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Methods of Separation and Purification

Separation techniques: choose filtration, crystallisation, distillation, separating funnel, or chromatography based on solubility and boiling points.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

Show all 10 objectives
  • describe methods of separation and purification for the components of mixtures, to include: — use of a suitable solvent, filtration and crystallisation or evaporation
  • describe methods of separation and purification for the components of mixtures, to include: — sublimation
  • describe methods of separation and purification for the components of mixtures, to include: — distillation and fractional distillation (see also 11.1(b))
  • describe methods of separation and purification for the components of mixtures, to include: — use of a separating funnel
  • describe methods of separation and purification for the components of mixtures, to include: — paper chromatography
  • suggest suitable separation and purification methods, given information about the substances involved in the following types of mixtures: — solid-solid
  • suggest suitable separation and purification methods, given information about the substances involved in the following types of mixtures: — solid-liquid
  • suggest suitable separation and purification methods, given information about the substances involved in the following types of mixtures: — liquid-liquid (miscible and immiscible)
  • interpret paper chromatograms including comparison with ‘known’ samples and the use of Rf values
  • explain the need to use locating agents in the chromatography of colourless compounds (knowledge of specific locating agents is not required)

Separation techniques are scored by one thing: choosing the correct method based on particle type, solubility, and boiling point. Guessing is a mark-loser.

1. Definition

A mixture contains two or more substances not chemically combined. Separation uses physical differences (e.g., solubility, boiling point) to obtain pure substances.

2. Key Ideas

  • First identify what you have: solid + liquid, solid + solid, liquid + liquid, or a mixture of substances dissolved in the same solvent.
  • Filtration separates an insoluble solid from a liquid.
  • Evaporation and crystallisation separate a soluble solid from a solution.
  • Separating funnel separates immiscible liquids.
  • Distillation separates liquids based on boiling point.
  • Chromatography separates substances based on different solubilities in the solvent and attraction to the paper.
The selection question you must answer first

“Is it soluble? Is it miscible? Which boiling point is lower?” If you cannot answer these, you cannot choose the technique.

3. Detailed Explanations

Quick Recall (vocabulary)
  • Soluble = dissolves in a solvent to form a solution; insoluble = does not dissolve.
  • Miscible liquids mix completely; immiscible liquids form layers.
  • Residue stays on filter paper; filtrate passes through; distillate is collected in distillation.

Concrete-first examples (then generalise using the table):

  • sand + water → filtration
  • salt + water → crystallisation / evaporation
  • oil + water → separating funnel

A. Choosing the Technique (Decision Table)

MixtureKey property usedTechnique
Insoluble solid + liquidInsolubilityFiltration
Soluble solid + liquidSolubility + evaporationEvaporation to dryness / Crystallisation
Two solids, one magneticMagnetismMagnetic separation
Two solids, one sublimesSublimationSublimation
Two immiscible liquidsImmiscibility (layers) + densitySeparating funnel
Solution (recover the solvent)Boiling pointSimple distillation
Two miscible liquids (close b.p.)Boiling pointsFractional distillation
Mixture of dyes/inksSolubility/attractionPaper chromatography

B. Filtration (Insoluble Solid + Liquid)

Used to separate an insoluble solid from a liquid.

  • The liquid that passes through is the filtrate.
  • The solid left on the filter paper is the residue.
  • Example: sand + water.
Filtration apparatusA filter funnel lined with filter paper sits over a conical flask. Insoluble residue remains on the paper while filtrate collects in the flask.filter paperresiduefiltrateinsoluble solid + liquid
Filtration: the insoluble solid remains as residue on the filter paper; the liquid passes through as filtrate.

C. Evaporation to Dryness (Soluble Solid + Liquid)

Used to obtain a soluble solid by heating until all the solvent evaporates.

  • Use this only if the solute does not decompose on heating (e.g., NaCl).

D. Crystallisation (Purifying a Soluble Solid)

Crystallisation is used when heating to dryness might decompose the solute, or when you want pure crystals.

Saturated solution

A saturated solution contains the maximum amount of solute that can dissolve at that temperature.

Process (mark-scheme steps):

  1. Heat the solution gently to concentrate it (do not boil to dryness).
  2. Stop heating when crystals just start to appear at the edge of the dish.
  3. Allow to cool so crystals form.
  4. Filter the crystals and dry them between filter papers.

Example: copper(II) sulfate crystals from copper(II) sulfate solution.

E. Separating Solid-Solid Mixtures

Magnetism: remove iron filings using a magnet.

Sublimation: used when one solid sublimes (solid → gas) and the other does not.

  • Examples of subliming solids used in separation questions include iodine and ammonium chloride.
Laboratory Warning

Iodine vapour irritates the eyes and respiratory system. Keep the apparatus in a fume cupboard, avoid breathing the vapour and follow your teacher’s instructions.

