General Separation and Purification Techniques

Why mixtures can be separated

A mixture contains substances that are together but not chemically bonded. For example, salty water contains dissolved salt and water. Its components can be separated because they have different physical properties: an insoluble solid can be trapped by a filter, substances can have different solubilities, and liquids can have different boiling points. First decide which substance you want to collect. That choice affects the method and the apparatus.

Filtration: collecting an insoluble solid or a liquid

Filter paper sits in a funnel above a collecting vessel. Pour the mixture carefully into the paper. Liquid and dissolved substances pass through as the filtrate, while an insoluble solid stays on the paper as the residue. For a mixture of sand and salt solution, filtration retains the sand but lets the salt solution through. It cannot remove dissolved salt from water: dissolved particles pass through the paper with the water.

Evaporation and crystallisation: obtaining a dissolved solid

If the wanted substance is a dissolved solid, heat the solution in an evaporating basin so that some solvent escapes as vapour. Evaporation to dryness can leave a solid behind, but it does not collect the solvent. The basin and its contents become hot, so heating must be controlled and the apparatus left to cool before handling.

To grow crystals, heat the solution gently to evaporate some solvent, then stop heating and allow the concentrated solution to cool. As it cools, less solute may remain dissolved, so crystals form. Filter off the crystals and dry them, for example between pieces of filter paper. This is different from heating until every drop has gone: strong heating may spoil some crystals. Filtration may therefore come before crystallisation to remove an insoluble impurity, and afterwards to collect the crystals.

Paper chromatography: separating substances in a solution

Chromatography is useful when several soluble substances are mixed, such as dyes in an ink. Draw a pencil starting line near the bottom of chromatography paper and place a small spot of the mixture on it. Stand the paper in a suitable solvent with the solvent level below the spot, and cover the container. The solvent rises through the paper, carrying substances at different speeds because they differ in how well they dissolve in the solvent and how strongly they are attracted to the paper. Remove the paper before the solvent reaches the top and mark the solvent front. Separate spots show components of the mixture; a single spot does not by itself prove that a substance is pure. Pencil is used because an ink starting line could dissolve and create misleading spots.

Distillation: collecting a liquid

Evaporation loses the solvent to the surroundings. Simple distillation instead collects a liquid from a mixture. Heat the mixture in a distillation flask. A component that vaporises passes through an unobstructed outlet into the inner tube of a condenser. Cold water flows around that tube, cooling the vapour so it condenses. The liquid then runs into an open receiving vessel as the distillate, while less volatile material remains in the flask. For salt solution, water can be collected this way while the salt remains behind. Distillation can also separate liquids with sufficiently different boiling points; the lower-boiling liquid tends to distil first.

The vapour route must remain open from the flask through the condenser to the receiver; a heated distillation apparatus must not be sealed. Choose a heating method appropriate to the mixture: a naked flame is unsuitable for flammable liquids. Wear eye protection and take care with hot glassware. In OCR practical work, these separation techniques may be used in different activities, including preparation of a pure, dry salt; the aim of each step determines which product is kept.

Choose the separation

• Filtration: insoluble solid from liquid; residue stays on paper, filtrate passes through.
• Evaporation: obtain a dissolved solid, but do not collect the solvent.
• Crystallisation: concentrate a solution, cool to form crystals, then filter and dry them.
• Paper chromatography: separate soluble components by their different movement with a solvent; keep the pencil baseline above the solvent.
• Simple distillation: vaporise and condense a liquid to collect the distillate; the less volatile material remains in the flask.

Methods can be combined. For sand in salt solution, filter first; then use crystallisation to obtain the salt or distillation to collect the water.