Specification Point 2.5:
Explain the difference between the use of ‘pure’ in chemistry compared with its everyday use and the differences in chemistry between a pure substance and a 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 substance.
- For example a beaker of pure sample of 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.
Diagram showing how carbon (blue) and oxygen (orange) can combine chemically to form the compound CO2 but also form a mixture of C atoms and O2 molecules
Melting Point of Pure Substances
Specification Point 2.6:
Interpret melting point data to distinguish between pure substances which have a sharp melting point and mixtures which melt over a range of temperatures.
- 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 melt or boil at different temperatures
- 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 food and 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 table below indicates melting point data for four different substances named A, B, C and D. Identify the substance that is a mixture.
- Substance C melts over a range of temperatures hence it is the mixture.
- Substances A, B and D have specific melting points hence they are pure substances.
Edexcel GCSE Chemistry Notes
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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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