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.

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.

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 held in a funnel. The liquid passes through the paper and is collected below; this liquid is called the filtrate. The solid particles are too large to pass through the paper, so they remain on it as the residue.

For example, a mixture of sand and water can be separated by filtration. Sand is insoluble in water, so the sand remains as the residue while the water passes through as the filtrate.

Filtration is a physical separation: it separates substances without producing a new substance.

Crystallisation

Crystallisation separates a dissolved solid from a solution by forming solid crystals. The solid is dissolved in a liquid called the solvent.

First, some of the solvent is evaporated. This makes the solution more concentrated because there is less solvent for the dissolved solid. The concentrated solution is then allowed to cool. As it cools, less of the solid can remain dissolved, so crystals form from the excess solid.

The crystals are separated from the remaining liquid, for example by filtration, and then dried. Some solid may remain dissolved in the liquid.

For example, salt crystals can be obtained from salt solution by evaporating some water and allowing the concentrated solution to cool. Crystallisation is a physical separation: the dissolved salt becomes solid again, but no new substance is formed.

Paper chromatography

Paper chromatography separates substances dissolved in a mixture by carrying them through paper with a solvent. The substances move at different rates, so they can become separated into distinct spots. For example, chromatography can separate the coloured substances in an ink.

How paper chromatography separates a mixture

A small spot of the mixture is placed on a pencil baseline near the bottom of a piece of paper. The paper is placed in a solvent with the baseline above the solvent level. This prevents the spot from being submerged in the solvent.

As the solvent rises through the paper, it carries dissolved substances with it. How far each substance travels depends on its relative attraction to the solvent and to the paper:

• A substance more strongly attracted to the solvent tends to travel further with it.
• A substance more strongly attracted to the paper tends to move more slowly and travel a shorter distance.

Because the substances can travel different distances, the original spot may separate into several spots on the paper. This is a physical separation: no new substances are produced.