Suitable solvent: if one solid dissolves in a solvent and the other does not (e.g., salt + sand: dissolve salt in water, filter sand, then evaporate/crystallise salt).

F. Separating Funnel (Immiscible Liquids)

Used to separate immiscible liquids (do not mix) like oil and water.

  • The denser liquid forms the lower layer and is drained first.
Separating funnel apparatusA separating funnel contains a less-dense upper layer and denser lower layer. The stopper is shown removed and the lower layer drains through the open tap into a beaker.stopper removedless-dense layerdenser layertap openimmiscible liquids form two layers
Separating funnel: remove the stopper, open the tap and collect the denser lower layer first.

G. Distillation (Separating by Boiling Point)

Simple distillation

Used to obtain a pure solvent from a solution (or to separate two miscible liquids with very different boiling points).

  • The liquid with the lower boiling point evaporates first.
  • The vapour is cooled in a condenser and collected as the distillate.
  • Example: pure water from seawater.
Simple distillation apparatusA heated round-bottom flask connects to a still head. The thermometer bulb is level with the condenser entrance. Vapour passes into a downward-sloping water condenser, with cooling water entering at the lower port and leaving at the upper port, before the distillate collects in an open receiver.electric heater / hot-water bathmixturethermometerbulb level with side armwater condenserwater outwater indistillatereceiver open to air
Simple distillation: the thermometer bulb sits beside the condenser entrance, and cooling water enters the condenser at the lower port.

Fractional distillation

Used to separate miscible liquids with close boiling points.

  • A fractionating column provides a large surface area for repeated evaporation and condensation.
  • The liquid with the lower boiling point reaches the condenser first.

Example: ethanol (78°C) from water (100°C); fractions of crude oil.

Fractional distillation apparatusA heated round-bottom flask connects through a fractionating column to a still head. The thermometer bulb is level with the condenser entrance. Vapour passes into a downward-sloping water condenser, with cooling water entering at the lower port and leaving at the upper port, before the distillate collects in an open receiver.electric heater / hot-water bathmixturefractionating columnthermometerbulb level with side armwater condenserwater outwater indistillatereceiver open to air
Fractional distillation: a fractionating column provides repeated condensation and vaporisation before vapour reaches the condenser.
Laboratory Warning

Ethanol and many organic solvents are flammable, and their vapours can ignite. Keep them away from naked flames, use the instructed heating method and wear eye protection.

H. Paper Chromatography

Used to separate and identify small amounts of substances (like dyes or medicines) that dissolve in the same solvent.

Paper chromatography setupPaper hangs in a covered beaker. The pencil start line and sample spots are above the solvent level, and separated spots lie below the marked solvent front.solvent frontpencil start linesolventseparated spotscovered container reduces solvent evaporation
Paper chromatography: use a pencil start line above the solvent level and mark the solvent front before it evaporates.

Principles (mark-scheme points):

  • Substances separate because they have different solubilities in the solvent and different attraction to the paper.
  • The start line must be drawn in pencil (graphite is insoluble).
  • The solvent level must be below the start line.
  • Use a lid to reduce solvent evaporation and keep conditions consistent.

R_f values

The retention factor (R_f) is a ratio used to compare substances.

R_f = (distance from start line to centre of spot)/(distance from start line to solvent front)

Rf rules

R_f has no units and is always between 0 and 1.

Locating agents

If the separated substances are colourless, a locating agent is used after the chromatogram is developed to make the spots visible.

One spot does not prove purity

One spot suggests one visible component in that solvent, but it does not guarantee purity. See Purity of Substances.

4. Common Mistakes

  • Using filtration for a soluble solid (no residue forms).
  • Boiling a crystallisation mixture to dryness (you can get powder or decomposition instead of crystals).
  • Forgetting to remove the stopper of the separating funnel before opening the tap (liquid “glugs” and the flow is not controlled).
  • Drawing the chromatography start line in ink.
  • Placing the chromatography start line below the solvent level.
  • Using simple distillation when boiling points are close (poor separation).

5. Exam Tips

Keywords that score

Name the property that makes the method work: “insoluble solid”, “miscible/immiscible liquids”, “lower boiling point”, or “different solubilities”. Use “filtrate”, “residue” and “distillate” for the correct fractions.

Purification vs separation

“Separation” gets you components. “Purification” means you must obtain a pure sample (e.g., crystallisation, distillation) and justify it.

If a question also asks how to check the product is pure, see Purity of Substances.

6. Worked Examples

Modelled example 1

Designing a Separation Strategy

Core

Problem

Describe a step-by-step method to obtain pure dry samples of iron filings, sand and sodium chloride from a mixture of all three.
Study the worked solution

  1. Remove the magnetic component

    Method

    Pass a magnet over the dry mixture to collect the iron filings.

    Reason

    Iron is magnetic, while sand and sodium chloride are not.

