Solutions, especially liquid solutions, generally have markedly
different properties than either
the pure solvent or the solute. For example, a solution of sugar
in water is neither
crystalline like sugar nor tasteless like water. Some of the properties
unique to solutions depend only
on the number of dissolved particles and not their identity. Such
properties are called
colligative properties. The colligative properties we will consider in
this SparkNote are vapor
pressure lowering, freezing point depression, boiling point elevation, and
osmotic pressure.
When a nonvolatile solute is dissolved in a solvent, the vapor pressure
of the resulting solution
is lower than that of the pure solvent. The amount of the vapor pressure
lowering is proportional to
the amount of solute and not its identity. Therefore, vapor pressure
lowering is a colligative
property. The equation that describes that phenomenon is called Raoult's
law.
Boiling point elevation is a colligative property related to vapor pressure
lowering. The boiling point
is defined as the temperature at which the vapor pressure of a liquid
equals the atmospheric pressure.
Due to vapor pressure lowering, a solution will require a higher
temperature to reach its boiling point
than the pure solvent.
Every liquid has a freezing point--the temperature at which a liquid
undergoes a phase change from
liquid to solid. When solutes are added to a liquid, forming a solution,
the solute molecules disrupt
the formation of crystals of the solvent. That disruption in the freezing
process results in a
depression of the freezing point for the solution relative to the pure solvent.
When a solution is separated from a volume of pure solvent by a
semi-permeable membrane that
allows only the passage of solvent molecules, the height of the solution
begins to rise. The value of
the height difference between the two compartments reflects a property
called the osmotic pressure of
a solution. As you know, if you add more solvent to a solution, the two
mix together to form a more
dilute solution. The same forces allowing that mixing serve to force
solvent molecules from the
pure solvent compartment across the membrane into the solution compartment
causing the change in
volume. The amount of osmotic pressure is directly related to the
concentration of the solute.
That is because more concentrated the solutions have greater potentials for
dilution.
