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GCSE Chemistry Revision

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GCSE Chemistry revision

Chromatography

Purity, formulations and chromatography

AQA 4.8.1.3 Foundation & Higher
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Revision summary

Key knowledge

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Introduction

  • Paper chromatography is an analytical technique used to separate the different substances within a mixture so that they can be identified.
  • It is commonly used to separate the different dyes found in an ink sample.
  • The pattern of spots produced at the end of the experiment is called a chromatogram.

Setting Up the Experiment

  • A pencil line called the baseline is drawn near the bottom of a piece of filter paper, and the ink sample is spotted onto it.
  • A pencil is used (not a pen) so that the line itself does not dissolve and interfere with the results.
  • A shallow amount of solvent, such as water or ethanol, is added to a beaker — shallow enough so that the baseline and ink spot are NOT submerged.
  • A lid is placed on top of the beaker to prevent the solvent from evaporating during the experiment.

How the Separation Occurs

  • The solvent travels up the filter paper by capillary action, carrying the dissolved dyes with it.
  • Each dye travels up the paper at a different rate, causing them to separate out into distinct spots.
  • Any substance that is insoluble in the chosen solvent will not move and will remain on the baseline.
  • The experiment is stopped when the solvent front has nearly reached the top of the paper, and the paper is then left to dry.

The Mobile Phase

  • The mobile phase is the substance in which molecules can move, and it is always a liquid or a gas.
  • In paper chromatography, the mobile phase is the solvent used (e.g. water or ethanol).
  • Chemicals that are more soluble in the solvent spend more time in the mobile phase and therefore travel further up the paper.

The Stationary Phase

  • The stationary phase is the substance or material in which molecules cannot move, and it is usually a solid or a very thick liquid.
  • In paper chromatography, the stationary phase is the filter paper itself.
  • Chemicals that are less soluble in the solvent, or more strongly attracted to the paper, spend more time in the stationary phase and travel a shorter distance.

Why Substances Separate

  • During the experiment, each chemical constantly switches between dissolving in the solvent (mobile phase) and binding to the paper (stationary phase).
  • The overall proportion of time a substance spends in each phase determines how far it travels up the paper.
  • Because the distance each substance travels depends on its own unique properties, the same substance will always produce the same chromatogram under the same conditions.

Calculating the Rf Value

  • Instead of comparing raw distances, chemists calculate a ratio called the Rf value to allow fair comparisons between experiments.
  • The Rf value is calculated using the formula: Rf = distance travelled by substance / distance travelled by solvent For example, if a substance travels 6 cm and the solvent travels 10 cm, the Rf value is 6 / 10 = 0.6.
  • Rf values are always between 0 and 1, and each substance has a characteristic Rf value that can be looked up in a data book to identify it.

Pure Substances vs Mixtures

  • A pure substance contains only one type of chemical, so it will not separate and will produce only a single spot on the chromatogram.
  • A mixture will produce multiple spots, showing that it contains more than one substance.
  • If a chromatogram shows only one spot, this is evidence that the sample may be a pure substance.

Factors Affecting Rf Values

  • The Rf value of a substance will change if a different solvent (mobile phase) or a different type of paper (stationary phase) is used.
  • When looking up Rf values in a data book, chemists must take into account which solvent and which type of paper were used in the experiment.
  • This means that Rf values are only directly comparable when the experimental conditions are identical.