Pure Substances & Mixtures (Edexcel GCSE Chemistry)

Revision Note

Stewart Hird

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Pure Substance vs Mixture

  • 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

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.