Choosing separation methods

Choose a separation method by asking which component you want to recover and which physical property distinguishes it from the others. Begin with simple mixtures, then combine methods when a mixture has several components.

Start with the sample and the goal

A mixture contains two or more substances not chemically combined. Separation uses physical differences (e.g., solubility, boiling point) to recover the components. Further purification may be needed to obtain a sufficiently pure sample.

What you need to know

• 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.

Choosing and explaining a method

Start with familiar examples, then use the table to select a method:

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

Choosing the technique (decision table)

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.

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).

Crystallisation (purifying a soluble solid)

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

To obtain crystals from a suitable solution:

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, rinse with a small amount of cold solvent if instructed, and dry them between filter papers.

In a suitable mixture, soluble impurities remain mainly in the solution left after crystallisation. Crystallisation can improve purity, but it does not guarantee that every impurity has been removed.

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

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. Choose this method only when the given substances have suitably different sublimation behaviour.

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).

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.

Distillation: choose which liquid to recover

To recover a solvent from a solution containing a non-volatile solute, use simple distillation: the solvent vaporises and is then cooled and collected as a liquid. For miscible liquids with relatively close boiling points, fractional distillation improves separation through repeated vaporisation and condensation in a fractionating column.

Learn the apparatus and follow each substance through it in simple and fractional distillation.

Paper chromatography: separate small dissolved samples

Paper chromatography separates substances through different solubilities in the solvent and different attractions to the paper. It is useful for small samples such as mixtures of dyes.

The paper chromatography lesson develops the method, locating agents, comparison with reference samples and retention-factor (R_f) calculations.

Avoiding 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).

Using the evidence

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

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.

Continue with this example in the chromatography lesson.

Continue with this example in the chromatography lesson.

Continue with this example in the distillation lesson.

Challenge 2

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

Air to enterOil to dissolve

Resulting flow

Smooth and controlledFaster mixing

Check my attempt

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, so drainage can slow, stop or become 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.

Next step

Try Strategy with a heat-sensitive solute independently.

Try it independently

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

Evaporating all the water also leaves any non-volatile impurities behind, while strong heating can remove water from hydrated copper(II) sulfate crystals. Use crystallisation: concentrate gently, allow the solution to cool, filter the crystals and dry them between filter papers. Soluble impurities can remain in the liquid left behind; small crystals or powder alone do not prove impurity.

Try the chromatography purity question in the chromatography lesson.

Practise and check

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