Separating mixtures
Chemistry · Key Stage 2 · The School
Match the method to the mixture
SIEVING separates by particle size — stones from sand. FILTERING separates an insoluble solid from a liquid — sand from water, using paper with holes too small for the sand. EVAPORATING recovers a DISSOLVED solid — salt from salt water — because the water leaves as vapour and the salt cannot.
Why filtering fails on salt water
This is the one people get wrong. Filter paper stops particles; dissolved salt has broken into pieces far smaller than the holes, so it goes straight through with the water. If a filter does not work, the substance is dissolved, not suspended — and evaporation is the answer.
Getting the water back too
Evaporating loses the water. If you want both, DISTIL: boil the solution, catch the steam, and cool it back to liquid in a separate container. That is how fresh water is made from sea water, and how it was worked out that the salt stays behind.
A beaker holds sand, salt and water together. First FILTER it: the sand stays in the paper, and salty water goes through — because the sand grains are bigger than the holes and the dissolved salt is not. Then EVAPORATE the filtrate gently: the water leaves and salt crystals are left in the dish. Three components, two methods, and the order matters — evaporating first would leave sand and salt mixed together.