Colligative properties

Colligative property- a property of a solution that depends on concentration of solute (the number of solute particles dissolved) but NOT on type of solute Colligative property- a property of a solution that depends on concentration of solute (the number of solute particles dissolved) but NOT on type of solute 3 colligative properties: 3 colligative properties: Vapor pressure reduction Vapor pressure reduction Boiling point elevation Boiling point elevation Freezing point depression Freezing point depression

Vapor pressure The pressure DIRECTLY above the surface of a liquid exerted by the molecules that have evaporated The pressure DIRECTLY above the surface of a liquid exerted by the molecules that have evaporated LOW vapor pressure = not much evaporation occurring LOW vapor pressure = not much evaporation occurring HIGH vapor pressure = lots of evaporation HIGH vapor pressure = lots of evaporation

Vapor pressure reduction Solute interferes with vaporization of water, therefore LOWERS the pressure of the vapor pressing down on surface Solute interferes with vaporization of water, therefore LOWERS the pressure of the vapor pressing down on surface

How does this influence boiling point & freezing point? Boiling Point: The temperature at which the vapor pressure of the liquid phase equals atmospheric pressure. Boiling Point: The temperature at which the vapor pressure of the liquid phase equals atmospheric pressure.

Boiling-Point Elevation Boiling point elevation: The difference in temperature between the boiling point of a SOLUTION and the boiling point of the pure solvent (). Boiling point elevation: The difference in temperature between the boiling point of a SOLUTION and the boiling point of the pure solvent (water). Because of the decrease in vapor pressure, additional kinetic energy must be added to raise the vapor pressure of the liquid phase of the solution to atmospheric pressure to initiate boiling. Because of the decrease in vapor pressure, additional kinetic energy must be added to raise the vapor pressure of the liquid phase of the solution to atmospheric pressure to initiate boiling.

Boiling point Elevation Since vapor pressure is lowered, boiling point is elevated Since vapor pressure is lowered, boiling point is elevated

Application: Adding salt to water while cooking pasta Water does not boil faster Water does not boil faster Water is boiling at a higher temperature so pasta cooks faster Water is boiling at a higher temperature so pasta cooks faster

Freezing-Point Depression Freezing-Point Depression: The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent (). Freezing-Point Depression: The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent (water). The presence of a solute in water disrupts the formation of the orderly pattern of ice. Therefore more kinetic energy must be withdrawn from a solution than from the pure solvent to cause the solution to solidify. The presence of a solute in water disrupts the formation of the orderly pattern of ice. Therefore more kinetic energy must be withdrawn from a solution than from the pure solvent to cause the solution to solidify.

Freezing Point Depression

Freezing point depression Since vapor pressure lowered, freezing point is LOWERED Since vapor pressure lowered, freezing point is LOWERED

Freezing point depression

Freezing-Point Depression The magnitude of the freezing-point depression is proportional to the number of solute particles dissolved in the solvent and does not depend upon their identity. The magnitude of the freezing-point depression is proportional to the number of solute particles dissolved in the solvent and does not depend upon their identity. Which would be a better salt for putting on icy roads, NaCl or CaCl 2 ? Which would be a better salt for putting on icy roads, NaCl or CaCl 2 ?

Therefore… Because freezing point of water goes down, we can make things like Because freezing point of water goes down, we can make things like ICE CREAM!!!! ICE CREAM!!!!

Ice cream directions Partner 1 -ice 1. Grab large ziplock bag 2. Fill ½ way with ice 3. Add 1 scoop of salt Partner 2- food 1. Grab small ziplock bag 2. Add 1 cup milk 3. 2 tablespoons sugar 4. 1 teaspoon vanilla 5. Squeeze air out of bag and shut tight!!! Smaller bag goes in larger bag– agitate & toss (about 10 minutes) Once milk has hardened, 1 person eats out of bag, 1 person eats out of bowl Do not throw away spoons or ice