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Year 7 & 8 TOC

Separating mixtures

Filtration and centrifuging

LO: Investigating and using a range of physical separation techniques such as filtration and centrifuging

Sometimes waiting or trying to decant a mixture isn’t enough. You may need to separate the particles through different methods. Filtering and centrifuge are two good method for separating large particles from small particles.

What are we learning?

In this section you’ll learn: 

  • What filtration is and how we use it
  • What centrifuging is and how we it


Have you ever cleaned rice or veggies in a colander? Notice how the rice or veggies don’t fall through the holes and only the water and dirt do.

Filters control what pass through by using different sized holes. In the case above water and the dirt and bacteria it collects can pass through the holes. Even the grate on the storm drain is an example of a filter.  Even a fly screen is an example of a filter as the holes are large enough to allow air flow in but not so large that they allow insects in.
The thing which passes through a filter is called a filtrate, while the things which get caught are called residue.
You may have heard of filter paper before. This is a very common type of paper which has holes too small to see with the naked eye. Different types of filter paper have different sized holes because they are trying to filter different things. Usually the solution can flow through the filter paper, but most solids get caught.
Sometimes filters can remove substances using chemicals instead of physically stopping them. Gas masks are a great example of this containing charcoal which attracts and holds some poisonous gases.


Have you ever been on the spinning wheel at the playground? When you’re on it and you’re spinning you start to feel yourself being pulled towards the outside. This is a perfect example of how centrifuges work. As the machine spins, heavy objects are pulled more than light objects. We can use the same principle to separate substances from mixtures. 


In a centrifuge we rotate the substances and they experience centrifugal force. This pushes the heavier substances from the centre to the walls of the tube. We can alter the amount of centrifugal force by changing the mass of the particles or the speed of rotation.

I.e. the faster we spin and the bigger the particles the greater the centrifugal force.

We use centrifuging for a number of different things, including separating cream from milk, washing machines and analysing blood.

ANalysing blood

Your blood isn’t just made up on one substance, it’s also a mixture. You blood contains red blood cells, white blood cells, water and nutrients. If we want to separate this mixture we need a centrifuge. 

As the blood spins, the red blood cells move to the bottom of the vile, the platelets above them and then the white blood cells and plasma at the very top. 

This is really helpful to medical research teams and doctors as different parts of the blood can be used for different purposes. 

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