Methods of Separation

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Separating Mixtures

• Mixtures can be separated by any means that makes use of the different physical properties of its constituents.
• The properties of the components of a mixture determine the best separation method.

1. Sieving
2. Filtration (diagram)
3. Decanting
4. Use of separation funnel (diagram)
5. Distillation (diagram)
6. Fractional distillation (basic diagram)
7. Electrostatic attraction
8. Magnetic susceptibility
9. Vaporisation
10. Crystallisation

Sieving

• A porous material is used to separate particles of different sizes
• Method is most common used to affect gross separations, e.g. liquids from suspended crystals or other solids
• To accelerate sieving, pressure is usually applied
• A series of sieves is stacked, with the screen of largest hole size at the top

Filtration

• Used to separate heterogenous mixtures composed of solids and liquids

• Uses a porous barrier to separate the solid from the liquid

• Liquid passes through, leaving the solid in the filter paper

• Filtration can be used to separate an insoluble substance from a soluble substance

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Separating Funnel

• Used to separate substances of different densities, i.e. immiscible liquids/substances
• Put into a separating funnel, which is then shook
• The materials will separate due to their different densities, with the most dense at the bottom
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Decantation

• Done to separate particulates form a liquid by allowing the solids to settle to the bottom of the mixture and pouring off the particle free part of the liquid
• Allows 2 immiscible liquids to separate and the lighter liquid is poured off

Distillation

• Used to separate homogenous mixtures
• Based on difference in boiling points of substances involved
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Evaporation

• Solution is heated, and becomes more concentrated
• Eventually, all of the solvent will disappear
• The solute will crystallise at the bottom
• Evaporation can be used to separate a solute from the solvent in a solution

Crystallisation

• Separation technique that results in the formation of pure solid particles from a solution containing the dissolved substance
• As one substance evaporates, the dissolved substance comes out of solution and collects as crystals
• Produces highly pure solids
• Rocky candy is an example of this

Centrifuging

• Centrifuges rotate containers of liquids to separate suspended materials with different densities.
  • The lighter materials move to the outside, as they are more affected by the speed
• Centrifuges separate different components of human blood or milk and to clarify solutions. A high speed separator can rotate at great speed to separate fat (cream) from milk.
• The spin drier in washing machines is a type of centrifuge that throws out the liquid by the “centrifugal force” of the rotation.

Magnetic Separation, i.e. a Magnet

• Can be used to separate a magnetic substance from a non-magnetic substance

Chromatography

• Separates components of a mixture based on ability of each component to be drawn across the surface of another material
• Mixture is usually liquid and is usually drawn across chromatography paper
• Separation occurs because various components travel at different rates
• Components with strongest attraction for paper travel the slowest
• Chromatography (colour writing) is used to separate small amounts of chemicals so that they can be analysed.
• Different substances or different components move at different speeds through a strip of wet paper, a gel or a gas.