Choosing separation methods
Different methods are used to separate different mixtures. The method chosen depends upon the properties of the components of the mixture.
| Separation method | Suitable for separating: |
| Filtration | An insoluble solid from a liquid |
| Crystallisation | A soluble solid from a solution |
| Simple distillation | A solvent from a solution |
| Fractional distillation | One or more liquids from a mixture of liquids |
| Chromatography | Coloured soluble substances |
| Separation method | Filtration |
|---|---|
| Suitable for separating: | An insoluble solid from a liquid |
| Separation method | Crystallisation |
|---|---|
| Suitable for separating: | A soluble solid from a solution |
| Separation method | Simple distillation |
|---|---|
| Suitable for separating: | A solvent from a solution |
| Separation method | Fractional distillation |
|---|---|
| Suitable for separating: | One or more liquids from a mixture of liquids |
| Separation method | Chromatography |
|---|---|
| Suitable for separating: | Coloured soluble substances |
Using chromatography to check purity
A The results of separating mixtures by chromatography. produced by paper chromatography or thin layer chromatography (TLC) can be used to distinguish between A substance that consists of only one element or only one compound. and A substance that consists of more than one element or compound. substances:
- a pure substance produces one spot on the chromatogram
- an impure substance produces two or more spots
In this chromatogram, brown ink is made of a mixture of red, blue and yellow substances. This is because the coloured substances in the brown ink travel the same distances (and have the same Rf values) as the reference substances.
Gas chromatography
In gas chromatography (GC):
- the Phase in chromatography that moves, usually a solvent or mixture of solvents. is an unreactive gas such as nitrogen or helium
- the Phase in chromatography that does not move, for instance, the paper in chromatography. is a very thin layer of an unreactive liquid
The stationary phase is held on an unreactive solid support such as silica beads. These are packed into a long, coiled ‘column’ inside a A device to switch radiators on and off depending on the temperature. oven. A detector measures the amount of each substance in a mixture as it leaves the column:
- different substances travel at different speeds through the column
- they leave at different times, called The time taken for a sample to pass through a chromatography apparatus.
The GC machine plots a graph of detector reading against retention time. Three main pieces of information can be gathered from a gas chromatogram:
- the number of compounds in the mixture - the number of peaks
- the amount of each compound present - the height of each peak (higher = more)
This gas chromatogram shows that:
- six different substances were in the mixture (there are six peaks)
- A was present in the smallest amount (it has the smallest peak)
- B and C were present in equal amounts
- F was present in the greatest amount (it has the largest peak)