Separation & Purification Techniques (Cambridge (CIE) O Level Chemistry): Revision Note

Filtration & Crystallisation

• The choice of the method of separation depends on the nature of the substances being separated

• All methods rely on there being a difference of some sort, usually in a physical property such as boiling point between the substances being separated

Separating a mixture of solids

• Differences in solubility can be used to separate solids

• For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities, e.g. to separate a mixture of sand and salt, water is a suitable solvent to dissolve the salt, but not the sand

 Filtration

• Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water). Centrifugation can also be used for this mixture

• Filter paper is placed in a filter funnel above another beaker

• The mixture of insoluble solid and liquid is poured into the filter funnel

• Filter paper will only allow small liquid particles to pass through in the filtrate

• Solid particles are too large to pass through the filter paper so will stay behind as a residue

Filtration of a mixture of sand and water

 Crystallisation

• Used to separate a dissolved solid from a solution, when the solid is more soluble in hot solvent than in cold (e.g. copper sulphate from a solution of copper (II) sulphate in water)

• The solution is heated, allowing the solvent to evaporate and leaving a saturated solution behind

• You can test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution

  • If the solution is saturated, crystals will form on the glass rod when it is removed and allowed to cool

• The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow

• Crystals are collected by filtering the solution

• They are then washed with distilled water to remove any impurities and then allowed to dry

Diagram showing the process of crystallisation

Distillation: Simple & Fractional

Simple Distillation

• Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of saltwater) or a pure liquid from a mixture of liquids

• The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask

• The vapour passes through the condenser, where it cools and condenses, turning into pure water which is collected in a beaker

• After all the water is evaporated from the solution, only the solid solute will be left behind

Diagram showing the distillation of a mixture of salt and water

• Simple distillation can be used to separate the products of fermentation, such as alcohol and water

• However, more effective separation is to use fractional distillation where the liquids are closer to boiling point and a higher degree of purity is required

Fractional distillation

• Used to separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two)

• The solution is heated to the temperature of the substance with the lowest boiling point

• This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker

• All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture

• For water and ethanol: ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker

• When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated

Fractional distillation of a mixture of ethanol and water

• An electric heater is safer to use when there are flammable liquids present

• The separation of the components in petroleum is achieved by fractional distillation on an industrial scale

• Fractional distillation of crude oil is not carried out in school laboratories due to the toxic nature of some of the components of the crude oil, but it can sometimes be simulated using a synthetic crude oil made specially for the demonstration

Assessing Purity

• Pure substances melt and boil at specific and sharp temperatures. Eg water has a boiling point of 100°C and a melting point of 0°C

• Mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures

• Melting and boiling point data can therefore be used to distinguish pure substances from mixtures

• An unknown pure substance can be identified by experimentally determining its melting point and boiling point and comparing them to literature values / data tables

  • Boiling points are commonly determined by distillation

• Melting point analysis is routinely used to assess the purity of drugs for example

• This is done using a melting point apparatus which allows you to slowly heat up a small amount of the sample, making it easier to observe the exact melting point

Melting point test using an oil bath  

• This is then compared to data tables

• The closer the measured value is to the actual melting or boiling point then the purer the sample is

• If the sample contains impurities:

  • The boiling point may appear higher than the sample's actual boiling point

  • The melting point may appear lower than the sample's actual boiling point