1. Colligative Properties of Solutions and Heterogeneous Mixtures
Sections 15.3 & 15.4

2. What are Colligative Properties?
Physical Properties that depend on the number of solute particles
Do NOT depend on the identity of the solute particles
Historically used to determine molar properties, especially molar mass

3. The Colligative Properties
Freezing Point Depression
Boiling Point Elevation
Vapor Pressure Lowering
Osmotic Pressure
Colligative Properties of Electrolytes

4. Freezing Point Depression
The freezing point of water is 0°C, but that melting point can be depressed by the adding of a solvent such as salt. The use of ordinary salt (NaCl) on icy roads in the winter helps to melt the ice from the roads by lowering the melting point of the ice because the solute particles interfere with the attractive forces among the solvent particles, preventing the solvent from entering the solid state at its normal freezing point.

5. Boiling Point Elevation
The boiling point of pure water is 100° C, and is the temperature at which its vapor pressure is equal to atmospheric pressure. The boiling point can be elevated by the adding of a solute such as salt. This means the solution must be heated to a higher temperature to supply the additional kinetic energy needed to raise the vapor pressure.
Ethylene glycol in antifreeze protects against freezing by lowering the freezing point and permits a higher operating temperature by raising the boiling point.

6. Vapor Pressure Lowering
Vapor Pressure is the pressure exerted in a container by liquid particles that have escaped the liquid’s surface and entered the gaseous state.
When a solute is dissolved in a solvent, the number of solvent molecules near the surface decreases, and the vapor pressure of the solvent decreases. This decreases the rate at which the solvent molecules of the liquid escape into the gas phase.
The Vapor Pressure of a Solution is Lower than that of the Pure Solvent

7. Osmotic Pressure
Depends on the number of solute particles in a given volume of solution. A solute is separated from a solution as it crosses a semi permeable barrier. Water can move in both directions across the membrane, the solute cannot. The additional water molecules on the solution side cause osmotic pressure.
The particles will move from an area of higher
solvent concentration to
lower solvent concentration

8. Colligative Properties of Electrolytes
Since colligative properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) should show greater changes than those of nonelectrolytes.

9. Heterogeneous Mixtures
Suspensions
A mixture containing particles that settle out if left undisturbed (cornstarch and water, fine sand and water)
Suspended particles can be filtered out

10. Heterogeneous Mixtures
Colloids
Mixture of intermediate size particles
Components cannot be separated by settling or filtration – they do not settle because they have polar layers surrounding them. The layers repel each other and prevent the particles from separating
examples: milk, beaten egg white, butter, mayonnaise

11. Characteristic Overview
Type of Mixture
Settle Out?
Relative Particle Size
Tyndall Effect
Suspension
Yes
Large
Colloid No
Small
Solution
Very Small
(dissolved)

12. Brownian Motion The jerky, random movements made by colloid particles
The dispersing medium particles form charged layers around the colloid particles. These charged layers repel each other and keep the particles from settling out.