Mixtures

Definition of Mixtures

A mixture is a combination of two or more substances that are physically combined, not chemically bonded. This means the substances keep their own individual properties and can be separated by physical methods. Unlike compounds, no new substances are formed in a mixture.

For example, if you mix sand and salt, both keep their properties and can be separated again.

Types of Mixtures

Mixtures can be classified into two main types:

• Homogeneous mixtures: These have the same composition throughout. You cannot see the different parts because they are evenly mixed. An example is salt dissolved in water (salt solution).
• Heterogeneous mixtures: These have visibly different parts or phases. You can often see the different substances. An example is a mixture of sand and iron filings.

The key difference is whether the components are evenly distributed or not.

Separation Techniques

Because mixtures are physical combinations, their components can be separated by physical methods. The main techniques you need to know are:

Filtration

Used to separate an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. The solid is trapped on the paper (residue), and the liquid passes through (filtrate).

Example: Separating sand from water.

Crystallisation

Used to obtain a soluble solid from a solution. The solution is gently heated to evaporate some water until crystals start to form, then left to cool so crystals grow.

Example: Obtaining salt crystals from salt water.

Simple Distillation

Used to separate a liquid from a solution when the liquid has a much lower boiling point than the solute. The solution is heated; the liquid evaporates, condenses in a condenser, and is collected separately.

Example: Separating pure water from salt water.

Fractional Distillation

Used to separate a mixture of liquids with different boiling points. The mixture is heated, and components evaporate at different temperatures, condensing separately in the fractionating column.

Example: Separating different gases in air or different liquids in crude oil.

For instance, air can be separated into oxygen, nitrogen, and other gases by fractional distillation because each gas has a different boiling point.

Properties of Mixtures vs Pure Substances

Mixtures differ from pure substances in several ways:

• Variable composition: The proportions of components in a mixture can vary, so mixtures do not have a fixed composition.
• Boiling and melting points: Mixtures usually melt and boil over a range of temperatures, unlike pure substances which have sharp melting and boiling points.
• Physical separation: Components of mixtures can be separated by physical methods such as filtration or distillation, while pure substances cannot be separated this way.

For example, pure water boils exactly at 1006C at normal pressure, but seawater (a mixture) boils over a range of temperatures due to the dissolved salts.

Learning Example: Separating Sand and Salt

If you have a mixture of sand and salt, you can separate them as follows:

1. Add water to the mixture and stir. Salt dissolves; sand does not.
2. Filter the mixture. Sand stays on the filter paper; salt solution passes through.
3. Heat the salt solution to evaporate water and crystallise the salt.

This uses filtration and crystallisation to separate the components physically.

Example: If you start with 100 cm8 of water and evaporate 50 cm8, the solution becomes saturated and salt crystals begin to form. The next step is to leave the solution to cool so more salt crystals grow, which can then be collected by filtration and dried.