Chromatography: suggest how chromatographic methods can be used for distinguishing pure substances from impure substances

The idea

Chromatography separates dissolved substances as a solvent front moves through a stationary phase. A pure substance produces a single spot in every solvent system because it has one fixed composition and one R_f value. An impure substance contains two or more substances, so it produces two or more spots, or a smeared streak, under the same conditions. To distinguish them, run a chromatogram of the sample alongside a known pure reference under identical conditions. If the sample gives one spot matching the reference, it is pure; if it gives extra spots or a streak, it is impure.

For example: A food colouring that gives one spot is a pure dye, while one giving three spots is a mixture of dyes.

What earns the marks

• A pure substance gives a single spot on a chromatogram because it contains only one substance with one R_f value.
• An impure substance gives two or more spots, or a streak, because it contains two or more substances that separate differently.
• The same chromatographic conditions, including solvent, stationary phase and temperature, must be used for a valid comparison.
• A known pure reference substance can be run alongside the sample; a matching single spot supports purity, while extra spots show impurity.
• Chromatography separates substances by their different solubilities in the mobile phase and attractions to the stationary phase, so each substance travels a characteristic distance.
• The number of spots indicates the minimum number of substances present, but two substances with similar R_f values may overlap.

Common mistakes and how to put them right

• Thinking that a pure substance always gives a spot at the solvent front.

  Put it right: A pure substance gives one spot at a characteristic R_f value, which is usually less than 1.

• Comparing spots from chromatograms run in different solvents.

  Put it right: R_f values and spot patterns are only comparable when the same solvent, stationary phase and conditions are used.

• Assuming that one spot always proves purity.

  Put it right: One spot shows only one detectable substance under those conditions; two substances with the same R_f value could overlap, so additional evidence may be needed.

• Confusing the mobile phase and stationary phase.

  Put it right: The mobile phase is the solvent that moves, and the stationary phase is the paper or plate that does not move.

Exam tips

• State clearly that a pure substance gives one spot and an impure substance gives more than one spot or a streak.
• Mention that the same solvent and conditions must be used when comparing a sample with a pure reference.
• Use the term R_f value correctly and link it to the distance travelled by the substance relative to the solvent front.
• If asked to suggest a method, describe running a chromatogram of the sample and a known pure sample side by side, then compare the spots.