Mixtures and how to separate them
Chemistry · Key Stage 3 · The School
Mixture or compound
In a MIXTURE the substances are not chemically joined, keep their own properties, and can be separated by physical means. In a COMPOUND the elements are chemically bonded in fixed proportions and the result behaves like neither — sodium is a violent metal, chlorine a poisonous gas, and sodium chloride is table salt. Separating a compound needs a chemical reaction, not a filter.
Method follows property
Each method exploits a difference. FILTRATION separates an insoluble solid from a liquid, by particle size. EVAPORATION recovers a dissolved solid, by boiling point. DISTILLATION recovers the liquid too, by condensing the vapour. CHROMATOGRAPHY separates dissolved substances by how strongly each sticks to the paper. Choose by asking which property differs.
Working out which to use
Sand in water: the sand is insoluble, so filter. Salt in water: dissolved, so evaporate — or distil if you want the water back. Inks in a pen: several dissolved dyes, so chromatography. Naming the property that differs before choosing the apparatus is what turns this from memorising into reasoning.
Sand and water: the sand is insoluble and the particles are big, so FILTER. Salt and water: dissolved, so filtering does nothing — EVAPORATE or crystallise. Water and ethanol: both liquids, different boiling points, so DISTIL. Coloured inks: components with different solubilities, so CHROMATOGRAPHY. Each method exploits one specific difference, so name the difference first and the method follows.
Real mixtures often need a sequence, and the order decides whether it works. Sand, salt and water together: FILTER first and the sand stays in the paper, then EVAPORATE the filtrate and the salt is left behind. Do it the other way and evaporating first leaves salt and sand mixed in the dish, with nothing left to tell them apart by. Ask of each step what it removes, and what it leaves for the next one.