Page 64 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry

Phase and Chemical Equilibria 49

about 21% of 1 atm or 0.21×760 Torr = 160 Torr. Thus, there is more than
ample O 2 in the air to sustain life in water on earth.

6.2.3. Colligative Properties

There are several properties, all resulting from lowering thepressureof the
solvent that can be observed in solutions. They are

• Elevation of boiling point;
• Depression of freezing point;
• Production of osmotic pressure.
In the simplest cases, one assumes that

(1) the solute is not volatile;
(2) the solute does not precipitate.

Thus, when a water solution freezes, the frozen solid is pure ice.

6.2.4. Elevation of Boiling Point. Depression
of Freezing Point
The easiest way to ascertain why there is a boiling point increase or a
freezing point decrease is to observe the variation of the chemical potential
with temperature.
Figure 6.3 shows three curves: the chemical potential of pure A in the
vapor phase, pure A in the liquid phase and of A in the solvent. Notice
that the solvent curve is lower than the liquid phase curve, as it should
be because of the lower chemical potential. The pure vapor phase curve
cuts the pure liquid curve at a lower temperature than the solution phase,
indicating that the boiling point of the solution is higher.
Figure 6.4 shows the variation of the chemical potentials with tem-
perature of pure A in the liquid phase, pure A in the solid phase and A in
solution. Again the solution curve is lower than the liquid curve, as it should
be. The pure liquid curve cuts the solution curve at a lower temperature
than the pure liquid curve, indicating that the solution freezing point is
lower.