Many useful substances occur naturally as mixtures and must be separated to obtain pure components.
Examples:
Drinking water is purified from seawater or river water.
Crude oil (a mixture of hydrocarbons) is separated into petrol, diesel, jet fuel etc.
Ink contains different dyes that can be separated.
The separation method depends on the PHYSICAL PROPERTIES of the substances:
Size of particles (filtration).
Boiling point (distillation).
Solubility in a solvent (chromatography).
Magnetic properties (magnetism).
Filtration and Crystallisation
FILTRATION:
Separates an INSOLUBLE SOLID from a LIQUID.
Mixture is poured through filter paper in a funnel.
Liquid (filtrate) passes through the tiny pores.
Solid (residue) is too large to pass through — trapped in filter.
Example: separating sand from water; collecting a precipitate.
EVAPORATION:
Removes a LIQUID (usually water) from a solution, leaving the dissolved solid.
The solution is heated — water evaporates.
The solid crystallises out as the solvent is removed.
Example: recovering salt from salt solution.
CRYSTALLISATION:
More controlled than simple evaporation.
Heat solution to saturate it, then allow to COOL SLOWLY.
As temperature falls, solubility decreases — pure crystals form.
Better for producing large, regular, pure crystals.
Example: growing copper sulfate crystals from solution.
Distillation and Chromatography
SIMPLE DISTILLATION:
Separates a SOLUBLE SOLID from a LIQUID (or separates two liquids with very different boiling points).
Liquid is heated → evaporates → vapour travels through condenser → cools → condenses as pure liquid (distillate).
Solid or less volatile substance remains in flask.
Example: obtaining pure water from seawater.
FRACTIONAL DISTILLATION:
Separates a MIXTURE OF LIQUIDS with different boiling points.
A fractionating column (tall, cooler at top) allows separation: lower boiling point substances rise higher and condense at the top; higher boiling point substances condense lower down.
Different fractions collected at different heights.
Example: separating crude oil; separating air into nitrogen, oxygen and argon.
CHROMATOGRAPHY:
Separates DISSOLVED SUBSTANCES based on how far they travel through a medium in a solvent.
A spot is placed on chromatography paper and placed in a solvent (mobile phase).
The solvent travels up, carrying dissolved substances with it.
Different substances travel at different rates — they separate into distinct spots.
Used for: separating ink dyes, identifying food colourings, forensic analysis.
Rf VALUE — identifies each substance:
Rf = distance moved by substance ÷ distance moved by solvent front
Each substance has a characteristic Rf value in a given solvent.
⚠️ Common Mistake
Filtration only removes INSOLUBLE solids. You CANNOT filter salt from water — salt is dissolved (soluble). Use evaporation or distillation instead. Also: in distillation you collect the VAPOUR that condenses — the liquid. Crystallisation collects the SOLID that forms as the solution cools.
📐 Variables
RfRetention factor (chromatography) (Rf) is measured in ()
📐 Key Equations
Rf = distance moved by substance ÷ distance moved by solvent front
📌 Key Note
Filtration: insoluble solid from liquid. Crystallisation: dissolved solid from solution (cool slowly). Simple distillation: liquid from solution. Fractional distillation: liquids with different boiling points. Chromatography: dissolved substances — Rf = substance distance ÷ solvent distance.
🎯 Matching Activity — Match the Separation Technique
Match each mixture to the best separation technique. — drag the symbols on the right to match the component names on the left.
Filtration
Drop here
Crystallisation
Drop here
Simple distillation
Drop here
Fractional distillation
Drop here
Chromatography
Drop here
Salt dissolved in water — heat to saturate, cool slowly to form pure crystals
Sand mixed with water — sand is insoluble and too large to pass through filter paper
Crude oil — separate into fractions by different boiling points
Pure water from seawater — boil off and condense the water vapour
Ink dyes — different dyes travel different distances in the solvent
⚽ FIFA Worked Examples
Rf Value Calculation
In a chromatography experiment, a dye spot moves 6.0 cm. The solvent front moves 9.0 cm. Calculate the Rf value.
F
Rf = distance moved by substance ÷ distance moved by solvent
I
Rf = 6.0 ÷ 9.0
F
No unit conversion needed — Rf has no units
A
Rf = 0.67
🧪 Required Practical
🔬 RP1 — Investigate paper chromatography to separate and identify colours in inks or food dyes. Measure Rf values and compare to known standards.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 3
1. A student wants to obtain pure water from seawater. Which technique should they use?
2. In a chromatography experiment, a spot travels 4.5 cm and the solvent front travels 9.0 cm. What is the Rf value?
3. Why is fractional distillation used to separate crude oil rather than simple distillation?
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