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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 1: 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 250 ml of water and you add sand to the water. It doesn't matter whether you add 20 g, 40 g, 100 g or any other 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.

Cereal

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Figure 1: A submicroscopic representation of a heterogeneous mixture. The gray circles are one substance (e.g. one cereal) and the white circles are another substance (e.g. another cereal). The background is the milk.
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Definition 2: 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.

Table 1: Examples of different heterogeneous mixtures

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).

The dissolving process

Coffee

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Salt dissolving in water

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Interesting Fact:

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 3: Homogeneous mixture

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

Example 1: Mixtures

Question

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

  1. sugar dissolved in water

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

Answer

Look at the definition

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

Decide whether or not you can see the components
  1. We cannot see the sugar in the water.

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

Decide whether or not the components are mixed uniformly
  1. The two components are mixed uniformly.

  2. 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.

Give the final answer
  1. Homogeneous mixture.

  2. Heterogeneous mixture.

Activity 1: Making mixtures

Make mixtures of sand and water, potassium dichromate and water, iodine and ethanol, iodine and water. Classify these as heterogeneous or homogeneous. Give reasons for your choice.

Make your own mixtures by choosing any two substances from

  • sand

  • water

  • stones

  • cereal

  • salt

  • sugar

Try to make as many different mixtures as possible. Classify each mixture and give a reason for your choice.

Figure 2: Potassium dichromate (top) and iodine (bottom)
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Exercise 1: Mixtures

Complete the following table:

Table 2

Substance

Non-mixture or mixture

Heterogeneous mixture

Homogeneous mixture

tap water

     

brass (an alloy of copper and zinc)

     

concrete

     

aluminium foil (tinfoil)

     

Coca Cola

     

soapy water

     

black tea

     

sugar water

     

baby milk formula

     
SubstanceNon-mixture or mixtureHeterogeneous mixtureHomogeneous mixture
tap water Mixture no yes
brass (an alloy of copper and zinc) mixture no yes
concrete mixture no yes
aluminium foil (tinfoil) non-mixture not applicable not applicable
Coca Cola mixture yes no
soapy water mixture yes no
black tea mixture no yes
sugar water mixture no yes
baby milk formula mixture no yes