Choosing a separation method
The method used to separate a mixture depends on the properties of its parts, especially solubility and boiling point.
- Filtration separates an insoluble solid from a liquid. The solid trapped on the paper is the residue; the liquid that passes through is the filtrate.
- Crystallisation obtains a soluble solid (e.g. a salt) from its solution by evaporating some water and letting crystals form on cooling.
- Simple distillation separates a solvent (e.g. pure water) from a solution: the liquid boils, the vapour is cooled in a condenser and collected.
- Fractional distillation separates two or more miscible liquids with different boiling points (e.g. ethanol and water) using a fractionating column.
Key idea / 要点
Chromatography separates soluble, coloured substances (e.g. dyes). Each substance is described by its Rf value:
Rf = distance moved by substance ÷ distance moved by solvent. Rf is always between 0 and 1.
Purity
A pure substance has a sharp, fixed melting point and boiling point. An impurity lowers and broadens the melting point, so measuring the melting range is a good test of purity. Colourless spots on a chromatogram can be revealed with a locating agent. Chromatography can also compare an unknown with known reference substances run alongside it: spots that travel the same distance are likely to be the same substance.
Worked example / 例题
On a chromatogram a spot moves 3.0 cm while the solvent front moves 12.0 cm. Rf = 3.0 ÷ 12.0 = 0.25.
Remember / 记住
- Filtration: insoluble solid from liquid.
- Crystallisation: soluble solid from solution.
- Pure substance melts sharply at one temperature.