CIE IGCSE Chemistry

Revision Notes

2.1.2 Criteria of Purity

Paper Chromatography

  • This technique is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink)
  • A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples
  • The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don´t wash into the solvent container
  • The solvent travels up the paper by capillary action, taking some of the coloured substances with it
  • Different substances have different solubilities so will travel at different rates, causing the substances to spread apart. Those substances with higher solubility will travel further than the others
  • This will show the different components of the ink / dye

Using paper chromatography to separate soluble mixtures, IGCSE & GCSE Chemistry revision notesAnalysis of the composition of ink using paper chromatography

Interpret Simple Chromatograms

  • If two or more substances are the same, they will produce identical chromatograms
  • If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
  • An impure substance will show up with more than one spot, a pure substance should only show up with one spot

Exam Tip

Paper chromatography is the name given to the overall separation technique while a chromatogram is the name given to the visual output of a chromatography run. This is the piece of chromatography paper with the visibly separated components after the run has finished.

Assessing Purity

  • Pure substances melt and boil at specific and sharp temperatures. Eg water has a boiling point of 100°C and a melting point of 0°C
  • Mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures
  • Melting and boiling points data can therefore be used to distinguish pure substances from mixtures
  • An unknown pure substance can be identified by experimentally determining its m.p and b.p and comparing to data tables
  • Mixtures melt over a range of temperatures as they contain two or more substances

Importance of Purity

  • A pure substance consists of only one substance and contains nothing else.
  • To have a pure substance for food and drugs is very important as impurities could be dangerous even in small amounts
  • Melting and boiling point analysis is routinely used to assess the purity of food and drugs
  • For example, if a sample of water melts at exactly 0°C and boils at exactly 100°C then the water is pure
  • If the melting and boiling points of the water aren’t these exact values then the water must be impure and contain other substances i.e. it must be a mixture
Extended Only

Retention Factor (Rf) Values

  • These values are used to identify the components of mixtures
  • The Rf value of a particular compound is always the same
  • Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions

Calculation

  • Retention factor = distance moved by compound  ÷ distance moved by solvent
  • The Rf value is a ratio and therefore has no units

Using Rf values to identify mixture components, IGCSE & GCSE Chemistry revision notesUsing Rf values to identify components of a mixture

Extended Only

Locating Agents

  • For chromatography to be useful the chemist needs to be able to see the components move up the paper, which is not the case for invisible samples such as proteins
  • Locating agents are substances which react with the sample and produce a coloured product which is then visible
  • The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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