a glass rod dipped in the solution forms crystals on cooling).
2. Step 2
  Remove from heat and allow the solution to cool slowly to room temperature. Blue copper sulfate crystals form as the solution becomes supersaturated.
3. Step 3
  Filter the mixture through a filter funnel and filter paper; the crystals are collected on the paper and the filtrate passes through.
4. Step 4
  Dry the crystals by pressing them between pieces of filter paper or leaving in a warm oven (below 100 °C to avoid losing water of crystallisation).
Answer
Evaporate to saturation → cool to crystallise → filter → dry between filter paper.
Examiner tip
Do not say 'boil to dryness' — this would decompose the crystals or give an impure powder. The word 'saturated' is important.
2Why distillation, not filtration, separates salt water
Extended
Question
Explain why simple distillation is used to obtain pure water from sea water rather than filtration.
Step-by-step solution
1. Step 1
  In sea water, salt (NaCl) is dissolved in water; it exists as Na⁺ and Cl⁻ ions, not as solid particles.
2. Step 2
  Filtration only separates insoluble solids from liquids; it cannot separate dissolved substances.
3. Step 3
  Distillation heats the sea water until water evaporates; the vapour is then cooled and condensed in a separate vessel, leaving the dissolved salt behind.
Answer
Salt is dissolved (not suspended) in sea water, so filtration cannot remove it; distillation evaporates the water, which is then condensed as pure liquid, leaving the salt in the flask.
3Fractional distillation of ethanol and water
Extended
Question
Describe how fractional distillation is used to separate a mixture of ethanol (boiling point $78\,°\text{C}$ ) and water (boiling point $100\,°\text{C}$ ).
Step-by-step solution
1. Step 1
  The mixture is heated in a flask. Ethanol has a lower boiling point (78 °C) and evaporates preferentially.
2. Step 2
  The vapour rises through a fractionating column packed with glass beads. Higher boiling-point components (water vapour) condense and drain back; lower boiling-point ethanol vapour passes through.
3. Step 3
  Ethanol vapour reaches the top of the column and enters the condenser, where it is cooled and collected as liquid ethanol.
4. Step 4
  Once most ethanol has distilled off, the temperature rises to 100 °C and water distils over and is collected separately.
Answer
Heat the mixture; ethanol (b.p. 78 °C) evaporates first and rises through a fractionating column; condenses and is collected; then water (b.p. 100 °C) distils over.
Examiner tip
Award marks for: fractionating column, different boiling points, lower boiling point substance distils first, temperature at thermometer.
4Choosing the correct purification method
Core
Question
A student has a mixture of sand and sodium chloride dissolved in water. Describe how to obtain a pure, dry sample of sodium chloride from this mixture.
Step-by-step solution
1. Step 1
  Filter the mixture: sand is insoluble and is retained on the filter paper; the sodium chloride solution (filtrate) passes through.
2. Step 2
  Evaporate the filtrate (sodium chloride solution) gently to saturation, then allow to cool to crystallise the sodium chloride.
3. Step 3
  Filter off the crystals and dry them between filter paper.
Answer
Step 1: filter to remove sand. Step 2: evaporate and crystallise to obtain NaCl crystals. Step 3: filter and dry the crystals.

Key Definitions and Keywords — Methods of Purification

Definitions to memorise and the exact keywords mark schemes credit for methods of purification answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Filtration
Examiner keyword
A separation technique that uses a filter (e.g. filter paper) to separate an insoluble solid from a liquid mixture; the solid is the residue, the liquid is the filtrate.
Crystallisation
Examiner keyword
Obtaining a pure solid solute from a solution by evaporating the solvent until saturated, then cooling to allow crystals to form, followed by filtration and drying.
Simple distillation
Examiner keyword
A technique that separates a liquid from a dissolved non-volatile solute, or liquids with very different boiling points, by heating, evaporation, and condensation.
Fractional distillation
Examiner keyword
A technique for separating miscible liquids with similar but different boiling points using a fractionating column; the component with the lowest boiling point distils first.
Residue
The solid material that remains on the filter paper after filtration.
Filtrate
The liquid that passes through the filter paper during filtration.

