Transcript Colligative properties - Bremen High School District 228

Colligative properties
Colligative properties
Colligative property- a property of a
solution that depends on concentration of
solute (the number of solute particles
dissolved) but NOT on type of solute
 3 colligative properties:

Vapor pressure reduction
 Boiling point elevation
 Freezing point depression

Vapor pressure
The pressure DIRECTLY above the
surface of a liquid exerted by the
molecules that have evaporated
 LOW vapor pressure = not much
evaporation occurring
 HIGH vapor pressure = lots of evaporation


http://www.youtube.com/watch?v=re9r0kzQp_
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Vapor pressure reduction

Solute interferes with vaporization of
water, therefore LOWERS the pressure of
the vapor pressing down on surface
How does this influence boiling
point & freezing point?

Boiling Point: The temperature at which
the vapor pressure of the liquid phase
equals atmospheric pressure.
Boiling-Point Elevation

Boiling point elevation: The
difference in temperature between
the boiling point of a SOLUTION
and the boiling point of the pure
solvent (water).
 When
a solute is dissolved in a
solvent, the boiling point of the
solvent will elevate (become
higher) than the normal boiling
point (will need more heat to boil)
Boiling point Elevation

Because the bp is higher, the vapor
pressure will be lowered
Application: Adding salt to water
while cooking pasta
Water does not
boil faster
 Water is boiling
at a higher
temperature so
pasta cooks
faster

More Examples – BP Elevation

Antifreeze =a liquid added to the cooling
system of a car to increases the BP of the water
in it so it doesn’t overheat as fast in summer.
Freezing-Point Depression

Freezing-Point Depression: The difference
in temperature between the freezing point
of a solution and the freezing point of the
pure solvent (water).

The presence of a solute in water disrupts the
formation of the orderly pattern of ice.
Therefore more kinetic energy must be
transferred from the solution (into the
surroundings) compared to the pure solvent to
cause the solution to solidify.
Freezing Point Depression
Freezing point depression
Example – FP Depression
Salt (solute) is added to melt ice (water =
solvent)
 Works by lowering the freezing point of water.
Ex: 32° F normal freezing point of water, now it
could be 15° F and still be in a liquid state.

Solid to liquid
Freezing-Point Depression

The magnitude of the
freezing-point depression
is proportional to the
number of solute particles
dissolved in the solvent
and does not depend upon
their identity.

Which would be a better salt
for putting on icy roads,
NaCl or CaCl2?
More Examples – FP Depression
•
Antifreeze =a liquid added to the cooling system of
a car to lower the FP of the water in it so it doesn’t
freeze solid.
•
Some animals are able to survive freezing
temperatures by producing natural antifreezes.
▫
Natural antifreezes are just higher levels of alcohols (ex.
glycerol) and sugars (ex. glucose) in the cells.
•
Examples are box turtles, painted turtles and
garter snakes, sugar maples and white spruce.
•
http://videos.howstuffworks.com/discovery/52100-frozen-planet-caterpillar-survives-frozen-death-video.htm
Therefore…

Because freezing point of water goes
down, we can make things like
ICE
CREAM!!!!