AQA GCSE Chemistry

Revision Notes

8.1.1 Pure Substances

Pure Substances

  • In everyday language we use the word pure to describe when something is natural or clean and to which nothing else has been added
  • In chemistry a pure substance may consist of a single element or compound which contains no other substances
  • For example a beaker of a sample of pure water contains only H2O molecules and nothing else
  • If salt were added to the beaker then a mixture is produced
  • A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined
  • The chemical properties of the substances in a mixture remain unchanged
  • Substances in mixtures can be separated by physical means
  • Air for example is a mixture of nitrogen, oxygen and some other gases such as carbon dioxide and argon

Elements, compounds & mixtures, IGCSE & GCSE Chemistry revision notes

Diagram showing how to represent elements, compounds and mixtures using particle diagrams

Distinguishing Purity

  • Pure substances melt and boil at specific and sharp temperatures e.g. pure 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 tend to lower the melting point and broaden the melting point range
  • Melting and boiling points data can therefore be used to distinguish pure substances from mixtures
  • Melting point analysis is routinely used to assess the purity of drugs
  • This is done using a melting point apparatus which allows you to slowly heat up a small amount of the sample, making it easier to observe the exact melting point
  • This is then compared to data tables
  • The closer the measured value is to the actual melting or boiling point then the purer the sample is

Cooling Curves

  • The influence of impurities can be more clearly seen on a heating / cooling curve
  • If the temperature of a liquid is measured as it cools and freezes the data can be used to produce a graph
  • The following graph shows the cooling curve for a sample of a compound
  • The horizontal part of the graph shows that the compound has a sharp melting point, so the compound is pure

Purity - Pure Substance Cooling Curve, IGCSE & GCSE Chemistry revision notes

Cooling curve for a pure substance

  • An impure sample of the compound would produce a gradual decrease in temperature as it freezes as shown in the graph below

Purity - ImPure Substance Cooling CurvePurity - ImPure Substance Cooling Curve

Cooling curve for an impure substance

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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