A pure substance is made up of only one kind of particles. All the constituent particles of a pure substance are the same in their chemical nature. These particles may be atoms or molecules. Pure substances have fixed melting or boiling point. In order to confirm that a colourless liquid is pure water, we can check its boiling point. Pure water boils at 100°C at atmospheric pressure. If this liquid also boils at 100°C then its is pure water.