    Working

    First product: separated iron filings.

  2. Exploit different water solubilities

    Method

    Add water and stir the remaining sand–salt mixture.

    Reason

    Sodium chloride dissolves but sand remains insoluble.

    Working

    Mixture becomes solid sand + sodium chloride solution.

  3. Separate residue and filtrate

    Method

    Filter, then wash and dry the residue.

    Reason

    Sand stays on the filter paper; dissolved salt passes through in the filtrate.

    Working

    Residue: pure dry sand; filtrate: sodium chloride solution.

  4. Recover the soluble solid

    Method

    Evaporate the water and dry the remaining sodium chloride.

    Reason

    Removing the solvent recovers the dissolved, heat-stable salt.

    Working

    Final samples: iron filings, sand and sodium chloride.

Guided practice 2

Calculating R_f

About 5 min

Problem

On a chromatogram, a dye spot moves 3.0 cm from the start line while the solvent front moves 10.0 cm. Calculate R_f.

Choose numerator, denominator and result

Correct ratio
Rf value

Hints

Hint 1: formula
Distance travelled by spot goes above distance travelled by solvent front.
Hint 2: units
Both distances use cm, so their units cancel.
View solution step by step

  1. Substitute the distances

    Method

    Divide the spot distance by the solvent-front distance.

    Reason

    This is the definition of retention factor.

    Working

    R_f = 3.0/10.0.

  2. Calculate and check

    Method

    State R_f = 0.30 without a unit.

    Reason

    The distance units cancel and a valid value lies between 0 and 1.

    Working

    R_f = 0.30.

Common misconception 3

Error Analysis in Chromatography

Find and correct the mistake

Learner setup

A learner draws the start line with blue ink and places the solvent above that line. Explain and correct both errors.

Protect the reference line and sample spots

Start-line material
Solvent level

View solution step by step

  1. Correct the baseline

    Method

    Draw the start line in pencil.

    Reason

    Blue ink dissolves and separates, adding unwanted spots that confuse the chromatogram; graphite is insoluble.

    Working

    Use a pencil reference line.

  2. Correct the solvent level

    Method

    Keep the solvent below the start line.

    Reason

    If submerged, the samples dissolve into the reservoir instead of moving up the paper with the solvent front.

    Working

    Start line and sample spots above the solvent surface.

Examiner practice 4

Choosing Simple vs Fractional Distillation

2 marks

Examination question

Choose the most suitable distillation method for separating ethanol and water, and justify it. [2 marks]

Name the method and property reason

View solution step by step

  1. Choose the method

    1 mark

    Method

    Select fractional distillation.

    Reason

    A fractionating column provides repeated evaporation and condensation.

    Working

    Method: fractional distillation.

  2. State the property reason

    1 mark

    Method

    Identify ethanol and water as miscible liquids with relatively close boiling points.

    Reason

    Simple distillation would give poorer separation when both vapours can be present over a close temperature range.

    Working

    Fractional distillation gives better separation of the two miscible liquids.

Challenge 5

Separating Funnel Method

Minimal support

Apparatus transfer

Before draining the lower layer from a separating funnel containing oil and water, why must the stopper be removed?

Connect air pressure to liquid flow

Removing stopper allows
Resulting flow

Hints

Hint 1: replacement
As liquid leaves the closed vessel, something must replace its volume.
Hint 2: pressure
Without incoming air, pressure above the liquid falls and flow becomes intermittent.
View solution step by step

  1. Allow air replacement

    Method

    Remove the stopper so air can enter the funnel.

    Reason

    Incoming air replaces the volume of liquid draining through the tap.

    Working

    Top open to atmosphere while the lower tap drains.

  2. Maintain control

    Method

    State that liquid then flows smoothly rather than glugging.

    Reason

    Pressure remains approximately equal to atmospheric pressure above the liquid.

    Working

    Remove stopper → air enters → controlled layer separation.

7. Mind Stretchers

Mind stretcher 1: Strategy With a Heat-Sensitive SoluteExtension

Question: A student has copper(II) sulfate solution and wants pure crystals. The student boils the solution to dryness. Explain why this is not the best method and state the correct method.

Show Answer

Boiling to dryness can give powder/small impure crystals and risks decomposition due to strong heating. The correct method is crystallisation: concentrate the solution gently until crystals start to form at the edge, then cool to crystallise, filter, and dry the crystals.

Mind stretcher 2: Chromatography “Purity” TrapExtension

Question: A student runs a sample and sees one spot. The student concludes “the sample is pure”. Correct this conclusion.

Show Answer

One spot means one visible component under those conditions (same solvent, same paper). Different substances can sometimes have the same R_f in one solvent, so one spot does not guarantee purity.

8. Quiz

Quiz Time!

Practise selecting and explaining separation methods, including chromatography and R_f calculations.

Start separation quiz

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