Purity of Substances (Oxford AQA IGCSE Combined Science Double Award)

Purity of Substances

• A change of state refers to a substance changing its state between solid, liquid or gas

• Changes of state happen at the melting and boiling points of a pure substance

• The purity of a substance affects its melting and boiling point

Changes of state

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 Physics and 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 the saltwater solution would be a mixture

• A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined

  • Air is a mixture of nitrogen, oxygen and some other gases such as carbon dioxide and argon

• The chemical properties of the substances in a mixture remain unchanged

• Substances in mixtures can be separated by physical means (e.g. filtration or distillation)

Compounds and mixtures

The heating and cooling curve of a pure substance

Distinguishing purity

• Pure substances melt and boil at specific  temperatures e.g. pure water has a boiling point of 100°C and a melting point of 0°C

Measuring the melting and boiling temperature of water

• 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 point 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