Common Mistakes and Misconceptions — Methods of Purification

The traps other students keep falling into on methods of purification questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Saying 'boil to dryness' to obtain crystals
0654 Examiner Report 2022
Why it happens
Students think all water must be removed; they do not realise that boiling to dryness gives a powder or impure residue, not pure crystals.
How to avoid it
Evaporate until saturated (test with glass rod), then cool slowly; this gives proper crystals.
✕Suggesting filtration to remove dissolved substances
Why it happens
Students confuse dissolved (invisible in solution) with suspended (cloudy). Filtration cannot remove dissolved solutes.
How to avoid it
Filtration: insoluble solids only. Distillation/crystallisation: dissolved substances.
✕Using simple distillation to separate liquids with similar boiling points
Why it happens
Students apply simple distillation to all liquid separations without considering boiling point differences.
How to avoid it
If boiling points differ by less than ~25 °C, use fractional distillation with a fractionating column.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Methods of Purification — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Experimental TechniquesCambridge IGCSE Coordinated Sciences 0654

Methods of Purification

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

PreviousNext

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Short Study Notes — Methods of Purification

Start with these resources to cover the key concepts, then work through the practice questions.

Page 1 / 0

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Methods of Purification , ready to print or save offline.

Methods of Purification — Cambridge IGCSE Coordinated Science 0654

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C2.4 — Describe and explain the methods of purification: filtration, crystallisation, simple and fractional distillation.
C2.5 — Choose the appropriate method for a given separation problem.

Filtration and Crystallisation

Filtration removes insoluble solids. Crystallisation purifies soluble solids from solution.

Filtration:

Used for: separating an insoluble solid from a liquid (or solution)
Method: Pour mixture through filter paper in a funnel. Insoluble solid remains as residue; liquid (filtrate) passes through.
Example: Separating sand from salt water. (The sand is filtered; the filtrate is salt water.)
Limitation: Cannot separate dissolved substances — they pass through filter paper.

Evaporation to dryness:

Used for: obtaining a dissolved solid from its solution when the solid is NOT damaged by heat
Method: Heat the solution; solvent evaporates; solid left behind
Example: Getting salt from salt water (fast, but crystals are small and may contain impurities)

Crystallisation:

Used for: obtaining PURE crystals of a dissolved solid
Method:
1. Heat solution to dissolve maximum solid (saturated solution)
2. Allow to cool slowly — large, pure crystals form as solubility decreases
3. Filter off the crystals
4. Dry gently (not too hot — to avoid melting crystals)
Why slow cooling? Slow cooling gives atoms time to arrange into a regular crystal lattice — larger, purer crystals.
Example: Purifying copper sulfate — blue crystals (CuSO₄·5H₂O) contain water of crystallisation.

Filtration: insoluble solid + liquid → solid on filter paper, filtrate passes through.
Crystallisation: slow cooling of hot saturated solution → large, pure crystals.
Evaporation to dryness: faster but produces smaller, less pure crystals.

Distillation — Separating Liquids

Simple distillation separates one liquid from a non-volatile solute. Fractional distillation separates miscible liquids with different boiling points.

Simple distillation:

Used for: separating a liquid from dissolved solids OR two liquids with very different boiling points (>25°C difference)
Method:
1. Heat mixture → liquid with lower BP vaporises first
2. Vapour travels through condenser (cooled by water)
3. Vapour condenses → pure liquid collected as distillate
Example: Getting pure water from salt water; getting water from an ink solution.

