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Year 7 & 8 TOC

Separating mixtures

Separation techniques

LO: Investigating separation techniques such as flotation, decantation, sedimentation and magnetic seperation 

Since mixtures are a combination of different types of substances there are different ways we can separate them back into their original substances. The technique we use to separate them depends properties of the substances. 

In this section you'll learn

  • What are physical and chemical properties
  • How to separate mixtures?
  • Separation techniques: 
    • Decanting
    • Sedimentation
    • Flotation
    • Magnetism & Electrostatics 

Physical & Chemical properties

Separating mixtures

There are many different methods of separation and depending on the properties of the substance different methods will be more or less effective. Let ‘s just say I give you a bag of M&Ms and ask you to sort them out into different colours. You won’t have to do anything fancy or complex to sort them out. 

While it might take a while, you can just pick out the different coloured M&Ms and place them in different piles. But what if the M&Ms where different sizes? and some of them were so small you couldn’t pick them up or even see them? You would need to try a different separating technique. 

What is density?

Before we start, you need to know about density.

One thing you will notice when you mix different substances, especially in liquids is that, some sink while others float? 

Floating or sinking has to do with density. Something’s density relates to how tightly packed it’s molecules are. If the substances is less dense than water, than it will float in the water, but if it is denser than water, it will sink in water. 

You can see in the image, how the different densities of the liquids allow them to stay separated. 


Drag the properties into their correct category. Are the a physical or chemical property? 

Separation techniques


A process where you carefully pour out the liquid out and leave any solids or sediments in the container.

A lot of people will use decanting to remove sediments from wine. You may have done this when you pour water out of a pot when making spaghetti.


As dense substances settle at the bottom of a  mixture they form sediments. 

  • Sedimentation is the process of using gravity to separate the sediments.
  • Sometimes substances aren’t dense enough to fall on their own, in these instances we use chemicals called flocculants to clump the suspended particles together, allowing them to sink.


Flotation is the opposite of sedimentation, instead of relying on things falling to the bottom, flotation relies on substances floating to the top. Once the substance is a the top, we can scoop it out of the mixture.

Sometimes, to help something float we need to increase it’s buoyancy, making it take up more space without becoming denser. We can do this by forming small air bubbles around it which attach to the solids making them rise to the surface. This technique is called DAF (dissolved air flotation).

Magnetism and Electrostatics

Magnetic separation uses magnets to attract some metals from other non magnetic materials. Anything made up of iron will be attracted to the magnet, making it great for separating iron and other magnetic metals from plastics, glass aluminium and paper. Electrostatics work much in the same way.

Electrostatics work in a similar way to magnets. They attract specific particles. They are used in cleaning to attract lint and fluff as well as photocopiers to attract toner to the paper.

In rubbish dumps, we use strong magnets to separate things containing iron from plastics and glass. This makes it easier to recycle materials.

Key points

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