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”

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

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 (C12H22O12) remains as one molecule

when dissolved.– Salts like MgCl2 dissociate in water so one

mole of MgCl2 actually forms 3 moles of ions.

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.

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.

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

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)

∆ Tb = Kbm (where Kb & Kf are the constants)∆ Tf = Kfm

Try It• If you have .625 m aqueous solution with a

nonvolatile, nonelectrolyte solute, what would the boiling point and freezing point be?

Try It• Calculate the freezing point and boiling

point of a solution that contains 105.4 g NaCl dissolved in 750.3 ml H2O.

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 dialysisuptake of nutrients by cells