Fractional distillation:

Used for: separating a mixture of miscible liquids with similar but different boiling points
Extra component: a fractionating column (filled with glass beads or rings) between flask and condenser
How it works: Vapour rises; condenses on cooler glass beads, re-vaporises, rises further. Components with lower BP reach top first and are collected. Components with higher BP condense lower and fall back.
Example 1: Separating ethanol (BP 78°C) from water (BP 100°C) in beer/wine
Example 2: Fractional distillation of liquid air (N₂, O₂, Ar) and crude oil (petrol, diesel, kerosene, etc.)

Key apparatus: round-bottom flask, fractionating column, Liebig condenser, conical flask as receiver, thermometer at top of fractionating column.

The volatile liquid vaporises, condenses in the cooled Liebig condenser and drips into the receiver as pure distillate.

Simple distillation: liquid from solid, or liquids with very different boiling points.
Fractional distillation: mixtures of liquids with similar boiling points — fractionating column needed.
Component with LOWEST boiling point collects first in distillate.

How it’s examined

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C2); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Methods of Purification , ready to print or save as PDF.

Step-by-step worked examples — Methods of Purification

Step-by-step solutions to past-paper-style questions on methods of purification , written exactly the way a tutor would explain them at the board.

1Obtaining copper sulfate crystals
Extended
Question
Describe how you would obtain pure, dry crystals of copper(II) sulfate from a solution of copper(II) sulfate.
Step-by-step solution
1. Step 1
  Heat the solution gently to evaporate water until the solution is saturated (test: a glass rod dipped in the solution forms crystals on cooling).
2. Step 2
  Remove from heat and allow the solution to cool slowly to room temperature. Blue copper sulfate crystals form as the solution becomes supersaturated.
3. Step 3
  Filter the mixture through a filter funnel and filter paper; the crystals are collected on the paper and the filtrate passes through.
4. Step 4
  Dry the crystals by pressing them between pieces of filter paper or leaving in a warm oven (below 100 °C to avoid losing water of crystallisation).
Answer
Evaporate to saturation → cool to crystallise → filter → dry between filter paper.
Examiner tip
Do not say 'boil to dryness' — this would decompose the crystals or give an impure powder. The word 'saturated' is important.
2Why distillation, not filtration, separates salt water
Extended
Question
Explain why simple distillation is used to obtain pure water from sea water rather than filtration.
Step-by-step solution
1. Step 1
  In sea water, salt (NaCl) is dissolved in water; it exists as Na⁺ and Cl⁻ ions, not as solid particles.
2. Step 2
  Filtration only separates insoluble solids from liquids; it cannot separate dissolved substances.
3. Step 3
  Distillation heats the sea water until water evaporates; the vapour is then cooled and condensed in a separate vessel, leaving the dissolved salt behind.
Answer
Salt is dissolved (not suspended) in sea water, so filtration cannot remove it; distillation evaporates the water, which is then condensed as pure liquid, leaving the salt in the flask.
3Fractional distillation of ethanol and water
Extended
Question
Describe how fractional distillation is used to separate a mixture of ethanol (boiling point $78\,°\text{C}$ ) and water (boiling point $100\,°\text{C}$ ).
Step-by-step solution
1. Step 1
  The mixture is heated in a flask. Ethanol has a lower boiling point (78 °C) and evaporates preferentially.
2. Step 2
  The vapour rises through a fractionating column packed with glass beads. Higher boiling-point components (water vapour) condense and drain back; lower boiling-point ethanol vapour passes through.
3. Step 3
  Ethanol vapour reaches the top of the column and enters the condenser, where it is cooled and collected as liquid ethanol.
4. Step 4
  Once most ethanol has distilled off, the temperature rises to 100 °C and water distils over and is collected separately.
Answer
Heat the mixture; ethanol (b.p. 78 °C) evaporates first and rises through a fractionating column; condenses and is collected; then water (b.p. 100 °C) distils over.
Examiner tip
Award marks for: fractionating column, different boiling points, lower boiling point substance distils first, temperature at thermometer.
4Choosing the correct purification method
Core
Question
A student has a mixture of sand and sodium chloride dissolved in water. Describe how to obtain a pure, dry sample of sodium chloride from this mixture.
Step-by-step solution
1. Step 1
  Filter the mixture: sand is insoluble and is retained on the filter paper; the sodium chloride solution (filtrate) passes through.
2. Step 2
  Evaporate the filtrate (sodium chloride solution) gently to saturation, then allow to cool to crystallise the sodium chloride.
3. Step 3
  Filter off the crystals and dry them between filter paper.
Answer
Step 1: filter to remove sand. Step 2: evaporate and crystallise to obtain NaCl crystals. Step 3: filter and dry the crystals.

