Pure and impure substances

Home learning focus

Learn about pure and impure substances, and different ways to remove impurities.

This lesson includes:

  • a learning summary
  • one video
  • one activity

Learn

When you see food and drink labelled as 'pure' on the shop shelves, have you ever wondered what this actually means? Well, if we take pure orange juice as an example, that means it contains the juice of oranges only, with nothing else added to it.

However, when scientists talk about pure and impure substances, they are referring to whether a substance contains all of the same atoms or molecules or not.

So that orange juice isn't actually pure! Orange juice is a mixture of different compounds, including citric acid, vitamin C and sugars.

Water

Water is everywhere and comes in lots of different types. These include:

  • rainwater
  • seawater
  • mineral water
  • sparkling water

They all contain the water molecule H₂O, but they are all impure mixtures.

This is because water is rally good at dissolving things, such as minerals and gases. These dissolved substances are called impurities.

Sparkling water, for example, is water that has carbon dioxide dissolved in it. This is what makes the water fizzy.

In fact, it is extremely hard to find completely pure liquid water anywhere in nature.

Gas dissolves into a liquid via diffusion.

Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

Label showing the different substances dissolved in mineral water.

This video from BBC Teach demonstrates how we can discover the different minerals dissolved in water.

Different minerals can be found in water

Mixtures

'Mixture' is just another word for any impure substance. A mixture contains two or more substances that are not chemically joined together.

This table shows some of the different examples of mixtures.

DescriptionExample
Mixture of elementsHelium and oxygen
Mixture of compoundsOrange juice
Mixture of elements and compoundsAir

Removing impurities

Impurities can be removed from a substance using a number of different separation techniques:

  • evaporation - used to removed dissolved impurities from solution by removing the liquid
  • distillation - used to separate out liquids from solution
  • chromatography - the method used to separate out different coloured dyes
  • filtration - used to remove undissolved impurities from solution
  • magnetism - used to remove magnetic substances from a mixture

Practise

There are lots of ways to try out your science skills.

Activity 1

Mixtures crossword

Practise your key terms using with this downloadable crossword from Beyond.

Pure and impure crossword

There's more to learn

Have a look at these other resources around the BBC and the web.

KS3 Chemistry
11-14 Chemistry
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