Separating mixtures — Chemistry, 7–10 years
A mixture contains different materials together, but they keep their own properties. Those properties can help us separate the mixture again.
A mixture can come apart
When different materials are together without becoming a new material, they form a mixture. The pieces may be visible, or one material may be spread through another. Because the ingredients keep properties such as size, weight or magnetism, we can sometimes take them apart.
Why separate a mixture?
Separating solves a practical problem: we may need one useful material without the others. People separate stones from grain, unwanted pieces from recycling, and solid dirt from water. The best method depends on the difference we can use, such as size, weight, attraction to a magnet or boiling point.
Sand and iron filings
1. Mix a spoonful of sand with a spoonful of iron filings. This is a mixture because both materials are still recognisable. 2. Wrap a magnet in paper. 3. Move it above the mixture; the iron is attracted and sticks to the paper. 4. Lift the paper and shake it over a second dish. The sand stays behind, so the materials are separated.
The strainer trap
A common mistake is to think a strainer separates every mixture. That feels sensible because it catches visible pieces, but a dissolved substance passes through with the liquid. A strainer can remove pasta from water; it cannot remove dissolved salt. For that, another method, such as evaporation, is needed.
Separating at home and outside
A kitchen sieve separates cooked rice or pasta from water, and a magnet can pick up steel objects from a pile. Recycling centres use screens, magnets and other machines to sort materials. These examples work because the materials have different properties; one method cannot separate every kind of mixture.
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