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Effects of Impurities on Boiling/Freezing Point

Definitions:

  • 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.

b=molality=\frac{moles\ solute}{kg\ solvent}

Freezing point eg: \Delta T=k_{F}\times b\times i

Boiling point eg: \Delta T=k_{B}\times b

  • k_{F} and k_{B} : constants that vary with solvent.
  • i: number of particles in the molecule of solute (NaCl=2, BaF_{2}=3

Example Problem:

  • Two containers of water boil at different temperature. What can be concluded about the sample with the higher boiling point?

higher\ boiling\ point\rightarrow higher\ \Delta T\rightarrow both\ water,\ same\ k_{B}

Therefore, the concentration of impurity (solute) must be higher in the container with the higher boiling point.