Separation of Mixtures

A solution is made up of a solvent and a solute. A solvent is a substance such as water that can dissolve a solute. The solvent ‘pulls apart’ the bonds that hold the solute particles together and the solute particles diffuse (spread randomly by hitting into each other) throughout the solvent to create a solution.

The solution is a mixture with evenly spread solvent and solute particles. These particles can be physically separated by evaporation.

When solids mix into a liquid and can no longer be seen, they have dissolved. Often the particles of the solute seemed to have disappeared, but they are all still present. They are now just in very small particles, too small to be seen by eye.

Mixtures can be separated by physical processes

Mixtures of substances are not chemically bonded (joined) to each other so they can be separated by physical techniques. The state of the various substances in the mixture, such as a liquid and solid or the physical properties of the substances, such as different boiling points will determine which method of separation will be used.

Filtering

Many drinks we purchase are solutions. Most of them are solutions of mainly sugar (solute) and water (solvent) with a small amount of flavouring, colouring and some minerals mixed in. We do not “see” the sugar because it is dissolved into the water and becomes too small to see.

This means a lot of sugar can be hidden in the liquid and we are unaware of the amount of sugar we take in, even in so-called healthy sports drinks. Filtering separates an insoluble solid in a mixture from the liquid completely.

The solvent molecules (liquid) and any dissolved molecules present in the solution can pass through the filter paper, which has small holes, while the solid particles cannot because they are too large and stay in the filter paper.

The solvent or solution containing dissolved substances passes through the filter paper is called filtrate. The solid particles that remain on the filter paper are called the residue.

Evaporating (by boiling)

Evaporating separates a dissolved solid from a liquid. The solvent (liquid) is lost into the surroundings. The liquid will evaporate but evaporation becomes faster at higher temperatures.

The solid remains because it has a higher (often very much higher) boiling point than the liquid.

Dissolving

When two solid substances are mixed together, they can be separated by dissolving. A solvent such as water can be added if only one of the substances is soluble. For example; if salt is mixed with dirt then adding water will dissolve the salt (which can later be separated by evaporation) and the remaining dirt can be removed from the solution by filtering.

The salt becomes the solute and will go through the filter as it is in solution.

Decanting

Decanting is simply pouring off a liquid without losing any of the denser substance (usually an insoluble solid) in the bottom of the container. Decanting separates a heavier substance from a lighter one.

Chemists are most often after the substance at the BOTTOM of the container.

Chromatography

Chromatography is a method used to separate the various substances in a mixture of dye or ink. Substances of the mixture will differ in how much they "stick" to things: to each other, and to other substances.

Some of the substances of the ink will stick more tightly to the paper fibres. They will spend less time in the water as it moves along the paper fibres, and they will not travel very far. Other components of the ink will stick less tightly to the paper fibres.

They will spend more time in the water as it moves along the paper fibres, and they will travel further through the paper.

Magnetism

Magnetism can be used to separate a magnetic substance (such as iron) from a mixture containing non-magnetic substances (such as sulfur or sand). The magnetic substance of the mixture is separated with the help of the magnetic attraction.

A magnet is moved over the mixture containing the magnetic substance e.g., iron filings. These get attracted to the magnet.

The process is repeated until the magnetic material is completely separated from the mixture. The non-magnetic substance is left behind.

Distillation

Distillation is a process of boiling a liquid until it forms a vapour and condensing, then collecting the liquid. The liquid collected is the distillate. The Liebig Condenser cools the vapour back into liquid. The purpose of distillation is separation of a mixture of two liquids.

This is possible if the two substances have different boiling points. The substance with the lower boiling point turns to gas and is collected while the other substance with a higher boiling point remains as a liquid in the flask

Mixtures can be easily separated physically – Summary