Separating Mixtures: Filtration, Crystallisation and Distillation

Because the components of a mixture are not chemically combined, they can be pulled apart by physical methods — no chemical reaction is needed. The trick is to choose a method that exploits a difference in physical properties between the components: a difference in solubility, in particle size, or in boiling point. This lesson, part of Topic C2 of OCR Gateway Science A, works through the four core separation techniques — filtration, crystallisation, simple distillation and fractional distillation — and shows how to choose the right one and combine them to purify a real mixture such as rock salt.

By the end of this lesson you should be able to describe filtration, crystallisation, simple distillation and fractional distillation, explain which physical property each one relies on, choose the right technique for a given mixture, and plan a multi-step separation.

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Filtration

Filtration separates an insoluble solid from a liquid (or from a solution). The mixture is poured through filter paper in a funnel: the liquid passes through the tiny holes but the solid particles are too big and stay behind.

• The solid trapped in the paper is the residue.
• The liquid that passes through is the filtrate.

For example, a mixture of sand and water can be filtered: the sand is the residue (it cannot fit through the paper) and the water is the filtrate. Filtration works because the two components differ in particle size — but only if the solid is insoluble. A dissolved solid passes straight through with the liquid, because dissolved particles are far too small to be caught, so filtration cannot remove salt from salt water.

Exam Tip: Filtration separates an insoluble solid from a liquid. Learn residue = the solid left in the paper, filtrate = the liquid that passes through. If the solid is dissolved, filtration will not work — it goes through with the filtrate.

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Crystallisation

Crystallisation separates a soluble solid from the solution it is dissolved in — for example getting copper sulfate crystals back from copper sulfate solution. It works by evaporating the solvent so that the dissolved solid is left behind as crystals.

Method (numbered):

1. Pour the solution into an evaporating basin.
2. Heat gently (often over a water bath) to evaporate some of the solvent. As the solvent leaves, the solution becomes more and more concentrated.
3. Stop heating when crystals start to form at the edges (the "point of crystallisation") — do not evaporate all the way to dryness.
4. Leave the solution to cool slowly. As it cools, more solid comes out of solution and crystals grow.
5. Filter off the crystals and dry them (e.g. by patting between filter paper or leaving in a warm place).

Cooling and evaporating slowly gives larger, well-formed crystals, because the particles have time to arrange themselves into a regular pattern. You must not evaporate a hydrated salt to dryness by strong heating, because that can drive off the water of crystallisation (or even decompose the salt) and spoil the crystals.

Exam Tip: Crystallisation gets a soluble solid back from its solution. The marks are: evaporate some (not all) solvent, then cool to crystallise, then filter and dry. Slow cooling/evaporation = bigger crystals.

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Simple Distillation

Simple distillation separates a liquid (the solvent) from a solution — for example pure water from salt water. It relies on the difference in boiling point between the liquid you want and the dissolved solid (which has a much higher boiling point and will not boil off).

The solution is heated until the solvent boils and turns to vapour. The vapour passes into a condenser — a tube surrounded by a cold-water jacket — where it cools and condenses back into a liquid. This pure liquid, the distillate, is collected; the dissolved solid is left behind in the flask.

Simple distillation is fine when there is a big difference in boiling point — such as between a liquid solvent and a dissolved solid. It is not suitable for separating two liquids whose boiling points are close, because both would boil off together; for that you need fractional distillation.

Exam Tip: Distillation does not make a new substance — it is a physical change. The water you collect was always water; it has just been boiled and condensed away from the salt. State that distillation relies on a difference in boiling point.

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Fractional Distillation

Fractional distillation separates a mixture of two or more miscible liquids (liquids that mix completely) that have different boiling points — for example ethanol from water, the different fuels in crude oil, or the gases in liquefied air.

The extra piece of apparatus is a fractionating column — a tall column, packed or with glass beads, fitted above the flask. It is hottest at the bottom and gets cooler towards the top. As the mixed vapour rises, it repeatedly condenses and re-evaporates on the packing, which separates the liquids by their boiling points:

• The liquid with the lowest boiling point reaches the top and distils over first.
• Liquids with higher boiling points condense lower down and run back into the flask until the temperature rises enough for them to reach the top.

For an ethanol–water mixture, ethanol (boiling point $78\,°\text{C}$78°C) distils over before water ($100\,°\text{C}$100°C). You watch the thermometer: the temperature holds steady at the boiling point of whichever liquid is distilling, then rises to the next plateau as that liquid runs out and the next begins.

Technique	Separates	Physical property used
Filtration	Insoluble solid from a liquid	Particle size (solubility)
Crystallisation	Soluble solid from its solution	Solubility (solid stays when solvent evaporates)
Simple distillation	A liquid (solvent) from a solution	Boiling-point difference (solid vs liquid)
Fractional distillation	Two or more miscible liquids	Different boiling points of the liquids

Exam Tip: Use fractional (not simple) distillation when separating liquids with similar boiling points. The key feature is the fractionating column (cooler at the top), and the liquid with the lowest boiling point comes off first.

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Choosing the Right Technique

When a question asks you to separate a mixture, work through what the components are and what differs between them. This decision flow captures the logic.

flowchart TD
  A["What is in the mixture?"] --> B{"Insoluble solid<br/>in a liquid?"}
  B -->|Yes| C["Filtration"]
  B -->|No| D{"Soluble solid<br/>dissolved in a liquid?"}
  D -->|"Want the solid"| E["Crystallisation"]
  D -->|"Want the liquid"| F["Simple distillation"]
  D -->|No| G{"Two or more<br/>liquids mixed?"}
  G -->|"Similar boiling points"| H["Fractional distillation"]

Worked Example 1 — A multi-step separation

Rock salt is a mixture of salt (soluble) and sand (insoluble). Describe how to obtain pure, dry samples of both the salt and the sand from rock salt.