Chromatography: Different compounds have different R_f values in different solvents, which can be used to help identify the compounds. The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

The idea

Chromatography separates dissolved substances as they travel at different rates through paper or a thin layer. A compound's R_f value depends on its solubility in the solvent and its attraction to the stationary phase, so the same compound can give different R_f values in different solvents. This means R_f alone does not prove identity; it must be compared with the R_f of a known pure sample run under the same conditions. A mixture may produce several spots because its components separate, and the pattern of spots can change with the solvent. A pure compound produces a single spot in every solvent, although the spot's position may differ between solvents.

For example: A pure dye gives one spot in water and one spot in ethanol, whereas an impure dye may give two spots in one solvent and three in another.

What earns the marks

• State that different compounds have different R_f values in a given solvent, so R_f can help identify a compound when compared with a known sample.
• Explain that R_f values depend on the solvent used, so a compound can have different R_f values in different solvents.
• Describe how a mixture separates into different spots because its components travel at different rates.
• State that a pure compound produces a single spot in all solvents, while a mixture may produce more than one spot.
• Explain that identification requires running a known pure sample under the same conditions and comparing R_f values or spot positions.
• Recognise that a single spot in one solvent does not by itself prove purity; the test should be repeated in a different solvent.

Common mistakes and how to put them right

• Claiming that a single spot in one solvent proves a compound is pure.

  Put it right by stating that purity is confirmed only if a single spot appears in all solvents tested.

• Assuming R_f is a fixed constant for a compound regardless of solvent.

  Put it right by stating that R_f varies with the solvent and conditions.

• Treating a matching R_f as proof of identity.

  Put it right by explaining that it supports identification when compared with a known sample under the same conditions.

Exam tips

• Use the phrase 'under the same conditions' when comparing R_f values with a known compound.
• Link the number of spots to the number of components: one spot suggests a pure compound, multiple spots indicate a mixture.
• When a question mentions different solvents, state that the R_f value may change but a pure compound still gives one spot in each solvent.