1.10 Separation Techniques

Prior learning: To fully understand this material you will need to have learnt about the different states of matter and the changes of state, solubility and solutions.

The appropriate method to use for separating the components in a mixture depends on the physical states of the things that are being separated. To summarise:

Technique What it does:
Filtration Separates an insoluble solid from a liquid or solution
Crystallisation Separates a solute (solid that has been dissolved) from a solution
Simple distillation Separates a liquid (the solvent) from the solution it is part of
Fractional distillation Separates the liquids in a mixture of liquids, based on their boiling points
Paper chromatography Separates dissolved dyes/pigments based on their different solubilities

We also need to understand a little about how each of these techniques works:

Filtration

The insoluble solid particles can’t pass through the tiny gaps in the filter paper, but the liquid particles can. As a result the solid (called the residue) remains in the filter paper, separated from the remaining liquid or solution which has passed through the filter paper. This liquid is termed the filtrate.

filtration diag

Crystallisation

crystallisation_diag

  • The solution is heated until enough of the solvent (water) has evaporated to make the solution saturated.
  • We can tell when the solution is saturated by dipping a glass rod into the solution to remove a drop and seeing if the drop goes cloudy and crystals start to form as it cools.
  • Once the solution is saturated, the Bunsen is turned off and the solution is allowed to cool – solubility decreases with temperature, and the solute that can’t remain dissolved forms crystals.
  • The crystals are removed from the remaining solution by filtering.
  • The crystals are dried (in a warm oven, or just left to dry).

Watch what happens once a saturated solution is allowed to cool.

Simple Distillation

The solution is heated in the flask. The solvent boils, becoming a vapour, which travels to the condenser. Here it is cooled and condenses, collecting as a pure liquid called the distillate. What remains in the flask is the same mixture, but containing less solvent – a more concentrated solution.

SimDist_diag

Fractional distillation

This technique works in a very similar way to simple distillation, but the idea of the fractionating column is to get a temperature gradient, cooler at the top and hotter at the bottom. This allows careful control of the temperature at which the distillate is being collected, allowing different liquids in a mixture to be separated, each turning into a vapour and being cooled and condensed in the condenser at their own individual boiling points.

FractDist_diag

Paper Chromatography

Mixtures of substances dissolved in a solvent, such as the dyes in ink or the food colourings used in sweets and other food products, can be separated to reveal which different dyes or additives have been used in the mixture.

chroma_paper

The process for doing this is paper chromatography.  It separates the components in the mixture based on differences in their solubility: the more soluble each component, the further it travels towards the top of the paper in the chromatography experiment. Insoluble components won’t move at all, remaining on the baseline (shown above as the pencil line with A, B and C marked along it).  The ink labelled B can be seen to be made from a blue, a pink and a dark purple pigment. The blue is the most soluble, and the purple least soluble.