What you'll learn
Chromatography is a technique for separating and identifying the substances in a mixture, such as the dyes in a food colouring or an ink. In this guide you will learn how paper chromatography works, the meaning of the mobile and stationary phases, how to calculate and use Rf values, how to tell pure substances from mixtures, and how to carry out and interpret the required practical. These ideas link to the wider topic of analysis and identifying substances.
Key terms and definitions
Chromatography — a method of separating substances in a mixture based on how they move through a medium.
Mobile phase — the moving solvent that carries the substances (e.g. water or ethanol).
Stationary phase — the material that stays still (the chromatography paper).
Rf value — the ratio of the distance moved by a substance to the distance moved by the solvent.
Solvent front — the furthest point the solvent reaches up the paper.
Pure substance — a single element or compound, not mixed with anything else.
Core concepts
How chromatography works
In paper chromatography, a spot of the mixture is placed on paper and a solvent moves up through it. Substances separate because they have different attractions to the two phases: the stationary phase (the paper) and the mobile phase (the solvent). A substance that is more attracted to the solvent (more soluble) and less attracted to the paper travels further. Substances with different solubilities therefore separate into distinct spots.
Mobile and stationary phases
The stationary phase is fixed in place — in paper chromatography, the chromatography paper. The mobile phase is the moving solvent that travels through the stationary phase, carrying the dissolved substances with it. The balance of attraction between the two phases determines how far each substance moves.
Carrying out the practical
To run a chromatogram correctly:
- Draw a pencil start line near the bottom (pencil because ink would dissolve and run).
- Place small spots of the mixtures (and reference substances) on the line.
- Stand the paper in a small depth of solvent, below the start line, so the spots are not submerged.
- Let the solvent rise until near the top, then mark the solvent front and let it dry.
Rf values
The Rf value identifies a substance and is calculated as:
Rf = distance moved by the substance ÷ distance moved by the solvent
Both distances are measured from the start line: the substance distance to the centre of its spot, and the solvent distance to the solvent front. Rf values are always between 0 and 1 and have no units. The same substance gives the same Rf in the same solvent, so Rf values can be compared with known references to identify substances.
Pure substances and mixtures
A pure substance produces a single spot in all solvents. A mixture separates into two or more spots. Chromatography can therefore show whether a sample is pure or a mixture, and how many substances it contains. Comparing spots with reference materials run alongside helps identify the components — for example, checking which dyes are in a food colouring.
Worked examples
Example 1: Calculating Rf
A spot moves 4 cm and the solvent moves 8 cm. What is the Rf value?
Rf = 4 ÷ 8 = 0.5. (No units; between 0 and 1.)
Example 2: Pure or mixture?
A sample produces three spots. Is it pure?
No — a pure substance gives a single spot. Three spots show the sample is a mixture of three substances.
Example 3: Why pencil for the start line
Why is the start line drawn in pencil?
Pencil (graphite) is insoluble in the solvent, so it will not run or interfere with the results, unlike pen ink which would dissolve.
Common mistakes and how to avoid them
Drawing the start line in pen. Always use pencil so it doesn't dissolve.
Starting with the spots in the solvent. The solvent must start below the spots, or they will wash off into the solvent.
Measuring Rf to the wrong points. Measure from the start line to the centre of the spot and to the solvent front.
Giving Rf units or values above 1. Rf is a ratio between 0 and 1 with no units.
Saying a mixture gives one spot. A pure substance gives one spot; a mixture gives several.
Exam technique for Chromatography
Identify the two phases — stationary (paper) and mobile (solvent).
Explain separation by different solubilities/attractions to the phases.
Calculate Rf correctly with the formula and measure to the right points.
Distinguish pure from impure by the number of spots.
Recall practical details — pencil line, solvent below the spots, mark the solvent front.
Quick revision summary
Chromatography separates the substances in a mixture using two phases: a stationary phase (the paper) and a mobile phase (the moving solvent). Substances separate because they have different attractions to these phases — a substance more soluble in the solvent and less attracted to the paper travels further. Always draw the start line in pencil, keep the solvent below the spots, and mark the solvent front before measuring. Identify substances using the Rf value = distance moved by substance ÷ distance moved by solvent, measured from the start line to the centre of the spot and to the solvent front; Rf is always between 0 and 1 with no units, and is constant for a substance in a given solvent. A pure substance gives a single spot, while a mixture gives several spots, so chromatography reveals how many substances are present and, by comparison with references, what they are. Name the phases, explain separation by solubility, calculate Rf accurately, and use spot counts to judge purity.