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

Impure compounds have a range of melting points and boiling points, as the different substances they contain melt or boil at different temperatures. They may even interfere with each other's melting and boiling points. 

However, pure compounds have definite melting points and boiling points. The presence of an impurity usually:

• Lowers the melting point
• Raises the boiling point

The greater the amount of an impurity, the bigger the differences from the true melting point and boiling point.

-BBC Bitesize