Chromatography
Paper chromatography, locating agents, and Rf values.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 12.3.1Describe paper chromatography and interpret chromatograms, including calculating Rf values.
5 minute read
Chromatography
Chromatography separates a mixture of dissolved substances by exploiting one thing: how strongly each substance is attracted to the paper (the stationary phase) versus how easily it travels with the solvent (the mobile phase).
The method
A spot of the mixture is placed on pencil (not pen — ink would itself dissolve and move) near the bottom of a sheet of chromatography paper, above the level of the solvent in a container. As the solvent front rises up the paper, each dissolved substance is carried along at its own rate, depending on how strongly it clings to the paper versus how soluble it is in the solvent — substances that are more soluble and less attracted to the paper travel further.
Interpreting a chromatogram
- A pure substance produces exactly one spot; a mixture produces more than one spot.
- Comparing spots against known reference substances run on the same chromatogram lets you identify unknown components — matching height (and hence Rf) suggests they are the same substance.
- A locating agent (a chemical spray, or exposure to UV light) is used to make colourless spots visible, since not every substance is naturally coloured.
Rf values
The Rf value (retardation factor) is calculated as Rf = distance moved by the substance ÷ distance moved by the solvent front. Both distances are measured from the same baseline (the original spot position). Rf has no units, since it is a ratio, and is always between 0 and 1. Rf values are a property of the substance and the solvent used, so they can be looked up and compared to identify unknowns.
Think of it like this
Chromatography is like a foot race through sticky mud: substances that are less "sticky" (less attracted to the paper) and more eager to move with the solvent race ahead, while substances that cling to the paper lag behind — the finishing position tells you which substance is which.
Worked examples
Method, step by step
A chromatogram shows the solvent front has moved 9.0 cm from the baseline. A spot from substance X has moved 3.6 cm. Calculate the Rf value of X.
- 1Rf = distance moved by substance / distance moved by solvent.
- 2Rf = 3.6 / 9.0.
- 3Rf = 0.40.
Rf = 0.40 (no units).
Common misconceptions
- Using pen instead of pencil to mark the baseline spot — pen ink dissolves in the solvent and would move up the paper along with the sample, ruining the chromatogram.
- Measuring the substance and solvent distances from different starting points — both must be measured from the same baseline (the original spot) for the Rf calculation to be valid.
- Thinking a single spot on a chromatogram always proves a substance is pure with total certainty — it strongly suggests purity but different substances can occasionally travel the same distance in a given solvent; using a second solvent adds confidence.
In the exam
- Rf value calculations must give an answer between 0 and 1 with no units — if your answer is outside that range, you have likely swapped the numerator and denominator.
- When asked how to identify an unknown substance by chromatography, mention running known reference substances alongside it and comparing Rf values (or spot positions), not just "looking at the spots".