Separation Techniques

Distillation
METHODS of SEPARATING MIXTURES and purifying substances
Simple Distillation

Distillation involves 2 stages and both are physical state changes.

(1) The liquid or solution mixture is boiled to vaporise the most volatile component in the mixture (liquid ==> gas). The ant-bumping granules give a smoother boiling action.

(2) The vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected.

This can be used to purify water because the dissolved solids have a much higher boiling point and will not evaporate with the steam, BUT it is too simple a method to separate a mixture of liquids especially if the boiling points are relatively close.


Fractional Distillation
Fractional Distillation
Fractional distillation involves 2 main stages and both are physical state changes. It can only work with liquids with different boiling points. However, this method only works if all the liquids in the mixture are miscible (e.g. alcohol/water, crude oil etc.) and do NOT separate out into layers like oil/water.

(1) The liquid or solution mixture is boiled to vaporise the most volatile component in the mixture (liquid ==>gas). The ant-bumping granules give a smoother boiling action.

(2) The vapour passes up through a fractionating column, where the separation takes place (theory at the end). This column is not used in the simple distillation described above.

(3) The vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected.

This can be used to separate alcohol from a fermented sugar solution.

It is used on a large scale to separate the components of crude oil, because the different hydrocarbons have different boiling and condensation points (see oil).

FRACTIONAL DISTILLATION THEORY:

Imagine green liquid is a mixture of a blue liquid (boiling point 80oC) and a yellow liquid (boiling point 100oC), so we have a coloured diagram simulation of a colourless alcohol and water mixture! As the vapour from the boiling mixture enters the fractionating column it begins to cool and condense. The highest boiling or least volatile liquid tends to condense more i.e. the yellow liquid (water). The lower boiling more volatile blue liquid gets further up the column. Gradually up the column the blue and yellow separate from each other so that yellow condenses back into the flask and pure blue distils over to be collected. The 1st liquid, the lowest boiling point, is called the 1st fraction and each liquid distils over when the top of the column reaches its particular boiling point to give the 2nd, 3rd fraction etc.

To increase the separation efficiency of the tall fractionating column, it is usually packed with glass beads, short glass tubes or glass rings etc. which greatly increase the surface area for evaporation and condensation.

In the distillation of crude oil the different fractions are condensed out at different points in a huge fractionating column. At the top are the very low boiling fuel gases like butane and at the bottom are the high boiling big molecules of waxes and tar.


Chromatography
Paper or Thin Layer Chromatography
This method of separation is used to see what coloured materials make up e.g. a food dye analysis.

The material to be separated e.g. a food dye (6) is dissolved in a solvent and carefully spotted onto chromatography paper or a thin layer of a white mineral material on a glass sheet. Alongside it are spotted known colours on a 'start line' (1-5).

The paper is carefully dipped into a solvent, which is absorbed into the paper and rises up it. The solvent may be water or an organic liquid like an alcohol (e.g. ethanol) or a hydrocarbon, so-called non-aqueous solvents. For accurate work the distance moved by the solent is marked on carefully with a pencil and the distances moved by each 'centre' of the coloured spots is also measured. These can be compared with known substances BUT if so, the identical paper and solvent must be used (See Rf values below).

Due to different solubilities and different molecular 'adhesion' some colours move more than others up the paper, so effecting the separation of the different coloured molecules.

Any colour which horizontally matches another is likely to be the same molecule i.e. red (1 and 6), brown (3 and 6) and blue (4 and 6) match, showing these three are all in the food dye (6).

The distance a substance moves, compared to the distance the solvent front moves (top of grey area on 2nd diagram) is called the reference or Rf valueand has a value of 0.0 (not moved - no good), to 1.0 (too soluble - no good either), but Rf ratio values between 0.1 and 0.9 can be useful for analysis and identification.

Rf = distance moved by dissolved substance (solute) / distance moved by solvent.

Some technical terms: The substances (solutes) to be analysed must dissolve in the solvent, which is called the mobile phase because it moves. The paper or thin layer of material on which the separation takes place is called the stationary or immobile phase because it doesn't move.

It is possible to analyse colourless mixture if the components can be made coloured e.g. protein can be broken down into amino acids and coloured purple by a chemical reagent called Ninhydrin and many colourless organic molecules fluoresce when ultra-violet light is shone on them. These are called locating agents.

Thin layer chromatograpy (t.l.c) is where a layer of paste is thinly and evenly spread on e.g. a glass plate. The paste consists of the solid immobile phase like aluminium oxide dispersesd in a liquid such as water. The plate is allowed to dry and then used in the same way as paper chromatography.


Crystallization


Filtration

Go to this link to view seperation diagrams:
http://www.gcsescience.com/e4-mixture-separation.htm
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