Separating mixtures · Grade 4
The substances in a mixture are not joined, so they can be separated by physical methods. Each method uses a difference between the substances: the size of the pieces (sieving), magnetism (a magnet), whether they dissolve (filtering), or their boiling points (distillation). Chromatography separates the dyes in an ink.
Key facts
- Sieving separates pieces of different sizes; a magnet pulls out iron and steel.
- Filtration separates an insoluble solid from a liquid: the solid left in the filter paper is the residue; the liquid that passes through is the filtrate.
- A solute dissolves in a solvent to make a solution. Evaporation removes the solvent and leaves the dissolved solid behind.
- Crystallisation: heat the solution to evaporate some of the water, then leave it to cool so crystals form. It gives pure crystals without overheating the salt.
- Simple distillation gets a solvent from a solution: the liquid boils, the vapour cools in the condenser and is collected as the distillate.
- Fractional distillation separates liquids with different boiling points: the liquid with the lowest boiling point is collected first.
- Paper chromatography separates dyes: each dye travels a different distance up the paper. A pure substance gives one spot and melts at one sharp temperature.