1. Colligative Properties The presence of a solute affects the properties of the solvent. –Example: Freezing point of water Colligative properties depend on the concentration (amount) of solute but not the identity of the solute. Two of the most common colligative properties are: 1. Freezing point depression 2. Boiling point elevation

2. Freezing Point Depression Adding a solute causes the freezing point of the solvent to become lower. The amount the freezing point is lowered is directly related to amount of solute present. Application – Salting roads in the winter The freezing-point depression, ∆t f, is the difference between the freezing points of the pure solvent and a solution in that solvent.

3. The molal freezing-point constant (K f ) is the freezing- point depression of the solvent in a 1-molal solution. ∆t f = K f m

4. Sample Problem What is the freezing-point depression of water in a solution of 9.00 g of glucose, C 6 H 12 O 6, in 200.0 g of water? (The molal freezing point constant for water is -1.86°C/m)

5. Boiling Point Elevation Adding a solute increases the boiling point of the solvent. The amount the boiling point is raised is directly related to amount of solute present. The boiling-point elevation, ∆t b, is the difference between the boiling points of the pure solvent and a solution of that solvent.

6. The molal boiling-point constant (K b ) is the boiling- point elevation of the solvent in a 1-molal solution. ∆t b = K b m Sample Problem The boiling point of an aqueous solution is 100.75°C. What is the molality of the solution? (The K b for water is 0.51°C/m)

7. Wrapping It Up Reflect on what you have learned concerning the today’s topic. Respond to the following… –What is the most important thing you learned today? –What is one question you would still like answered? –What is a way what you have learned today connects with what you knew before?