Separation and purification

Using a suitable solvent

This separates a soluble solid from an insoluble one. Sand and salt is the standard pair.

Add water and stir. The salt dissolves and the sand does not, because sand is insoluble in water. The mixture is then filtered, and the two components leave by different routes.

The choice of solvent is the whole art. It must dissolve the substance you want to move and leave the other behind, so a question that gives solubility data is telling you which solvent to pick.

Filtration

This separates an insoluble solid from a liquid.

Apparatus: a filter funnel with a folded filter paper, sitting in a conical flask or beaker.

It works because the pores in the filter paper are large enough for the liquid molecules and any dissolved particles to pass through, but too small for the particles of the insoluble solid.

• The solid caught on the paper is the residue.
• The liquid that passes through is the filtrate.

With a sand and salt mixture stirred into water, sand is the residue and salt solution is the filtrate. Washing the residue with a little distilled water rinses off the last of the dissolved salt.

Crystallisation

This obtains a pure solid solute from its solution, and it is the method used when the solid would decompose or lose its water of crystallisation if the solution were simply boiled dry.

1. Heat the solution in an evaporating basin to drive off some of the solvent, until the solution is saturated. Test for saturation by dipping a cold glass rod in: if crystals form on the rod as it cools, the solution is ready.
2. Stop heating and leave the solution to cool slowly. Solubility falls as the temperature falls, so the solute can no longer stay dissolved and crystals form.
3. Filter to collect the crystals.
4. Wash them with a little cold distilled water, which removes the solution clinging to them without dissolving much of the crystal.
5. Dry them between filter papers, or in a warm oven.

Two details carry marks. Cool slowly, because slow cooling gives larger and purer crystals. Do not evaporate to dryness, because every impurity dissolved in the solution would then be left behind with the product, and a hydrated salt such as copper(II) sulfate would lose its blue water of crystallisation and turn white.

Simple distillation

This separates the solvent from a solution, giving pure liquid and leaving the dissolved solid behind. Pure water from seawater is the standard example.

The apparatus, in order:

• A flask holding the solution, with anti-bumping granules to keep the boiling smooth.
• A thermometer with its bulb level with the side-arm, so it reads the temperature of the vapour leaving, not the liquid boiling.
• A Liebig condenser, a tube inside a water jacket. Cold water enters at the lower end and leaves at the top, so the jacket stays full and the coldest water meets the coldest vapour.
• A beaker or receiving flask for the distillate.

The solution is heated, the solvent boils and its vapour travels into the condenser, where it is cooled and condenses back to a liquid that runs into the receiver. The dissolved solid stays in the flask because its boiling point is far higher, so it never enters the vapour.

Fractional distillation

This separates two or more miscible liquids with different boiling points.

The apparatus is the same as for simple distillation with one addition: a fractionating column, packed with glass beads or rings, standing between the flask and the condenser.

The column is hottest at the bottom and coolest at the top. Vapour rising through it repeatedly condenses on the beads and evaporates again, and each of those cycles enriches the vapour in the component with the lower boiling point. By the time the vapour reaches the top it is almost entirely that component.

Worked example. A fermentation mixture contains ethanol and water. Ethanol boils at 78 °C and water at 100 °C.

Heat the mixture. The thermometer rises and steadies at 78 °C while ethanol distils over and is collected. When almost all the ethanol has gone, the reading climbs sharply towards 100 °C, which is the signal to change the receiver, because water is now coming across.

The steady thermometer reading is what tells you which liquid is arriving. That is exactly the same principle that separates crude oil into fractions in 11.3, and liquid air into nitrogen and oxygen.

Choosing the right technique

Read the question for two things: which component you are asked to obtain, and whether the other one is soluble. Crystallisation and simple distillation act on the same mixture and keep opposite halves of it, so choosing between them depends entirely on which one the question wants.

Common mistakes

• Swapping residue and filtrate.
• Saying filtration separates a dissolved solid from its solution. Dissolved particles pass straight through the paper.
• Evaporating a solution to dryness when crystals are wanted.
• Cooling a saturated solution quickly, which gives small, impure crystals.
• Washing crystals with warm water, which dissolves them.
• Placing the thermometer bulb in the boiling liquid instead of level with the side-arm.
• Sending the condenser water in at the top.
• Using simple distillation for two miscible liquids, which does not separate them properly.

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