The impure water starts boiling at above 100 degrees centigrade.
The boiling point of pure water is 100 degrees centigrade at sea level.
The temperature continues rising as impure water boils.
Impure water therefore boils over a range of temperature.It is concluded that impurities raise the boiling point of liquids.
Pure substances melt and boil at constant temperatures that are specific for a particular substance.
Melting point and boiling point are therefore used to determine
 the purity of substances.