Chromatography – IGCSE Chemistry Definition

Chromatography is a core practical technique tested on Paper 6 (alternative to practical) and frequently on Paper 2 and Paper 4 as well. Examiners ask candidates to interpret chromatograms, calculate Rf values, and describe the method. This is one of the most mark-rich practical topics.

The 0620 definition

Chromatography is a technique for separating and identifying the components of a mixture of dissolved substances, based on their different rates of movement through a stationary phase (e.g. paper) under the influence of a moving phase (solvent).

Method for paper chromatography

1. Draw a pencil line (baseline) near the bottom of the chromatography paper — pencil because it is insoluble and will not dissolve in the solvent
2. Place a small spot of the mixture on the baseline using a capillary tube
3. Place spots of known reference substances alongside (for comparison)
4. Lower the paper into the solvent so the baseline is above the solvent level — the spots must not be submerged
5. Allow the solvent to rise up the paper (capillary action)
6. Remove the paper when the solvent front is near the top
7. Mark the solvent front position in pencil immediately
8. Allow the paper to dry; circle any visible spots

Why substances separate

Different substances in the mixture have:

• Different solubilities in the solvent (mobile phase)
• Different attractions to the paper (stationary phase)

More soluble substances travel further with the solvent. Less soluble substances stay closer to the baseline.

Interpreting chromatograms

• One spot from a sample: the sample is a pure substance
• Multiple spots: the sample is a mixture
• Matching position with a reference: identifies that component

Calculating Rf values

Rf = distance moved by substance / distance moved by solvent front

See Rf value for full details.

Worked exam question

A student uses paper chromatography to investigate a food dye. The dye produces three spots. (a) What does this tell you about the dye? [1] (b) Explain why a pencil line is used for the baseline, not ink. [1] (c) Explain why the baseline must be above the solvent level. [1]

Mark scheme

(a) The dye is a mixture (of three components/substances) [1]

(b) Pencil is insoluble in the solvent, so it will not dissolve and interfere with the results; ink would dissolve and move with the solvent [1]

(c) If the baseline is below the solvent, the spots would dissolve into the solvent instead of being carried up the paper [1]

Common exam mistakes

• Using pen instead of pencil for the baseline — this is both a practical error and an exam mistake if you suggest it in a method.
• Letting the spots dip into the solvent — the spots must be above the solvent level.
• Saying chromatography “proves” what a substance is — it only identifies substances by comparison with known references run at the same time.