Pure Substance vs Mixture

Specification Point 1.9:

Understand that a pure substance has a fixed melting and boiling point, but that a mixture may melt or boil over a range of temperatures

Pure substance: 

• Contains only one substance, with no other substances mixed together
• Has fixed melting and boiling point as they consist of only one substance so all molecules will have the same melting and boiling points
• E.g. Water has a boiling point of  100°C and a melting point of 0°C

Mixture: 

• A combination of two or more substances (elements and/or compounds) that are not chemically joined together
• Has a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures

Testing purity:

• The physical property of boiling/melting point can therefore be used to test if a substance is pure or not.
• For example, if a sample of water melts at exactly 0°C and boils at exactly 100°C then the water is pure.
• If the melting and boiling points of the water aren’t these exact values then the water must be impure and contain other substances i.e. it must be a mixture.