Separation methods use differences in physical properties without normally changing substance identities. Filtration separates an insoluble solid from a liquid because solid particles are retained while liquid passes through. Evaporation removes a volatile solvent to leave a non-volatile solute, while simple distillation collects the solvent by boiling and condensation. Dissolved salt is not removed by ordinary filter paper.
Fractional distillation separates miscible liquids with different boiling points using repeated vaporisation and condensation. Chromatography separates components because their interactions with stationary and moving phases differ. A component that spends more time moving with the solvent generally travels further. For a simple planar chromatogram, Rf equals distance travelled by a spot divided by distance travelled by the solvent front, both measured from the starting line.
Example 1
Which process recovers liquid water from salt water?
Simple distillation: water vaporises and condenses into a separate container, while non-volatile salt remains.
Example 2
A chromatographic spot travels 3.0cm and the solvent front 6.0cm. Calculate Rf.
Rf=6.03.0=0.50. It has no unit because the two distances use the same unit.
Watch out:Dissolved ions pass through ordinary filter paper; recover water by distillation or salt by solvent evaporation.
Separation rules and physical properties practice | Year 11 Chemistry | Ascent Arcade