2. Colligative Properties

3. Vapor Pressure Lowering l The bonds between molecules keep molecules of a liquid from escaping into the vapor state. l In a solution, some of the solvent is busy keeping the solute dissolved. l This lowers the vapor pressure.

4. Vapor Pressure Lowering l Electrolytes form ions when dissolved. NaCl  Na + + Cl - 2 ions CaCl 2  Ca +2 + 2Cl - 3 ions l Calcium chloride would lower the vapor pressure more because it dissociates into more ions.

5. Boiling Point Elevation l The vapor pressure determines the boiling point. l The lower the vapor pressure, the higher boiling point. l It turns out that the boiling point of a solution is higher than the boiling point of the pure solvent.

6. Boiling Point Elevation l Salt water boils above 100ºC. KBr  K + + Br - 2 ions MgF 2  Mg +2 + 2F - 3 ions l Magnesium fluoride would raise the boiling point more because it dissociates into more ions.

7. Freezing Point Depression l Solids form when molecules make an orderly pattern. l The solute molecules break up the orderly pattern. l This makes the freezing point lower.

8. Freezing Point Depression l Salt water freezes below 0ºC. K 2 SO 4  2K + + SO 4 -2 3 ions NaCl  Na +1 + Cl - 2 ions l Potassium sulfate would lower the freezing point more because it dissociates into more ions.

9. Molality l Molality is another unit for concentration. m = l 1 kg = 1000 g moles of solute kilogram of solvent

10. Example l What is the molality of a solution with 9.3 mole of NaCl in 450 g of water? moles of solute kilogram of solvent m = 0.450 kg 9.3 mol m = 21 m

11. Problem l What is the molality of a solution with 11.3 mole of KBr in 650. g of water? (17.4 m)

12. Problem l What is the molality of a solution with 142 g of KBr in 750. g of water? (1.59 m)

13. Why Molality? l The size of the change in boiling point is determined by the molality.  T b = K b x m x n  T b is the change in the boiling point l K b is a constant determined by the solvent l m is the molality of the solution. l n is the number of ions the solute falls into when it dissolves.

14. What about Freezing? l The size of the change in freezing point is determined by the molality.  T f = -K f x m x n  T f is the change in the boiling point l K f is a constant determined by the solvent. l m is the molality of the solution. l n is the number of ions the solute falls into when it dissolves.

15. Example l What is the boiling point of a solution made by dissolving 1.20 moles of NaCl in 750. g of water? K b for water is 0.51. moles of solute kilogram of solvent m = 0.750 kg 1.20 mol m = 1.60 m

16. Example l What is the boiling point of a solution made by dissolving 1.20 moles of NaCl in 750. g of water? K b for water is 0.51. KbKb m Δ T b =(1.60)0.51 ΔT b = 1.63 °C n(2) So T b = 100 + 1.63 = 101.63 °C

17. Example l What is the freezing point of a solution made by dissolving 1.45 moles of CaCl 2 in 850. g of water? K f for water is 1.86. moles of solute kilogram of solvent m = 0.850 kg 1.45 mol m = 1.71 m

18. Example l What is the freezing point of a solution made by dissolving 1.45 moles of CaCl 2 in 850. g of water? K f for water is 1.86. - K f m Δ T f =(1.71)1.86 ΔT f = - 9.54 °C n(3) So T f = 0 + - 9.54 = - 9.54 °C

19. Colloids l Sometimes, mixtures are partway between true solutions and heterogeneous mixtures. l Such mixtures, called colloids, contain particles that are evenly distributed through a dispersing medium, and remain distributed over time rather than settling out.

20. Colloids l Familiar colloids include fog, smoke, homogenized milk, and ruby-colored glass.

21. Colloids l A beam of light can be used to distinguish a colloidal suspension from a true solution. This is referred to as the Tyndall Effect. l The Tyndall Effect is caused by reflection of light by very small particles in suspension in a trasnparent medium.

22. Colloids l A beam of light can be used to distinguish a colloidal suspension from a true solution. This is referred to as the Tyndall Effect. l The Tyndall Effect is caused by reflection of light by very small particles in suspension in a trasnparent medium.

23. Colloids l The Tyndall effect is seen below using a laser pointer. The glass on the left contains colloidal silver and the one on the right is water from the tap after the bubbles have settled out.