1. Colligative Properties Physical properties affected by dissolved solute particles Type of solute doesn’t matter. Number of particles does matter. Colligative means “depending on the collection”

2. Colligative Properties Occur with nonvolatile solutes – ones that have little tendency to become a gas 4 colligative properties: –Vapor pressure lowering –Boiling point elevation –Freezing point depression –Osmotic pressure

3. Colligative Properties Electrolytes are molecules that ionize or fall apart when in solution; nonelectrolytes do not fall apart or ionize Electrolytes will have a greater impact on the 4 colligative properties Why? –Sugar (C 12 H 22 O 12 ) remains as one molecule when dissolved. –Salts like MgCl 2 dissociate in water so one mole of MgCl 2 actually forms 3 moles of ions.

4. Vapor Pressure Lowering Pressure in a CLOSED container exerted by the particles of liquid that have escaped and become gas particles Greater number of escaped particles = greater vapor pressure The addition of a solute LOWERS the vapor pressure – WHY? The solute molecules get in the way of the solvent molecules trying to escape at the surface of the liquid.

5. Boiling Point Elevation A liquid boils when it’s particles can escape as a gas (or when the vapor pressure is equal to the atmospheric pressure) Ex: salt in water when cooking pasta antifreeze in radiators When a solute is added, the boiling point is driven higher – WHY? The solute molecules get in the way of the solvent molecules trying to escape at the surface of the liquid.

6. Freezing Point Depression A liquid freezes when its particles do not have enough energy to overcome the intermolecular forces between them. So the molecules are pulled closer together and take on a defined shape. When a solute is added, the freezing point is driven lower – WHY? The solute molecules get in the way and make it harder for the solvent molecules to move closer and become a solid Ex: salt & sand on icy roads ethylene glycol on icy airplanes

7. Calculating b.p. elevation and f.p. depression Because the type of solute doesn’t matter, you only need to know the amount of solute Elevation & depression happen at a constant rate depending on the amount of solute added (number of moles) ∆T b = K b m (where K b & K f are the constants) ∆T f = K f m

8. Try It If you have.625 m aqueous solution with a nonvolatile, nonelectrolyte solute, what would the boiling point and freezing point be?

9. Try It Calculate the freezing point and boiling point of a solution that contains 105.4 g NaCl dissolved in 750.3 ml H 2 O.

10. Osmotic Pressure Osmosis is a natural process where the solvent flows across a membrane – always flows from where there is more solvent to where there is less solvent. Adding solute to one side of the membrane will cause more solvent to flow that way. This increases the osmotic pressure on that side of the membrane. Ex: kidney dialysis uptake of nutrients by cells