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Separation techniques Chromatography from BBC
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Mixtures of liquids can be separated according to their properties. The technique used depends on whether the liquids dissolve in each other, and so are miscible, or if they are immiscible. Fractional distillation is a technique used to separate liquids according to their boiling points. Chromatography is used to separate mixtures of coloured compounds.

Separation of liquids

Liquids can be described in two ways – immiscible and miscible. The separation technique used for each liquid depends on the properties of the liquids.

Immiscible liquids

Separating oil and water in a separating funnel

Oil and water can be separated using a funnel

Immiscible means that the liquids don't dissolve in each other – oil and water are an example. It is possible to shake up the liquids and get them to mix but they soon separate. Separating immiscible liquids is done simply using a separating funnel. The two liquids are put into the funnel and are left for a short time to settle out and form two layers. The tap of the funnel is opened and the bottom liquid is allowed to run. The two liquids are now separate.

Miscible liquids

Miscible liquids are harder to separate as they dissolve in each other. Miscible liquids are often separated using fractional distillation. This is possible as miscible liquids have different boiling points.

Fractional distillation of liquid air

You need to be able to explain how nitrogen and oxygen are obtained from the air.

About 78 per cent of the air is nitrogen and 21 per cent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Liquefying the air

Fractional distillation

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say that the air has been liquefied.

Here's what happens as the air liquefies:

  1. Water vapour condenses, and is removed using absorbent filters
  2. Carbon dioxide freezes at –79ºC, and is removed
  3. Oxygen liquefies at –183ºC
  4. Nitrogen liquefies at –196ºC

The liquid nitrogen and oxygen are then separated by fractional distillation.

Chromatography can be used to separate mixtures of coloured compounds. Mixtures that are suitable for separation by chromatography include inks, dyes and colouring agents in food.

Simple chromatography is carried out on paper. A spot of the mixture is placed near the bottom of a piece of chromatography paper and the paper is then placed upright in a suitable solvent, eg water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.


The colours separate and move up the paper at different rates

Rf values

Different chromatograms and the separated components of the mixtures can be identified by calculating the Rf value using the equation:

Rf = distance moved by the compound ÷ distance moved by the solvent

The Rf value of a particular compound is always the same - if the chromatography has been carried out in the same way. This allows industry to use chromatography to identify compounds in mixtures.

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