Colligative Properties
• Physical properties affected by dissolved
solute particles
• Type of solute doesn’t matter.
• Number of particles does matter.
• Colligative means “depending on the
collection”
Colligative Properties
• Occur with nonvolatile solutes – ones that
have little tendency to become a gas
• 4 colligative properties:
– Vapor pressure lowering
– Boiling point elevation
– Freezing point depression
– Osmotic pressure
Colligative Properties
• Electrolytes are molecules that ionize or
fall apart when in solution; nonelectrolytes
do not fall apart or ionize
• Electrolytes will have a greater impact on
the 4 colligative properties
• Why?
– Sugar (C12H22O12) remains as one molecule
when dissolved.
– Salts like MgCl2 dissociate in water so one
mole of MgCl2 actually forms 3 moles of ions.
Vapor Pressure Lowering
• Pressure in a CLOSED container exerted
by the particles of liquid that have escaped
and become gas particles
• Greater number of escaped particles =
greater vapor pressure
• The addition of a solute LOWERS the
vapor pressure – WHY?
• The solute molecules get in the way of the
solvent molecules trying to escape at the
surface of the liquid.
Boiling Point Elevation
• A liquid boils when it’s particles can escape
as a gas (or when the vapor pressure is
equal to the atmospheric pressure)
• Ex: salt in water when cooking pasta
antifreeze in radiators
• When a solute is added, the boiling point is
driven higher – WHY?
• The solute molecules get in the way of the
solvent molecules trying to escape at the
surface of the liquid.
Freezing Point Depression
• A liquid freezes when its particles do not have
enough energy to overcome the intermolecular
forces between them.
• So the molecules are pulled closer together and
take on a defined shape.
• When a solute is added, the freezing point is driven
lower – WHY?
• The solute molecules get in the way and make it
harder for the solvent molecules to move closer and
become a solid
• Ex: salt & sand on icy roads
ethylene glycol on icy airplanes
Calculating b.p. elevation
and f.p. depression
• Because the type of solute doesn’t matter,
you only need to know the amount of
solute
• Elevation & depression happen at a
constant rate depending on the amount of
solute added (number of moles)
∆ Tb = Kbm
∆ Tf = Kfm
(where Kb & Kf are the constants)
Try It
• If you have .625 m aqueous solution with a
nonvolatile, nonelectrolyte solute, what
would the boiling point and freezing point
be?
Try It
• Calculate the freezing point and boiling
point of a solution that contains 105.4 g
NaCl dissolved in 750.3 ml H2O.
Osmotic Pressure
• Osmosis is a natural process where the
solvent flows across a membrane – always
flows from where there is more solvent to
where there is less solvent.
• Adding solute to one side of the membrane
will cause more solvent to flow that way.
• This increases the osmotic pressure on that
side of the membrane.
• Ex: kidney dialysis
uptake of nutrients by cells