1. Drill – 4/11/08
What two factors determine if a substance is in the solid, liquid, or gas phase?
How would you define boiling point? Melting point?

2. Colligative Properties
Chapter 13, Section 2

3. What is a colligative property?
A property that depends on the concentration* of solute particles – NOT the identity of the particle.
Vapor Pressure Reduction
Boiling Point Elevation
Freezing Point Depression
Osmotic Pressure
* Concentration will be in terms of molality (m)

4. Vapor pressure is pressure caused by molecules in the gas phase that are in equilibrium with the liquid phase. (in a closed system particles go back and forth between phases)

5. Vapor Pressure Reduction
The pressure of the vapor over a solvent is reduced when a solute is dissolved in it. Vapor pressure reduction is directly related to concentration of the solution.

6. Why does this occur?
Increasing solute particles reduces the proportion of solute to solvent. Fewer solvent particles will be available to leave the solution and enter the gas phase.
The solution will remain a liquid over a larger temp. range.

7. Phase Diagram
Shows us what phase the solvent will be in at a given temperature and pressure.
If we add a solute that lowers the vapor pressure, then our phase diagram adjusts…

9. Boiling Point Elevation
The boiling point of a solvent is raised when a solute is dissolved in it. It is directly related to the concentration of the solution.
Boiling point – the temp. at which vapor pressure of a liquid is equal to atmospheric pressure

10. The change in boiling point is proportional to the molality of the solution:
Tb = Kb  m
Kb is the molal boiling point elevation constant, a property of the solvent. It has the units ºC/m.
Tb is the boiling point elevation. It is the difference between the boiling points of the pure solvent and that of the solution.

11. Freezing Point Depression
The freezing point of a solvent is lowered when a solute is dissolved in it. Freezing point depression is directly related to concentration of a solution.
Freezing point – the temp. where vapor pressure of the solid and liquid phases are the same.

12. The change in freezing point can be found similarly:
Tf = Kf  m
Here Kf is the molal freezing point depression constant of the solvent.

13. Osmotic Pressure () The pressure required to prevent osmosis.
Osmosis is the flow of particles from a dilute solution to a concentrated one across a semi permeable membrane.