Interpreting Chromatograms

Paper chromatography is used to separate and identify the soluble substances in a mixture — for example the different coloured dyes in a felt-tip pen, or food colourings in a sweet.

The mixture is placed as a small spot on a sheet of chromatography paper, the bottom of the paper dips into a solvent (often water or ethanol), and the solvent slowly soaks up the paper. As it passes the spot, it dissolves the substances and carries them upwards.

Each substance has its own balance between how well it dissolves in the solvent and how strongly it sticks to the paper:

• A substance that is more soluble in the solvent is carried further up the paper.
• A substance that is less soluble (or sticks to the paper) stays lower down.

Because the components travel at different speeds, they end up separated at different heights — and a single mixed spot turns into several separate spots.

Key idea: the substances are not changed — chromatography is a physical separation, just spreading the components out so we can see and identify them.

The method has two details examiners ask about again and again — get them right and they are easy marks.

1. Draw the start line (baseline) in PENCIL. Pencil is graphite, which is insoluble in the solvent, so the line stays put. If you drew it in ink, the ink would dissolve and run up the paper with the solvent — ink is itself a mixture of dyes — and ruin the result.

2. Put the solvent level BELOW the baseline. The paper dips into only a shallow depth of solvent. The solvent must start below the spots. If the solvent level were above the baseline, the spots would simply dissolve straight into the solvent in the tank and wash away instead of travelling up the paper.

The basic steps:

1. Draw a baseline near the bottom of the paper in pencil.
2. Add small, concentrated spots of the mixture (and any reference samples) on the baseline; let each spot dry.
3. Stand the paper in a container with a shallow depth of solvent, solvent level below the baseline.
4. Leave it (covered) until the solvent has risen most of the way up. The line the solvent reaches is the solvent front.
5. Remove the paper and let it dry — the separated spots can now be read.

The finished paper is called a chromatogram. The single most important rule:

The number of spots tells you the number of components in the mixture.

So:

• One spot → the substance is pure (a single substance).
• Two or more spots → the substance is a mixture of that many components.
• A spot that has not moved from the baseline has stayed put — it is insoluble in that solvent.

Worked reading. If a sample of green food colouring is run and produces two spots — a blue one and a yellow one — then the green colouring is a mixture of two dyes (a blue dye + a yellow dye), not a single pure substance.

Chromatography does not just count components — it can identify them. You do this by running the unknown mixture alongside known reference substances on the same piece of paper, in the same solvent, at the same time.

Then you compare heights:

• A spot in the mixture that has travelled to the same height as a known reference spot is the same substance as that reference.
• A reference that has no matching spot in the mixture is not present in the mixture.

This works because, on a given paper in a given solvent, every substance always travels to the same characteristic height — a bit like a chemical "fingerprint".

Important conditions. The comparison is only valid if the knowns are run on the same paper, in the same solvent, at the same time. Different solvents move substances to different heights, so heights from two separate chromatograms cannot be compared directly.