# Colligative Properties Colligative Properties Are properties that depend

• Slides: 15

Colligative Properties

Colligative Properties…. Are properties that depend on the number of dissolved particles only. The type of dissolved particles does not matter

Freezing Point • The temperature at which a solution freezes from a liquid to a solid • Freezing point of pure water is 0 o C • Adding a solute to pure water lowers the freezing point below zero

Freezing Point is lowered when: • Salt is added to roads in winter, so they don’t get icy even if the temp is below zero • Antifreeze is added to engine fluids so they don’t freeze inside your engine and your car still starts when the temp is below zero

Take 1000 g of pure water And add one mole of dissolved particles, and you will lower the freezing point by 1. 86 degrees C. Remember: it does not matter what the dissolved particles are, only how many there are!!!

Boiling Point • The temperature at which a solution boils from a liquid to a gas • Boiling point of pure water is 100 o C • Adding a solute to pure water raises the boiling point above 100 degrees C

Boiling Point is raised when: • Salt is added boiling water to cook pasta faster (at a higher temperature) so the pasta does not get “soggy”. Instead it comes out “al dente” • Antifreeze is added to engine fluids so they don’t boil inside your engine and your car won’t overheat when temp is above 100 degrees C.

Take 1000 g of pure water And add one mole of dissolved particles, and you will raise the boiling point by 0. 52 degrees C. Remember: it does not matter what the dissolved particles are, only how many there are!!!

Some solutes are more effective than others at raising and lowering boiling and freezing points • I mole of sugar (covalent compound) dissolves in water to form 1 mole of dissolved sugar C 6 H 1206(aq) • 1 mole of Na. Cl (ionic compound) will dissolve to form 2 moles of dissolved particles: 1 mole of Na+ (aq) and I mole of Cl- (aq)

Some solutes are more effective than others at raising and lowering boiling and freezing points • 1 mole of Ca. Cl 2 (ionic compound) will dissolve to form 3 moles of dissolved particles: 1 mole of Ca+ (aq) and 2 moles of Cl- (aq) • In general, ionic compounds form more dissolved particles, so they are a better choice for adding to the water to change boiling or freezing points.

Vapor Pressure: Table H some molecules at the surface of a liquid may have enough energy to evaporate even if the liquid is below its boiling point. These escaping gas particles exert a pressure called VAPOR PRESSURE.

Vapor Pressure • The more easily the molecules evaporate, the higher the vapor pressure • The weaker the intermolecular forces, the more easily the molecules can separate and go into the gas phase, therefore the higher the vapor pressure

Vapor Pressure • The more easily the molecules evaporate, the higher the vapor pressure • The stronger the intermolecular forces, the less easily the molecules can separate and go into the gas phase, therefore the lower the vapor pressure

Using Table H • Which liquid has the highest vapor pressure at 50 degrees? • The lowest? • Which liquid has the weakest intermolecular forces? • The strongest?

Vapor pressure The boiling point of a liquid is defines as the temperature at which the vapor pressure of the liquid equals atmospheric pressure (1 atm or 101. 3 k. Pa) In other words, at that temp all the molecules change to gas phase (boil), not just the molecules at the surface