Colligative Properties
properties of solutions, packet page 7
You might need to take additional notes as
well
Colligative Properties

Properties of solutions (compared to the pure
solvent)

Depend upon the number of solute molecules
or ions (concentration), but NOT upon the
identity of the solute.

Include freezing point depression, boiling point
elevation, vapor pressure, and osmotic
pressure.
You should be able to:
1. Define the CP
 2. How does concentration of the solute
affect the CP?
 3. Explain the mechanism of how the solute
affects the CP.
 4. Draw a molecular picture illustrating the
effect of the solute on the solvent.

How do you determine how
many particles there are?
*Covalent compounds do NOT ionize so they stay as
one particle.
*Ionic compounds will dissociate into ions. Add the
total resulting number of ions:

NaCl  Na+ + Cl-

Na2SO4  2Na+ + SO42-

(NH4)3PO4  3NH4+ + PO43
Notice that polyatomic ions do not break apart further.
What is vapor pressure?
Vapor pressure is the pressure produced
when vapor particles above a liquid
collide with the container walls
 Some molecules from the liquid
phase will vaporize if

The molecules are moving fast
enough/sufficient energy
 The molecules are at the surface


Vapor pressure increases with
The vapor pressure of a solution is lower
than the vapor pressure of the pure solvent.
 Solvent molecules are attracted to the
solute particles, so fewer particles escape to
the gas phase  fewer particles means
lower pressure.
 Solute particles also reduce the number of
solvent particles at the surface, making
vaporation more difficult.

What is boiling point?

What is a boiling point?


The temperature at which a liquid becomes a gas
When does boiling happen?
When the vapor pressure above a solution equals
the atmospheric pressure
 Vapor pressure increases as temperature
increases, because molecules are moving faster
and changing to gas phase

Boiling point elevation (BPE)
The boiling point of a solution is higher
than the boiling point of the pure solvent.
 Solvent molecules are attracted to the
solute particles, and that attraction (IMF)
requires energy to break.
 Solute particles also reduce the number of
solvent particles at the surface, making
evaporation more difficult.

Boiling point elevation is caused
by lowered vapor pressure
Solutes block solvent
particles at the surface
from escaping, which
reduces vapor pressure
More kinetic energy is
required for solvent
particles to evaporate,
to raise the vapor
pressure to atmospheric
pressure (boil)
More energy needed
means higher
temperature to boil
Air pressure at different altitudes
affects the boiling point of water

If water boils at a lower temperature, why do food packages
recommend increased cooking times?
Freezing point depression (FPD)
When a solute is present, the orderly pattern
that the pure solvent normally forms
becomes disrupted.
 More kinetic energy must be lost from the
solution for it to solidify (freeze).
 This lowers the freezing point of the
solution.

Solutes interfere with solid
structure pattern during freezing
Some animals adapted to utilize
freezing point depression
Rainbow smelt survive winter
under ice by producing antifreeze proteins and glycerol
(non-electrolyte)
Spring peeper frogs can increase
blood [glucose] by massive
breakdown of liver glycogen to
survive temporary freezes
Osmosis
Osmosis is the net flow of solvent molecules
from higher solvent concentration to lower
solvent concentration.
 Net flow stops once the mass of liquid on the
higher side exerts a pressure
equal to the flow.
 This pressure is known as
osmotic pressure.

Osmotic pressure

When the concentrations of solutes are different
on either side of a semi-permeable membrane,
water is pressured to flow across the membrane
to try to equalize the concentrations. [Note:
they will never be equal.]
Osmotic pressure example
Consider two beakers. One contains pure water,
and the other contains water and sugar (red).
If these two beakers were connected by a tube that
contained a semi-permeable membrane, water
molecules would flow from the side where [water]
is higher to the side where [water] is lower.
This flow of water, from the higher [water] lower
[water], results in a change in volume of the two
sides. The side that also has sugar has a larger
volume.
Osmotic pressure in dialysis

As the solute concentration increases, more
solvent will flow into the solution,
increasing the osmotic pressure

More liquid on one side means more pressure
Osmosis in
blood cells
A cell membrane is
a semi-permeable
membrane. Water
can flow in and out
of the cell based on
the solutes in its
environment.
Electrolytes

Compounds that conduct an electric current in
aqueous solution or in a molten (liquid) state.

An electrolyte solution contains charged
particles (ions) which can move.

Any salt (ionic compound) dissolved in water
is an electrolyte

NaCl, KI, Mg(NO3)2, etc.
The degree to which a compound
conducts depends on ionization

Strong electrolytes—a large portion of the
solute exists as ions, e.g. aqueous solutions of
most ionic compounds (ionic)

Weak electrolytes—these are solutions in
which only a small portion of the solute exists
as ions (WEAK acids and bases)

Non-electrolytes—compounds that do NOT
conduct electricity either when molten or in
aqueous solution (covalent, organic, gases,
distilled water)