Chromatography: Chromatography can be used to separate mixtures and can give information to help identify substances. Chromatography involves a stationary phase and a mobile phase. Separation depends on the distribution of substances between the phases.

The idea

Chromatography separates the components of a mixture as they move between two phases. The stationary phase is fixed, for example the water held in paper or a solid coating on a plate. The mobile phase moves, for example a solvent rising up paper or a gas carrying vapour through a column. A sample is placed at the start line and the mobile phase travels through the stationary phase. Each substance distributes itself between the phases to a different extent, so components travel different distances and separate. The distance moved relative to the solvent front gives an Rf value, which can be compared with known values to help identify a substance. A pure substance usually gives a single spot, while a mixture gives more than one spot.

What earns the marks

• State that chromatography separates the components of a mixture, not a compound.
• Identify the stationary phase as the fixed phase and the mobile phase as the moving phase in a named example such as paper chromatography.
• Explain that separation depends on how each substance distributes itself between the stationary and mobile phases.
• Describe how to obtain and use an Rf value, calculated as distance moved by the substance divided by distance moved by the solvent front, to help identify a substance.
• Interpret a chromatogram: a single spot suggests a pure substance, while multiple spots indicate a mixture.

Common mistakes and how to put them right

• Confusing the phases by calling the solvent the stationary phase.

  Put it right by stating the solvent that moves is the mobile phase and the paper or plate is the stationary phase.

• Measuring Rf from the wrong baseline or forgetting to measure to the centre of the spot.

  Put it right by measuring from the start line to the centre of the spot and from the start line to the solvent front.

• Claiming chromatography identifies a substance on its own.

  Put it right by saying Rf values must be compared with known values under the same conditions.

Exam tips

• Always label the stationary phase and mobile phase in a diagram before explaining separation.
• Show the Rf calculation with both distances and keep the solvent front distance as the denominator.
• Use the phrase 'distribution between the phases' to explain why components move different distances.