- 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.
Diagram showing how to represent elements, compounds and mixtures using particle diagrams
Melting & Boiling Point
- 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 closer the measured value is to the actual melting or boiling point then the purer the sample is.
- 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.
- This 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.
Cooling curve for a pure compound
- An impure version of the compound would produce a gradual decrease in temperature as it freezes as shown in the graph below.
Cooling curve for an impure compound
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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.