Page 85 - Basic physical chemistry for the atmospheric sciences

Solution chemistry and aqueous equilibria 7 1

the case of a solid, the equilibrium concentration (or solubility) of <i
gas in a liquid involves a balance between randomness and energy, but
in the case of a gas these two driving forces act in the opposite
directions to those in which they act on a solid. Consequently, for a
gas, increasing the temperature, which favors the more random state,
will decrease the solubility.
The solubilities of solids and liquids are not affected much by pres­
sure. By contrast, the solubility of any gas in a solvent increases with
increasing pressure. This is because increasing pressure decreases the
randomness of the gas phase, and hence decreases the difference in
randomness between the gas phase and the aqueous solution. Car­
bonated drinks (soda pop) depend on this fact. Bottled at pressure in
excess of I atm, considerable quantities of C02 can be dissolved in the
liquid. When the pressure is reduced to atmospheric by taking the cap
off the bottle, the C0 2 rapidly comes out of solution in the form
s
of bubble .
A n approximate relationship between pressure and solubility is
given by Henry's law
(4.3)

where C is the solubility of the gas in the solution, p the pressure of
g
the gas over the solution, and kH s a temperature-dependent propor­
i
tionality constant called the Henry's law constant. Tabulated solubili­
ties of gases are generally based on a gas pressure of I atm above
the liquid.

4.4 Colligative properties
Some properties of a solution depend on the concentration of the
solution but not on the particular identity of the solute. These are
called colligative properties. Three colligative properties are discussed
below: vapor-pressure lowering, boiling-point elevation, and freezing­
point depression.
The effect of the concentration of a solution on the vapor pressure
of the solvent is given approximately by Raoult 's law, which states:
the vapor pressure (p A) of a solvent (A) above a solution is equal to
the product of the vapor pressure of the pure solvent p� and the mole
fraction o f the solvent in the solution (I/IA). That is,

.
(4 4 )