C7 Revision 21st Century Science, Chemistry

Chromatography- an analysis method, used to separate substances within a mixture. This can then be used to identify substances individually.

Each type of chromatography has two phases:

Mobile- the molecules can move, liquid or gas

Stationary- the molecules can move, solid

Components within the mixture separate out as the mobile phase moves across the stationary phase.

How quickly  a chemical travels depends on how it distributes itself between the two phases. This is the reason why different chemicals separate out and end up at different points.

Constantly, each chemical will move between the two phases.

The molecules of each chemical move constantly between the two phases.

They reach a dynamic equilibrium, when the amount leaving the stationary phase for the mobile phase, is the same as the amount that is leaving the mobile phase of the stationary phase.

However, this does not necessarily mean that there is the same amount of chemical in each phase.

Remember:

Equilibrium-here the relative (%) quantities of reactants and products will reach a certain balance, and will stay there.

Dynamic equilibrium- this means that the reactions are taking place in both directions. However, the overall affect is nil, and this is because the forward and reverse reactions cancel each other out.

Within paper chromatography, a spot of the substance that is being tested is put onto a baseline of the paper. Then, the bottom of the paper is placed within a beaker which contains a solvent. Here. the stationary phase is the chromatography paper (often filter paper).

Below is what happens:

1) The solvent moves up the paper

2) Chemcials within the sample dissolve in the solvent, and move between it and the paper.