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2.2.1 Pure Substances & Mixtures - Coggle Diagram
2.2.1 Pure Substances & Mixtures
Intro
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
For example a beaker of a sample of pure water contains only H2O molecules and nothing else
The chemical properties of the substances in a mixture remain unchanged
In chemistry a pure substance may consist of a single element or compound which contains no other substances
Substances in mixtures can be separated by physical means
In everyday language we use the word pure to describe when something is natural or clean and to which nothing else has been added
Air for example is a mixture of nitrogen, oxygen and some other gases such as carbon dioxide and argon
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Distinguishing Purity
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
Melting and boiling points data can therefore be used to distinguish pure substances from mixtures
This is then compared to data tables
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
The closer the measured value is to the actual melting or boiling point then the purer the sample is
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
Cooling Curves
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 influence of impurities can be more clearly seen on a heating / cooling curve
The horizontal part of the graph shows that the compound has a sharp melting point, so the compound is pure
An impure sample of the compound would produce a gradual decrease in temperature as it freezes as shown in the graph below
Graph 1 (comments)
Graph 2 (comments)