1.7 describe techniques for the separation of mixtures, including simple distillation, fractional distillation, filtration, crystallisation and paper chromatography.

Simple distillation

This is used to separate a liquid from dissolved solids when we wish to collect the liquid, e.g., pure water from sea water.

Distillation involves boiling the liquid so it turns to vapour and then condensing the vapour back to liquid, which is collected as the distillate.  The dissolved solids remain in the flask.

Fractional distillation

The addition of a fractionating column helps separate liquids with different boiling points, e.g., a mixture of ethanol and water.

The liquid with the lower boiling point evaporates more easily and rises through the fractionating column first.  The thermometer at the top shows the boiling point of the component that is coming through to be condensed in the condenser.  The distillate, known as a fraction, is collected in the flask.

The liquid with the higher boiling point will condense in the fractionating column and drip back into the flask at first.  This will come through later as the temperature rises.

Filtration

Used to separate a liquid and an insoluble solid.  The insoluble solid forms the residue in the filter paper.  The liquid, or a solution containing a dissolved solid, passes through the filter paper and is collected as the filtrate.

An example is separating unreacted copper (II) oxide from a solution of copper (II) sulfate after reaction with sulfuric acid (see section on preparing salts: 4.7)

Crystallisation

An evaporating basin is used to separate a soluble solid from a liquid when we do not wish to collect the liquid. The last part of the evaporation must be carried out slowly to form decent crystals.

Chromatography

Used for analysis. It separates a mixture of coloured dyes or inks which show up as dots on chromatography paper at different heights. The solvent, e.g. water, must be below the sample spot as shown opposite.