Separation techniques and practical chemistry
Separation techniques and practical chemistry tests how physical differences are used to separate mixtures and how pure substances are identified.
Choosing a method
Separation depends on a physical difference: particle size, solubility, boiling point, density, magnetism or adsorption. Filtration separates an insoluble solid (residue) from a liquid or solution (filtrate) using filter paper in a funnel. Crystallisation recovers a soluble solid: warm to evaporate solvent until crystals just appear, then cool and filter. Do not evaporate hydrated salts to dryness, because heating can drive off water of crystallisation or decompose the salt.
Distillation and chromatography
Simple distillation separates a solvent from a solution; fractional distillation separates miscible liquids with different boiling points using a fractionating column. A separating funnel separates immiscible liquids by density. Chromatography separates coloured or UV-visible substances according to how strongly they adsorb to the stationary phase versus dissolve in the mobile phase. The retardation factor is Rf = distance moved by spot ÷ distance moved by solvent front, measured from the baseline.
Worked example
A chromatogram has a solvent front 8.0 cm from the baseline and a blue spot 3.2 cm from the baseline. Rf = 3.2 ÷ 8.0 = 0.40. If a known blue dye run on the same paper gives 0.40, the spot is likely that dye. The measurement must be from the start line to the centre of the spot, and the solvent front must be marked before it evaporates.
Traps
Rf cannot exceed 1; a value above 1 means the spot or front was measured wrongly. Do not measure from the bottom edge of the paper. A pure substance gives one spot, but one spot does not always prove purity because two substances can have the same Rf. Filtration does not separate dissolved salt from water, and a centrifuge is used for fine precipitates.