1. Colligative Properties of Solutions
14.4: Pgs

2. Colligative Properties
Physical properties of solutions differ from the solvent they were made from
Differences are due to the number of solute particles in solution (not the type of solute)
These differences in physical properties are called colligative properties
Colligative properties include:
vapor pressure
freezing point
boiling point
Osmotic pressure

3. 1. Vapor Pressure Review: What is vapor pressure? Volatile vs.
Nonvolatile substances
Define volatile vs. nonvolatile

4. 1. Vapor Pressure
Ions and water molecules interact so that less are able to leave the surface, creating a lower vapor pressure.

5. 1. Vapor Pressure
Solutions that contain nonvolatile solutes will have a LOWER vapor pressure than the pure solvent
Glucose and sodium chloride are both nonvolatile solutes
Example – salt water solution

6. 1. Vapor Pressure
The decrease in vapor pressure (and all colligative properties) is proportional to the number of solute particles in the solution
NaCl fully ionizes into 2 ions, while the glucose molecules do not break apart when dissolving
Each NaCl will have a GREATER effect on vapor pressure because it dissociates into more particles than glucose

7. 2. Boiling-Point
Since adding a solute to water decreases the vapor pressure, what will happen to the boiling point?
It will increase!
Called boiling-point elevation

8. Calculations with boiling point
Δ T = Kb m i
Kb = (called the boiling-point-elevation constant), depends only on the solvent
m = molal concentration of the solution
i = number or parts the solute breaks into

9. 3. Freezing-Point Depression
Presence of solute in water disrupts the freezing pattern
Need to withdraw MORE energy from a solution for it to solidify
Called freezing-point depression

10. Calculations with freezing point
Δ T = Kf m i
Kf = (called the freezing-point-elevation constant), depends only on the solvent
m = molal concentration of the solution
i = number or parts the solute breaks into

11. Phase diagram showing the effect of nonvolatile solute on freezing point and boiling point.
Purple = solution
Blue = pure solvent

12. Table of Kb and Kf

13. A problem to consider….
An aqueous solution is m in glucose. What is the boiling point and freezing point for this solution?
Table 12.3 gives Kb and Kf for water as oC/m and 1.86 oC/m, respectively. Therefore,
This screen needs to be all white so we can show work on it!
The boiling point of the solution is oC and the freezing point is –0.0413oC.

14. 4. Osmotic Pressure Solute concentration effects osmosis rate
Osmosis: when a solvent moves from an area of high concentration to an area of lower concentration through a semipermeable membrane that allows only the solvent to pass.

15. An experiment with osmosis:
Osmotic Pressure = amount of additional pressure caused by water molecules what moved into the concentrated solution
Must overcome the gas pressure above the column to get the liquid to rise up
Osmotic pressure once again depends on the number of particles and not the type

16. Let’s Make Ice Cream! Part 2 – Individual Groups:
Measure ½ cup milk, 1-2 Tbsp sugar and ¼ tsp vanilla and put into small Ziplock bag
Seal and squeeze out air
Place sealed small bag in large bag (both lab groups go in one large bag)
Seal and squeeze air out of bag
Shake/mix mixture until a thick ice cream appearance
Takes about 5 – 8 minutes.
EAT!
Part 1 – Table Group:
Mass large Ziplock bag
Fill it 1/3 full with ice cubes and mass again.
Determine mass of ice cubes used
Put thermometer in ice, and record temp when it stabilizes
Mass 100g of salt in a paper cup and add to Ziplock bag, stirring occasionally
Record lowest temp reached
On day after colligative properties notes day