Key Definitions and Keywords — Methods of Purification

Definitions to memorise and the exact keywords mark schemes credit for methods of purification answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Filtration
Examiner keyword
A separation technique that uses a filter (e.g. filter paper) to separate an insoluble solid from a liquid mixture; the solid is the residue, the liquid is the filtrate.
Crystallisation
Examiner keyword
Obtaining a pure solid solute from a solution by evaporating the solvent until saturated, then cooling to allow crystals to form, followed by filtration and drying.
Simple distillation
Examiner keyword
A technique that separates a liquid from a dissolved non-volatile solute, or liquids with very different boiling points, by heating, evaporation, and condensation.
Fractional distillation
Examiner keyword
A technique for separating miscible liquids with similar but different boiling points using a fractionating column; the component with the lowest boiling point distils first.
Residue
The solid material that remains on the filter paper after filtration.
Filtrate
The liquid that passes through the filter paper during filtration.

Common Mistakes and Misconceptions — Methods of Purification

The traps other students keep falling into on methods of purification questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Saying 'boil to dryness' to obtain crystals
0654 Examiner Report 2022
Why it happens
Students think all water must be removed; they do not realise that boiling to dryness gives a powder or impure residue, not pure crystals.
How to avoid it
Evaporate until saturated (test with glass rod), then cool slowly; this gives proper crystals.
✕Suggesting filtration to remove dissolved substances
Why it happens
Students confuse dissolved (invisible in solution) with suspended (cloudy). Filtration cannot remove dissolved solutes.
How to avoid it
Filtration: insoluble solids only. Distillation/crystallisation: dissolved substances.
✕Using simple distillation to separate liquids with similar boiling points
Why it happens
Students apply simple distillation to all liquid separations without considering boiling point differences.
How to avoid it
If boiling points differ by less than ~25 °C, use fractional distillation with a fractionating column.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Methods of Purification — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Methods of Purification

Short Study Notes in the form of Slides

Short Study Notes — Methods of Purification

Detailed Notes

What you’ll learn

How it’s examined

1Obtaining copper sulfate crystals

2Why distillation, not filtration, separates salt water

3Fractional distillation of ethanol and water

4Choosing the correct purification method

Filtration

Crystallisation

Simple distillation

Fractional distillation

Residue

Filtrate

✕Saying 'boil to dryness' to obtain crystals

✕Suggesting filtration to remove dissolved substances

✕Using simple distillation to separate liquids with similar boiling points

Practice questions

Past paper style quiz

Assess your understanding

Methods of Purification — frequently asked questions

Methods of Purification

Short Study Notes in the form of Slides

Short Study Notes — Methods of Purification

Detailed Notes

What you’ll learn

How it’s examined

1Obtaining copper sulfate crystals

2Why distillation, not filtration, separates salt water

3Fractional distillation of ethanol and water

4Choosing the correct purification method

Filtration

Crystallisation

Simple distillation

Fractional distillation

Residue

Filtrate

✕Saying 'boil to dryness' to obtain crystals

✕Suggesting filtration to remove dissolved substances

✕Using simple distillation to separate liquids with similar boiling points

Practice questions

Past paper style quiz

Assess your understanding

Methods of Purification — frequently asked questions