Mixtures and Separating Techniques

A mixture is two or more elements or compounds that are not chemically combined. Because there is no chemical bond joining them, the chemical properties of each substance stay unchanged.

Air is a mixture of gases (mainly nitrogen and oxygen). Sea water is a mixture of water and dissolved salts. Crude oil is a mixture of many different hydrocarbon compounds.

The big contrast is with a compound. In a compound the elements are chemically combined in fixed proportions and can only be separated by a chemical reaction. In a mixture the proportions can vary and the parts can be separated by physical processes alone.

A separation is physical if it does not make any new substances. Filtration, distillation and chromatography all rearrange particles without changing them chemically, so no reaction takes place.

Because these methods rely only on physical differences such as particle size, solubility and boiling point, choosing the right one comes down to matching the method to the property that differs between the substances.

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. The small liquid particles pass through the tiny holes, while the larger insoluble solid particles are trapped.

The trapped solid left in the paper is called the residue, and the liquid that passes through is called the filtrate.

Term	Meaning
Residue	The insoluble solid trapped on the paper
Filtrate	The liquid that passes through the paper

A good example is separating sand from a mixture of sand and water: the sand cannot dissolve, so it stays on the paper as residue while the water runs through as filtrate. Filtration only works when one substance is insoluble; it cannot separate a dissolved solid, because dissolved particles are small enough to pass straight through the paper.

Crystallisation separates a soluble solid from its solution by evaporating the solvent so that crystals form.

The solution is warmed gently in an evaporating basin. As the solvent (usually water) evaporates, the solution becomes more concentrated. It is heated until the point of crystallisation, then left to cool. As it cools, the solid can hold less in solution, so pure crystals form and can be filtered off and dried.

Getting salt back from salt water uses crystallisation: the water evaporates away and salt crystals are left behind.

Heat gently and stop before the solution is fully dry. Overheating can break up crystals or decompose some compounds, spoiling the shape and purity of the product.

Crystallisation is the method of choice when the solid you want would be damaged by strong heating or when you want well-formed crystals rather than a powder. It is the reverse situation to filtration: here the useful substance is the dissolved one, not the insoluble one.

Distillation uses evaporation followed by condensation to collect a liquid. It exploits differences in boiling point.

Simple distillation separates a solvent from a solution (or any liquid from dissolved solids). The solution is heated; the liquid with the lower boiling point evaporates, travels into a condenser where it cools back to a liquid, and is collected. The dissolved solid stays behind in the flask. This is how pure water can be obtained from salt water.

Fractional distillation separates two or more liquids that are mixed together and have different boiling points. A fractionating column is fitted above the flask. As vapours rise, the column has a temperature gradient (hottest at the bottom, coolest at the top).

Feature	Simple distillation	Fractional distillation
Separates	A liquid from dissolved solids	Two or more miscible liquids
Key equipment	Flask + condenser	Flask + fractionating column + condenser
Based on	One liquid boiling off	Different boiling points of each liquid

Worked reasoning — a mixture of ethanol (boils at 78 °C) and water (boils at 100 °C). Heat the mixture. Ethanol vapour and some water vapour rise up the column. Water vapour condenses lower down (the column is cooler higher up) and drips back. Only ethanol vapour, with the lower boiling point, reaches the top first, so ethanol is collected first, ahead of the water.

Chromatography separates the substances in a mixture, most often coloured substances such as the dyes in an ink or food colouring.

There are two parts: a stationary phase (the chromatography paper) and a mobile phase (the solvent that moves through it). A spot of the mixture is placed on a pencil start line, and the bottom of the paper is stood in solvent. As the solvent soaks up the paper, it carries the dissolved substances with it.

Each substance moves a different distance because each is held by the paper and attracted to the moving solvent by different amounts:

• A substance that is more strongly attracted to the solvent and less strongly held by the paper moves far up.
• A substance that is more strongly held by the paper moves only a short way.

A mixture separates into several spots, so you can see how many substances it contains. A pure substance gives a single spot. The start line is drawn in pencil because pencil (graphite) is insoluble and will not run up the paper and interfere with the results.

How far each spot travels is compared to how far the solvent travels using the Rf value. The detailed Rf calculation and required practical are covered in the dedicated chromatography lesson.

Exams often give you a mixture and ask which technique separates it. Match the method to the property that differs.

You want to separate…	Property that differs	Method
Insoluble solid from a liquid	Solubility	Filtration
Soluble solid from its solution	Solubility + volatility	Crystallisation
A pure liquid from dissolved solids	Boiling point	Simple distillation
Two or more mixed liquids	Boiling point	Fractional distillation
Coloured substances in a mixture	Attraction to paper vs solvent	Chromatography

Worked example — rock salt (a mixture of salt and sand). Salt is soluble, sand is not. First add water and stir so the salt dissolves. Filter: the sand is the residue, the salty water is the filtrate. Then crystallise the filtrate: evaporate the water to leave pure salt crystals. Two techniques in sequence, each chosen for the property it exploits.

1. Confusing filtration and crystallisation

Filtration removes an insoluble solid; crystallisation recovers a dissolved solid. Salt water cannot be separated by filtration because the salt is dissolved and passes through the paper.

2. Mixing up residue and filtrate

The residue is the solid left on the paper; the filtrate is the liquid that passes through. Read the question to see which one the answer needs.

3. Saying distillation makes a new substance

Distillation is a physical separation; it does not make new substances. The liquid that evaporates and condenses is the same substance, just purified.

4. Drawing the chromatography start line in ink

The start line must be pencil. Ink would dissolve in the solvent and rise up the paper, ruining the separation. Pencil is insoluble.

5. Choosing simple distillation for two mixed liquids

Simple distillation separates a liquid from dissolved solids. Two liquids that are mixed together need fractional distillation so the fractionating column can separate them by boiling point.