Mixtures and separation — Chemistry, 11–13
A mixture contains substances together without turning them into one new substance. Because the parts keep their own properties, methods such as filtering, sieving and evaporation can separate them.
Together, but not joined
In a mixture, different substances are together but their particles are not chemically joined into a new substance. Seawater is a mixture of water and dissolved salts; trail mix is a mixture of nuts, fruit and other pieces. Each part still keeps some of its own properties.
Why separate mixtures?
Useful materials are often found mixed with things we do not want. People needed ways to obtain clean drinking water, salt, metals and medicines. The method must match a difference between the parts, such as particle size, boiling point, magnetism or solubility.
Separating sand and salt
Mix sand and salt, then add water. The salt dissolves but the sand does not, so pour the mixture through a filter to catch the sand. Finally, heat the salty water carefully so the water evaporates and the salt remains. You used solubility, filtration and evaporation in order.
A filter is not a magic net
It is easy to expect a filter to remove anything unwanted from a liquid. A filter only catches particles larger than its holes; dissolved salt particles pass through with the water. To recover that salt, you need another difference, such as water evaporating more easily than salt.
Cleaning water
Water-treatment plants use several separation steps because dirty water contains different unwanted materials. Screens catch large rubbish, filters remove smaller particles, and other processes can reduce dissolved substances or kill microbes. No single filter can do every job, so each step uses a different property.
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