C1bL10 Analysing Chemicals

Key Words

Chemical analysis - looking at the composition of chemicals.

Chromatography - separating chemicals by placing them on paper and dipping one end into a solvent.

Retention Number  - A number that compares the speed at which a chemical is carried up the paper.

Solvent - A substance that dissolves another substance.

Grade E

Chemicals in food can be analysed by use of a technique called chromatography.  You place a sample of the mixture you want to analyse onto a piece of chromatography paper.  You also drop samples of different  known materials.  You then place the end of the paper into a solvent.

You can see that the dyes have moved. 

Grade C

After a period of time, the paper is removed from the solvent and is allowed to dry.  The food dye is a mixture of A and D.  There is no C or D present, so we can say that the chemicals that make up the dye are A and D, but not B or C.

Each spot is a separate chemical.  The spots are all of different heights, and that is characteristic of the chemical.  There is a second green spot that appears in the food dye, but is not A, B, C, or D.  Another analysis is needed to identify it.

Grade A

The retention factor is a comparison between the distance moved by the solvent and the distance moved by the dye.

Retention factor = distance moved by the dye

                                distance moved by the solvent

Retention factor has no units.  It must also be less than 1.  In this case, the solvent has moved 8.5 cm, and the dye in the green spot 6 cm.

Retention factor (Rf) = 6 cm = 0.71

                           8.5 cm

The retention factor is characteristic of the chemical.