Effects of Impurities on Boiling/Freezing Point
- Boiling Point Elevation: the boiling point of a sample is raised when impurities are present.
- Freezing Point Depression: the freezing point of a sample is lowered when impurities are present
Example: salt water, boil above 212°F (100°C) and freeze 0°C (32°F)
How it is useful!
- Salt is used to lower freezing point of ice/water in winter
- In chemistry it is used to determine solute concentration
The absolute value of the difference in pure and impure boiling/freezing point is directly proportional to the molality of the sample.
Freezing point eg:
Boiling point eg:
- and : constants that vary with solvent.
- i: number of particles in the molecule of solute
- Two containers of water boil at different temperature. What can be concluded about the sample with the higher boiling point?
Therefore, the concentration of impurity (solute) must be higher in the container with the higher boiling point.