Mixtures

A mixture forms when two or more substances are together but are not chemically bonded to each other. Those substances may be elements, compounds, or both. They are present side by side rather than joined into a new substance.

Because no chemical reaction has combined the substances into something new, each substance keeps its own chemical properties. The substances in a mixture still behave as themselves. For example, in seawater, the water is still water and the salt is still salt.

This is why mixtures can be separated by physical processes. Separating a mixture does not create any new substances. It only sorts out the substances that were already there.

A useful way to recognise a mixture is to look for these features:

• two or more substances are present together;
• they are not chemically bonded;
• each substance keeps its own properties;
• the substances can be separated physically.

Mixtures: examples

• Air is a mixture of gases.
• Seawater is a mixture of water and dissolved salts.
• Sand mixed with salt is a mixture of two solids.

Choosing separation techniques

A separation technique works by using a physical difference between the substances in a mixture. To choose a method, identify what the mixture contains, which substance you want to collect, and what physical difference can separate it from the others.

Choosing a method for the mixture

• Filtration: Use this to separate an insoluble solid from a liquid. The liquid passes through the filter, while the solid is retained. A dissolved solid cannot be separated from its solution by filtration because it passes through with the liquid.
• Crystallisation: Use this to obtain a dissolved solid from a solution. Some solvent is removed, and crystals of the solid form as the solution becomes more concentrated or cools. The method uses the solid’s solubility in the solvent.
• Simple distillation: Use this to collect a solvent from a solution, separating it from dissolved solids. The solvent boils and turns into vapour; the vapour is then cooled and condensed back into liquid. The dissolved solids remain behind.
• Fractional distillation: Use this to separate a mixture of liquids with different boiling points. The liquids vaporise and condense at different temperatures, allowing them to be collected separately.
• Chromatography: Use this to separate dissolved substances that move differently in a solvent. As the solvent moves through a stationary material, the substances travel at different rates and separate.

The desired product matters: for a solution of a solid in a liquid, choose crystallisation to obtain the solid or simple distillation to collect the solvent.

Distillation

Distillation separates substances using differences in their boiling points. A mixture is heated so that a substance vaporises, and the vapour is then cooled until it condenses back into a liquid. The collected liquid is called the distillate.

Simple distillation

In simple distillation, the substance with the lower boiling point vaporises first. Its vapour passes into a condenser, where it cools and becomes liquid again. The liquid is then collected separately.

Simple distillation can be used to obtain a solvent from a solution. For example, when salty water is heated, water vaporises while the dissolved salt remains in the original container. The water vapour condenses in the condenser, so pure water can be collected.

Fractional distillation

Fractional distillation is used to separate a mixture of liquids with different boiling points. It uses the same processes of vaporisation and condensation, but includes a fractionating column between the heated mixture and the condenser.

As vapours rise through the column, they repeatedly condense and vaporise. The higher-boiling liquid is more likely to condense and return down the column, while vapour richer in the lower-boiling liquid continues towards the condenser. This makes the separation of the liquids more effective.

Both forms of distillation are physical separation methods: the substances are separated, but no new substances are made.

Filtration

Filtration separates an insoluble solid from a liquid. An insoluble solid does not dissolve in the liquid, so it remains as solid particles in the mixture.

The mixture is poured through filter paper, usually held in a funnel. The liquid passes through the paper and is collected below as the filtrate. The solid particles are retained on the paper as the residue.

For example, when a mixture of sand and water is filtered, the sand is the residue and the water is the filtrate.

A substance that has dissolved in the liquid is not separated by filtration: it passes through the filter paper with the liquid and remains in the filtrate.

Crystallisation

Crystallisation separates a dissolved solid from a solution by forming solid crystals. The dissolved substance is the solute, and the liquid in which it is dissolved is the solvent.

Obtaining crystals from a solution

1. Gently heat the solution so that some of the solvent evaporates. This makes the solution more concentrated.
2. Leave the concentrated solution to cool. As it cools, less solute can remain dissolved, so some of it forms crystals.
3. Separate the crystals from the remaining solution and dry them.

For example, copper sulfate crystals can be obtained by concentrating copper sulfate solution and then allowing it to cool.

Crystallisation is a physical separation process: it recovers the dissolved substance rather than creating a new substance.

Paper chromatography

Paper chromatography separates dissolved substances in a mixture. A solvent moves through the paper, carrying the substances with it. Different substances travel different distances, so they separate.

How paper chromatography separates substances

A small spot of the mixture is placed on a pencil baseline near the bottom of a strip of chromatography paper. The paper is then placed in a solvent, with the baseline and spot above the solvent level. This prevents the spot from dissolving directly into the solvent at the bottom.

The solvent moves up the paper and passes through the spot. Substances in the mixture dissolve in the solvent and are carried up the paper. Each substance also has an attraction to the paper, which holds it back.

A substance that is more soluble in the solvent, or less strongly attracted to the paper, tends to travel further. A substance that is less soluble, or more strongly attracted to the paper, tends to travel a shorter distance. The substances therefore form spots at different positions on the paper.

For example, a spot of ink may separate into distinct spots of different dyes. Each separated spot shows where a dye has travelled.