Fundamental Chemistry and Hydraulics - Mathematics and Statistics for the Environment - page 349

Key Point: Dissolving a solid in a liquid is usually an endothermic process (i.e., heat is absorbed),
so raising the temperature will fuel this process. In contrast, dissolving a gas in a liquid is usually
an exothermic process (i.e., it evolves heat), so lowering the temperature generally increases the
solubility of gases in liquids.
Note: Thermal pollution is a problem because of the decreased solubility of O 2 in water at higher
temperatures.
Pressure has only an appreciable effect on the solubility of gases in liquids. For example, carbon-
ated beverages such as soda water are typically bottled at significantly higher atmospheres. When
the beverage is opened, the decrease in the pressure above the liquid causes the gas to bubble out
of solution. When shaving cream is used, dissolved gas comes out of solution, bringing the liquid
with it as foam.
14.1.2.5 Colligative Properties
Properties of a solution that depend on the concentrations of the solute species rather than their
identity include the following:
• Lowering vapor pressure
• Raising boiling point
• Decreasing freezing point
• Osmotic pressure
True colligative properties are directly proportional to the concentration of the solute but entirely
independent of its identity.
• Lowering of vapor pressure— With all other conditions identical, the vapor pressure of
water above the pure liquid is higher than that above sugar water. The vapor pressure above
a 0.2- M sugar solution is the same as that above a 0.2- M urea solution. The lowering of
vapor pressure above a 0.4- M sugar solution is twice as great as that above a 0.2- M sugar
solution. Solutes lower vapor pressure because they lower the concentration of solvent mol-
ecules. To remain in equilibrium, the solvent vapor concentration must decrease (hence the
vapor pressure decreases).
• Raising the boiling point— A solution containing a nonvolatile solute boils at a higher tem-
perature than the pure solvent. The increase in boiling point is directly proportional to the
increase in solute concentration in dilute solutions. This phenomenon is explained by the
lowering of vapor pressure already described.
• Decreasing the freezing point— At low solute concentrations, solutions generally freeze or
melt at lower temperatures than for the pure solvent.
Key Point: The presence of dissolved “foreign bodies” tends to interfere with freezing; therefore,
solutions can only be frozen at temperatures below that of the pure solvent.
Key Point: We add antifreeze to the water in our radiators to both lower its freezing point and
increase its boiling point.
• Osmotic pressure— Water moves spontaneously from an area of high vapor pressure to an
area of low vapor pressure. If allowed to continue, in the end all of the water would move
to the solution. A similar process will occur when pure water is separated from a concen-
trated solution by a semipermeable membrane (i.e., it only allows the passage of water
molecules). The osmotic pressure is the pressure that is just adequate to prevent osmosis.