Melting and boiling points of pure substances and mixtures

  • melting point is the temperature at which a solid changes into a liquid.
  • boiling point is the temperature at which a liquid changes into a gas.

Pure substances have specific melting and boiling points. Mixtures melt and boil over a range of temperatures.

The graphs below show the cooling curves for a pure sample of a compound called salol (C13H10O3) and an impure sample.

A cooling curve for salolIn a sample of pure salol, the temperature stays the same as it changes state. Graph of the freezing and melting range of a sunstance, between 40 c and 50 c.In a sample of impure salol (salol mixed with other substances), the temperature falls gradually as it freezes.

We can use these differences between melting and boiling points to distinguish between pure substances and mixtures.