Chemistry - Mixtures

 We see mixtures all the time in our everyday lives. A stew, for example, is a mixture of different foods such as meat and vegetables; sea water is a mixture of water, salt and other substances, and air is a mixture of gases such as carbon dioxide, oxygen and nitrogen.

Definition: Mixture

A mixture is a combination of two or more substances, where these substances are not bonded (or joined) to each other and no chemical reaction occurs between the substances.

In a mixture, the substances that make up the mixture:

• are not in a fixed ratio

Imagine, for example, that you have 250mL of water and you add sand to the water. It doesn’t matter whether you add 20g, 40g, 100g  or anyother mass of sand to the water; it will still be called a mixture of sand and water.

• keep their physical properties

In the example we used of sand and water, neither of these substances has changed in any way when they are mixed together. The sand is still sand and the water is still water.

• can be separated by mechanical means

To separate something by “mechanical means”, means that there is no chemical process involved. In our sand and water example, it is possible to separate the mixture by simply pouring the water through a filter. Something physical is done to the mixture, rather than something chemical.

We can group mixtures further by dividing them into those that are heterogeneous and those that are homogeneous.

Heterogeneous Mixtures

A heterogeneous mixture does not have a definite composition. Cereal in milk is an example of a heterogeneous mixture. Soil is another example. Soil has pebbles, plant matter and sand in it. Although you may add one substance to the other, they will stay separate in the mixture. We say that these heterogeneous mixtures are non-uniform, in other words they are not exactly the same throughout.

Definition: Heterogeneous mixture

A heterogeneous mixture is one that consists of two or more substances. It is non-uniform and the different components of the mixture can be seen.

Heterogeneous mixtures can be further subdivided according to whether it is two liquids mixed, a solid and a liquid or a liquid and a gas or even a gas and a solid. These mixtures are given special names which you can see in table below.

Phases of matter • Name of mixture • Example

liquid-liquid • emulsion • oil in water

solid-liquid • suspension • muddy water

gas-liquid • aerosol • fizzy drinks

gas-solid • smoke • smog

Homogeneous Mixtures

A homogeneous mixture has a definite composition, and specific properties. In a homogeneous mixture, the different parts cannot be seen. A solution of salt dissolved in water is an example of a homogeneous mixture. When the salt dissolves, it spreads evenly through the water so that all parts of the solution are the same, and you can no longer see the salt as being separate from the water. Think also of coffee without milk. The air we breathe is another example of a homogeneous mixture since it is made up of different gases which are in a constant ratio, and which can’t be visually distinguished from each other (i.e. you can’t see the different components).

Did You Know?

An alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, where the resulting material has metallic properties. For example steel is an alloy made up mainly from iron with a small amount of carbon (to make it harder), manganese (to make it strong) and chromium (to prevent rusting).

Definition: Homogeneous mixture

A homogeneous mixture is one that is uniform, and where the different components of the mixture cannot be seen.

Question

For each of the following mixtures state whether it is a homogeneous or a heterogeneous mixture:

A. sugar dissolved in water

B. flour and iron filings (small pieces of iron)

Step 1: Look at the definition

We first look at the definition of a heterogeneous and homogeneous mixture.

Step 2: Decide whether or not you can see the components

a) We cannot see the sugar in the water.

b)We are able to make out the pieces of iron in the flour.

Step 3: Decide whether or not the components are mixed uniformly

a) The two components are mixed uniformly.

b) In this mixture there may be places where there are a lot of iron filings and places where there is more flour, so it is not uniformly mixed.

Step 4: Give the final answer

A. Homogeneous mixture.

B. Heterogeneous mixture.