Pure Substance vs Mixture (Edexcel IGCSE Chemistry)

• 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

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

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

Cooling curve for an impure substance