1. Colligative properties
The properties of a solution change when you change its concentration... (duh!)
Any property of a solution that changes with its concentration is known as...
Colligative properties

2. Types of colligative properties
Boiling point elevation
Freezing point depression
Osmotic pressure increase
Increased electrical conductivity

3. Less air up here = low air pressure
More air down here = high air pressure

4. Increased motion of H2O molecules due to increased heat causes the pressure within water (vapor pressure) to increase as well.
When vapor pressure is greater than the air pressure forcing it into the container, the water molecules change into a gas.
Air pressure pushing down on the water
Vapor pressure pushing up into the air

5. Boiling point elevation
But what if there were not as many water molecules on the surface of the water to vaporize?
Boiling point elevation

6. Boiling point elevation
concentration of a solution =  boiling point of the solution
Which means:
A concentrated solution needs more heat energy to raise its vapor pressure and begin to boil.
Why?
The greater amount of solute molecules (specifically on the surface of the solution) reduces the amount of solvent molecules that can vaporize out of the solution!

7. Tb (solution) - Tb (pure solvent) = Kbm
How to calculate the boiling point of solutions of various concentrations:
Tb (solution) - Tb (pure solvent) = Kbm
Tb = boiling point
(The Tb(pure solvent) for water is 100°C)
Kb = 0.52°C/m for water
m = the effective molality (m) of the solute
(The effective molality (m) is a measurement of the number of moles of solute per kilogram of solvent.)

8. Why molality (m)?
Since a solution’s boiling point (Tb) can be changed by the amount of its solute, we need to know how many particles will be floating around inside the solution.

9. How to calculate molality:
If the solute is a covalent compound, use its regular molality as the effective molality (m)
For example, if you have a 0.5 m solution of C6H12O6, its effective molality is 0.5 m
***Remember! Covalent compounds do not break apart from each other within a solution (unlike ionic compounds.)
If the solute is an ionic compound, multiply its regular molality by the number of ions within the compound.
For example, if you have a 0.8 m solution of MgCl2, the effective molality would be 2.4 m (0.8 x 3 ions).

10. What is the boiling point of a 1.25m solution of C6H12O6?
Time to practice!
What is the boiling point of a 1.25m solution of C6H12O6?
Tb (solution) - Tb (pure solvent) = Kbm
Tb (solution) – 100 = 0.52 (1.25)
Tb (solution) – 100 = 0.65
Tb (solution) – =
Tb (solution) = °C

11. What is the boiling point of a 1.0 m solution of Ca(NO3)2?
Time to practice!
What is the boiling point of a 1.0 m solution of Ca(NO3)2?
Tb (solution) - Tb (pure solvent) = Kbm
Tb (solution) – 100 = 0.52 (3.0)
Tb (solution) – 100 = 1.56
Tb (solution) – =
Tb (solution) = °C
Since this is an ionic compound, we multiply the 1.0 m regular molality by three (3) which is the number of ions in Ca(NO3)2

12. It’s called “boiling point elevation” for a reason!
Remember!
It’s called “boiling point elevation” for a reason!

13. Freezing point depression
How does the additional solute in a solution affect its freezing point?
Freezing point depression

14. Freezing point depression
concentration of a solution = freezing point of the solution
Which means:
Solutions freeze at lower temperatures than pure solvents
Why?
The extra solute molecules “get in the way” of the solvent molecules. These extra solute particles keep the liquid from freezing at the normal freezing point of solvents.

15. Tf (pure solvent) – Tf (solution) = Kfm
How to calculate the freezing point of solutions of various concentrations:
Tf (pure solvent) – Tf (solution) = Kfm
***Take note that the Tf for the pure solvent and for the solution have been reversed from the boiling point elevation equation.
Tf = freezing point (The Tf(pure solvent) for water is 0°C)
Kf = 1.86°C/m for water
m = the effective molality of the solute

16. The same rules apply for calculating molality!
For covalent compounds
use its regular molality as the effective molality.
For ionic compounds
multiply its regular molality by the number of ions within the compound

17. What is the freezing point of a 1.25m solution of C6H12O6?
Time to practice!
What is the freezing point of a 1.25m solution of C6H12O6?
Tf (pure solvent) – Tf (solution) = Kfm
0 – Tf (solution) = 1.86(1.25)
0 – Tf (solution) = 2.3
-Tf (solution) = 2.3
Tf (solution) = -2.3 °C

18. What is the freezing point of a 1.0 m solution of Ca(NO3)2?
Time to practice!
What is the freezing point of a 1.0 m solution of Ca(NO3)2?
Tf (pure solvent) – Tf (solution) = Kfm
0 – Tf (solution) = 1.86(3.0)
0 – Tf (solution) = 5.6
-Tf (solution) = 5.6
Tf (solution) = -5.6°C
Since this is an ionic compound, we multiply the 1.0 m regular molality by three (3) which is the number of ions in Ca(NO3)2

20. Colligative property #3: Osmotic pressure increase
Osmotic pressure = pressure applied to a solution to keep water from moving across a semipermeable membrane (osmosis).
concentration of a solution = osmotic pressure of the solution

21. Definitions... Semipermeable membrane Osmosis
Any “barrier” that will only allow solvents
(not solutes) to move through them.
Osmosis
Water (solvent) flows from the solution with the lower solute concentration into the solution with higher solute concentration.
This flow will continue until the concentrations on both sides are equal.

22. Which direction will the solvent (water) flow?
Practice time...
Imagine having two containers of equal volume separated by a semipermeable membrane.
Solution A has only 10 molecules of NaCl while Solution B has 100 molecules of NaCl.
Which direction will the solvent (water) flow?

23. Practice time...
Solution A = lower solute concentration so it will allow water to pass through the membrane!
The excess solute in Solution B “plugs up the holes” within the membrane causing less water to flow.

24. Colligative property #4:
Increased conductivity of electricity
number of electrolytes = conductivity of the solution
Electrolytes = dissolved ionic compounds; able to move electrons and charges around the solution due to their unbalanced numbers of electrons