1. Unit 4: Solutions Lesson 6

2. C11-4-11: Perform a lab to demonstrate freezing-point depression and boiling-point elevation. C11-4-12: Explain freezing-point depression and boiling-point elevation at the molecular level.

3. Colligative property – property that depends only upon the number of solute particles, and not upon their identity

4. Vapor pressure – pressure exerted by a vapor that is in dynamic equilibrium with its liquid in a closed system Solutions that contain solutes that are non-volatile will always have a lower vapor pressure than the pure solvent

5. Example: aqueous solution of NaCl Na & Cl ions are dispersed throughout the liquid water Surrounded by layers of associated water molecules  shells of water solvation Formation of shells reduces the number of solvent molecules with enough kinetic energy to escape as vapor Solution has a lower vapor pressure than the pure solvent at the same temperature

6. Ionic solutes that dissociate (e.g. salts) have greater effects on the vapor pressure than non-dissociating solute such as glucose Decrease in a solution’s vapor pressure is proportional to the number of particles the solute makes in solution Example: NaCl  2 molecules Glucose  1 molecule CaCl 2  3 molecules NaCl lowers vapor pressure 2x as much as glucose CaCl 2 lowers vapor pressure 3x as much as glucose

7. When a substance freezes the particles rearrange themselves to take on an orderly pattern Solutes in water have shells of solvation that disrupt this process Freezing requires more kinetic energy to be drawn from the solvent than would be drawn from the solvent in pure form Lowers the freezing point of the solution below that of the pure solvent This difference in temperature = freezing point depression Proportional to the number of solute particles dissolved in the solvent and does not depend upon their identity

8. Salting to reduce ice formation Materials in road salt, lower the freezing point of the ice  less ice Ethylene Glycol (anti-freeze) Added to water in the cooling systems of vehicles to lower the freezing point of the water to below zero Wood frog Uses glycerol to lower the freezing point within cells Allows fluid to freeze, but not the cells

9. Boiling point – temperature at which the vapor pressure of the liquid phase = atmospheric pressure Adding a non-volatile solute to a liquid solvent decreases the vapor pressure and raises the boiling point The decrease in vapor pressure requires additional kinetic energy for the solvent particles to overcome the attractive forces keeping them in the liquid

10. Boiling-Point Elevation – difference in temperature between the boiling point of a solution and the boiling point of the pure solvent Proportional to the number of solute particles dissolved in the solvent

11. Making candy or fudge Combine sugar & some other ingredients to water & boil the liquid As the water boils some becomes vapor  concentration of sugar in the mixture increases As the concentration increases so does the boiling point Colligative property  more solute particles = higher boiling point Use candy thermometer to tell you when you have the proper ratio of sugar:water to make a solid