The only technique here that separates several substances at once — and it works on a difference of degree, not of kind.
The property being used
Every other technique in this chapter runs on a yes-or-no difference: magnetic or not, dissolved or not, boils or does not. Chromatography is different. It separates substances that all dissolve — just to different extents.
The rule that explains every chromatogram
Substances that are more soluble in the solvent travel longer distances from the starting point.
Substances that are insoluble in the solvent remain at the starting point.
Many inks and dyes are mixtures of coloured substances, so paper chromatography is used to separate them.
Separating the substances in ink
Four substances went in as one black spot. The grey one never leaves the start line — that is your evidence it is insoluble in water.
How the experiment is set up1. A drop of ink is placed near the bottom of a strip of filter paper, on the start line. The strip is dipped into water, which is the solvent. 2. Water travels up the paper, carrying the substances in the ink with it. They separate because they are carried different distances. 3. The paper with the different coloured spots is called a chromatogram.
Chromatography is used to separate small amounts of substances that dissolve to different extents in a particular solvent.
Reading a chromatogram in the exam
You will be shown spots at different heights and asked which substance is most soluble. The answer is always the one furthest from the start line. And a spot still on the start line is not a mistake — it is the signature of a substance that is insoluble in that solvent.
Careful with the wording: the start line must sit above the water. If it were below, the ink would simply dissolve into the water in the tube instead of travelling up the paper.
Where it is used
Identifying colours in dyes — paper chromatography reveals which coloured substances a dye is made of.
Identifying coloured substances in food products — canned food and confectionery.
Detecting water pollutants — environmental agencies use it to check that water is clean.
Science at work: the Singapore Food Agency
Scientists at the SFA test food that is imported or sold in local supermarkets to make sure it is safe to eat. One method they use is gas chromatography, which works on the same principle but uses a gas as the solvent instead of a liquid.
In Greek, chrom means "colour" and graphy means "writing".