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Year 7 & 8 TOC

Separating mixtures


LO: Investigating and using a range of physical separation techniques such as evaporation, distillation and crystallisation

Think about salt water. Salt water is a mixture of salt dissolved into water. Using filtration, flotation or sedimentation won’t work to separate the two. So what can you do? 

Here, we’ll talk about evaporation, distillation and crystallisation. 


When you heat up water, you’ll notice it starts to boil and steam starts coming off it. At this point the water is turning from a liquid to a gas. This is means the liquid is evaporating. Different liquids have different boiling points (this is why it’s important to know the properties of a substance first). 

Water boils at 100°C, while petrol boils at 95°C and olive oil at 300°C. If we wanted to separate these 3 liquids from each other we could boil the mixture at different temperatures. Petrol would evaporate first, changing from a liquid to a gas and moving out of the mixture. 

Chiefs use this a lot in the kitchen. If they want to thicken up a sauce, they let the sauce sit over the stove to evaporate the water in it. 

Coke is a liquid, it’d probably be pretty hard to remove the water from the mixture through any other means, other than evaporation. 

The video shows you what is left after you remove all the water from a can of coke. 


Okay, let’s just say you wanted to remove the water from coke but you wanted to keep the water. How can you do that if it evaporates? 

Distillation is a way to collect the gas produced by evaporation and then cool it back down to a liquid again. The process of cooling down a gas to a liquid is called condensation

As we heat salt water, the water converts into a gas. The gas rises up and is sent through a condenser. As the gas travels it cools down, becoming a liquid again (condensing). At the end we have salt and water in separate beakers.

If we want to separate 2 different liquids from each other, this is called fractional distillation.

Crude oil is a mixture removed from the earth. This mixture is a combination of different liquids and minerals all with different boiling points. 

Crude oil is heated and the petrol in the mixture is evaporated. As the gas rises it is cooled back into a liquid and captured in a new container. 


Crystallisation is a technique which chemists use to purify solid compounds. If we take a glass of water (solvent) and add sugar, salt or alum (crystallising solutes) to it and then heat it up, as it slowly cools, the solutes in the solution slow down and begin to bump into each other. This is called Nucleation. As they bump into each other the solutes molecules, eventually form large crystals. This process is called crystallisation. 

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