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
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 is everywhere and comes in lots of different types. These include:
- 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.
'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.
|Mixture of elements||Helium and oxygen|
|Mixture of compounds||Orange juice|
|Mixture of elements and compounds||Air|
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
There are lots of ways to try out your science skills.
There's more to learn
Have a look at these other resources around the BBC and the web.