Separation Techniques

Summary: Methods for separating mixtures: filtration, crystallisation, simple distillation, fractional distillation, and paper chromatography. The method chosen depends on the physical properties of the substances being separated. Tags: igcse chemistry separation practical purification Created: 2026-07-14 Last Updated: 2026-07-14

Decision Guide

What are you trying to separate?
├─ Insoluble solid from a liquid
│   → Filtration
│
├─ Soluble solid from a liquid (want the solid)
│   → Crystallisation
│
├─ A liquid from a solution (want the liquid/solvent)
│   → Simple Distillation
│
├─ Two or more miscible liquids with different boiling points
│   → Fractional Distillation
│
├─ Coloured solutes (dyes, inks) dissolved in a solvent
│   → Paper Chromatography
│
└─ Solid-solid mixture where one is magnetic
    → Magnetic Separation (use a magnet)

Filtration

Separates: Insoluble solid from a liquid (e.g., sand from water, BaSO₄ precipitate from solution)

Equipment

• Filter funnel, filter paper, conical flask, beaker

Method

1. Fold filter paper into a cone and place in filter funnel
2. Pour the mixture through the filter paper
3. Residue = insoluble solid that stays on the filter paper
4. Filtrate = liquid that passes through

Why It Works

• Filter paper has pores large enough for water molecules/ions to pass through but too small for the solid particles

IGCSE Examples

• Separating sand from salty water
• Collecting BaSO₄ precipitate in gravimetric analysis
• Removing excess CuO after reacting with acid (Method A salt prep)

Crystallisation

Separates: Soluble solid from a solution (e.g., salt from salty water, CuSO₄ crystals from CuSO₄ solution)

Method

1. Heat the solution to evaporate some of the solvent (NOT all of it)
2. Heat until the crystallisation point — when a drop on a cold glass rod forms crystals, or crystals first appear at the edge of the solution
3. Allow to cool slowly at room temperature — crystals form as solubility decreases with temperature
4. Filter, wash with cold distilled water, and dry between filter paper

Why NOT Evaporate to Dryness?

• Some salts decompose on strong heating (e.g., hydrated CuSO₄ → anhydrous CuSO₄)
• Evaporation to dryness gives a powder, NOT crystals
• Impurities would be mixed with the solid

Key Exam Words

• “Heat to the crystallisation point”
• “Allow to cool slowly” (slow cooling = larger, purer crystals)
• “Wash with cold distilled water”

Simple Distillation

Separates: A pure liquid (solvent) from a solution (e.g., pure water from seawater, ethanol from fermentation mixture)

Equipment

• Round-bottomed/pear-shaped flask, thermometer, condenser (Liebig), receiving flask, anti-bumping granules, heat source

Method

1. Heat the solution in the flask with anti-bumping granules (prevent violent boiling)
2. The substance with the lower boiling point evaporates first
3. The vapour passes into the condenser — cold water enters at the bottom, leaves at the top (counter-current flow for efficient cooling)
4. The vapour condenses back to liquid in the condenser
5. The pure liquid (distillate) is collected in the receiving flask
6. The thermometer reads the boiling point of the distilling vapour

Key Points

• Thermometer bulb must be at the opening to the condenser (measures temperature of vapour entering condenser, NOT the liquid)
• Water in at the bottom of the condenser jacket (ensures the condenser is always full of cold water)

IGCSE Examples

• Obtaining pure water from seawater/inK
• Separating ethanol (bp 78°C) from water (bp 100°C)

Fractional Distillation

Separates: Two or more miscible liquids with different boiling points (e.g., ethanol + water, crude oil fractions, liquid air)

Equipment

• Same as simple distillation PLUS a fractionating column packed with glass beads/spirals

Why the Fractionating Column?

• Provides a large surface area for condensation and re-evaporation
• Creates a temperature gradient — hotter at the bottom, cooler at the top
• As vapour rises, it cools; higher-boiling substances condense and drip back down
• Lower-boiling substances reach the top and enter the condenser

Method

1. Heat the mixture
2. Vapours rise through the fractionating column
3. Lower-boiling liquid reaches the top first and distills over
4. Higher-boiling liquid condenses on the glass beads and drips back into the flask
5. The thermometer at the top reads the boiling point of the vapour passing into the condenser

IGCSE Applications

Crude Oil Fractions (in order of increasing boiling point)

Paper Chromatography

Separates: Small amounts of coloured solutes (dyes, inks, food colourings) dissolved in a solvent

Method

1. Draw a pencil baseline (~2 cm from bottom of chromatography paper) — pencil is insoluble so won’t interfere
2. Apply small spots of the sample(s) on the baseline using a capillary tube
3. Place the paper in a beaker with a small amount of solvent — solvent level must be below the baseline (so spots don’t dissolve into the solvent)
4. Allow solvent to rise up the paper (capillary action)
5. Remove when solvent front is near the top; mark the solvent front immediately with pencil
6. Allow to dry; observe the chromatogram

Rf Value (Retention Factor)

Rf = distance moved by substance / distance moved by solvent front

• Rf is always between 0 and 1
• Each substance has a characteristic Rf for a given solvent and temperature
• Used to identify unknown substances by comparison with known standards

What It Tells You

• 1 spot → substance is pure
• Multiple spots → substance is a mixture
• Same Rf as a known sample → substance is likely the same

Why Substances Separate

• Different attractions to the stationary phase (paper/water) vs mobile phase (solvent)
• More soluble in solvent → travels further (higher Rf)
• More attracted to paper → travels less (lower Rf)

Choosing a Technique (Summary Table)

Common Question Types

Type 1: “Describe how you would obtain…”

• Frequency: ~40% of papers, 3-5 marks
• Method name → equipment → steps → reason
• Must justify WHY that technique is chosen

Type 2: Rf Calculation (1-2 marks)

• Measure distances, calculate Rf = d_substance / d_solvent
• Answer to 2 significant figures

Type 3: Chromatogram Analysis (2-3 marks)

• “Is X pure?” → if more than 1 spot, it’s a mixture
• “Does Y contain Z?” → compare Rf values

Common Mistakes

• Using pen instead of pencil to draw baseline — ink dissolves and runs, contaminating the chromatogram
• Solvent level above the baseline — spots wash off into solvent
• Thermometer bulb in the liquid in distillation — should be at the condenser opening
• Condenser water in at the top — must enter at bottom for complete filling (counter-current)
• Forgetting anti-bumping granules in distillation
• Evaporating to dryness instead of crystallising — get powder not crystals

Key Facts to Memorize

• Filtration: residue = solid on paper; filtrate = liquid through paper
• Crystallisation: heat to crystallisation point → cool slowly → filter → wash with COLD distilled water → dry
• Simple distillation: condenser water in at BOTTOM, out at TOP; thermometer at condenser opening
• Fractional distillation: fractionating column provides large surface area, temperature gradient; liquid with LOWER bp distills first
• Chromatography: pencil baseline (not pen), solvent below baseline, Rf = distance substance / distance solvent
• Pure substance → sharp melting/boiling point, single spot in chromatography

Related Notes

• Experimental Techniques — Overview of all experimental methods
• Paper Chromatography — Deeper dive into chromatography
• Fuels and Petroleum — Fractional distillation of crude oil
• Salt Preparation Methods — Uses crystallisation and filtration
• Composition of Air — Fractional distillation of liquid air for N₂
• IGCSE-Chem-Index

IGCSE Chemistry (0620/0971) wiki. Core practical topic — heavily assessed in Papers 5/6.