Colligative Properties
Colligative Properties….
Are properties that depend on the number of
dissolved particles only.
The type of dissolved particles does not
matter
Freezing Point
• The temperature at which a solution
freezes from a liquid to a solid
• Freezing point of pure water is 0o C
• Adding a solute to pure water lowers the
freezing point below zero
Freezing Point is lowered when:
• Salt is added to roads in winter, so they
don’t get icy even if the temp is below zero
• Antifreeze is added to engine fluids so
they don’t freeze inside your engine and
your car still starts when the temp is below
zero
Take 1000g of pure water
And add one mole of dissolved particles,
and you will lower the freezing point by
1.86 degrees C.
Remember: it does not matter what the
dissolved particles are, only how many
there are!!!
Boiling Point
• The temperature at which a solution boils
from a liquid to a gas
• Boiling point of pure water is 100o C
• Adding a solute to pure water raises the
boiling point above 100 degrees C
Boiling Point is raised when:
• Salt is added boiling water to cook pasta
faster (at a higher temperature) so the
pasta does not get “soggy”. Instead it
comes out “al dente”
• Antifreeze is added to engine fluids so
they don’t boil inside your engine and your
car won’t overheat when temp is above
100 degrees C.
Take 1000g of pure water
And add one mole of dissolved particles,
and you will raise the boiling point by 0.52
degrees C.
Remember: it does not matter what the
dissolved particles are, only how many
there are!!!
Some solutes are more effective
than others at raising and lowering
boiling and freezing points
• I mole of sugar (covalent compound)
dissolves in water to form 1 mole of
dissolved sugar C6H1206(aq)
• 1 mole of NaCl (ionic compound) will
dissolve to form 2 moles of dissolved
particles: 1 mole of Na+ (aq) and I mole of
Cl- (aq)
Some solutes are more effective
than others at raising and lowering
boiling and freezing points
• 1 mole of CaCl2 (ionic compound) will
dissolve to form 3 moles of dissolved
particles: 1 mole of Ca+ (aq) and 2 moles
of Cl- (aq)
• In general, ionic compounds form more
dissolved particles, so they are a better
choice for adding to the water to change
boiling or freezing points.
Vapor Pressure: Table H
some molecules at the surface of a liquid
may have enough energy to evaporate even
if the liquid is below its boiling point. These
escaping gas particles exert a pressure
called VAPOR PRESSURE.
Vapor Pressure
• The more easily the molecules evaporate,
the higher the vapor pressure
• The weaker the intermolecular forces, the
more easily the molecules can separate
and go into the gas phase, therefore the
higher the vapor pressure
Vapor Pressure
• The more easily the molecules evaporate,
the higher the vapor pressure
• The stronger the intermolecular forces, the
less easily the molecules can separate
and go into the gas phase, therefore the
lower the vapor pressure
Using Table H
• Which liquid has the highest vapor
pressure at 50 degrees?
• The lowest?
• Which liquid has the weakest
intermolecular forces?
• The strongest?
Vapor pressure
The boiling point of a liquid is defines as the
temperature at which the vapor pressure
of the liquid equals atmospheric pressure
(1 atm or 101.3 kPa)
In other words, at that temp all the
molecules change to gas phase (boil), not
just the molecules at the surface