Chemistry 2.7 - 2.8 - Separating mixtures

Properties guide the choice

A mixture contains two or more substances that are together but not chemically bonded
to one another. Separation is therefore a physical change: no new substance has to be
made. The particles of each component keep their chemical identity, but a difference in
physical property lets apparatus send the components to different places.

Before selecting apparatus, answer three questions:

1. What must be collected? A solid, a liquid, or separated samples of several
   substances?
2. What kind of mixture is it? Look at states, solubility and whether liquids mix.
3. Which physical property differs? Useful differences here are insolubility, solubility,
   boiling point and movement through chromatography paper with a chosen solvent.

The target matters. From sodium chloride solution, crystallisation is chosen to collect
sodium chloride crystals, whereas simple distillation is chosen to collect liquid water.
The mixture has not changed, but the required product has.

Worked selection example

A cloudy sample contains sand, sodium chloride and water. Sand is insoluble; sodium
chloride is dissolved.

1. Filter first, using the sand's insolubility. Sand is retained while sodium chloride
   solution passes through.
2. Decide what is wanted from the filtrate. Crystallise it to collect sodium chloride, or
   distil it to collect water.

One technique could not recover every component because two different property differences
must be used.

Filtration traps an insoluble solid

Filtration separates an insoluble solid from a liquid. A folded filter paper sits inside
a filter funnel above a conical flask or beaker. The mixture is poured into the paper slowly
so it does not rise above the paper's edge.

Two named products leave the process:

• the residue is the insoluble solid retained on the filter paper;
• the filtrate is the liquid and any dissolved substances that pass through.

At particle level, a solution is not a suspension of visible solid pieces. Its dissolved
particles are dispersed among the solvent particles, so the solution passes through the
paper. Insoluble solid particles large enough to be retained collect as residue. This is why
filtering sodium chloride solution does not remove the dissolved sodium chloride.

A workable method

1. Fold a correctly sized filter paper into a cone, place it in the funnel and dampen it so
   it sits against the funnel.
2. Support the funnel over a labelled clean container.
3. Swirl the mixture so the solid is transferred, then pour slowly down a glass rod. Keep
   the liquid below the paper's rim.
4. Allow the filtrate to drain. If the solid is wanted, rinse it with a small amount of a
   liquid in which it is insoluble, then dry it.

There is no independent and dependent variable in this separation procedure: it makes a
product rather than testing how one quantity affects another. The operational controls are
still important. Use intact, correctly fitted paper; transfer the whole mixture; use a clean
receiver; and allow enough time for drainage.

If solid appears in the filtrate, the paper may be torn, badly fitted or overfilled. Repeating
the filtration with intact paper can improve the separation. If the required residue is
partly soluble in the wash liquid, excessive washing would reduce the amount recovered.

Crystallisation recovers a soluble solid

Crystallisation is used when a dissolved solid is wanted from a solution. It is not
filtration: there are no insoluble solid particles to trap at the start. First the solution
is concentrated by evaporating some solvent; then it is cooled so crystals can form.

For many solid solutes, less solute can remain dissolved at a lower temperature. As the
concentrated solution cools, excess solute particles leave the solution and arrange into a
regular solid crystal. The solute does not evaporate and return as crystals: it remains in
the dish while some solvent escapes.

Worked method: copper sulfate crystals from solution

Copper sulfate becomes substantially less soluble as water cools, so it is a useful example of cooling crystallisation. Sodium chloride changes solubility much less with temperature and is usually recovered mainly by evaporation.

1. Pour the solution into an evaporating basin and place it on a heatproof mat or suitable
   supported heating arrangement.
2. Heat gently to evaporate some water. Stop when crystals begin to appear at the edge,
   or when a drop placed on a cool glass rod forms crystals.
3. Leave the concentrated solution to cool undisturbed. More crystals form as the solution
   cools.
4. Filter off the crystals, rinse them with a very small amount of cold solvent if suitable,
   and dry them between clean filter papers or in another appropriate way.

The endpoint is not a fixed time: it is the observation that the solution has become
sufficiently concentrated. Heating to dryness is poor practice because concentrated
solutions can spit, some solutes can decompose, and it does not allow controlled crystal
growth. Wear eye protection, heat gently, keep faces away from the basin and allow hot
apparatus to cool before handling it.

Crystallisation normally leaves some solute dissolved in the remaining liquid, so recovery
is not complete. Further evaporation could recover more solid, but over-heating increases
spitting and decomposition risk. A specific improvement is to evaporate only to the first
sign of crystallisation, then cool fully before filtering: this increases crystal formation
without boiling the basin dry.

Distillation collects liquids

Distillation uses a difference in boiling point to separate and collect liquids. It has
two physical state changes:

Liquid is heated to form vapour; cooling the vapour produces liquid distillate.

In simple distillation, the mixture is heated in a flask. Vapour rich in the more
volatile, lower-boiling component leaves the flask, enters a condenser, loses energy and
condenses. The receiver collects this liquid as the distillate. A non-volatile dissolved
solid remains in the heated flask.