Colligative Properties Freezing Point Depression Boiling Point Elevation Vapor Pressure Lowers

Colligative Properties Properties that depend ONLY on the number of solute particles Do NOT depend on their identity

Ionic vs. Molecular Solutes Ionic solutes produce two or more ion particles in solution NaCl Na+ + Cl- Molecular solutes (covalent compounds) do not dissociate in solution C2H6 Ionic solutes have more particles, so they effect the colligative properties more than molecular solutes. The effect is proportional to the number of particles of the solute in the solution.

Learning Check How many particles do each of the following give upon solvation? NaCl CaCl2 Glucose (C6H12O6)

Vapor Pressure The particles in the vapor phase exert pressure, just like any other gas.

Vapor Pressure Lowering The solute particles are surrounded by and attracted to particles of solvent. WHY? This reduces the kinetic energy of the solvent particles, causing fewer particles to escape into the space above the liquid. The result is a lower vapor pressure.

Freezing Point Depression When a solute is added to a pure liquid, the resulting solution has a lower freezing point than the pure liquid. Ex: NaCl (salt) added to water Water freezes at 0oC, but when salt is added it will not freeze until a negative temperature. The more salt that is added, the lower the temperature must be.

Boiling Point Elevation When a solute is added to a pure liquid, the resulting solution has a higher boiling point than the pure liquid. Ex: NaCl (salt) added to water Water boils at 100oC, but when salt is added it will not boil until a higher temperature. The more salt that is added, the higher the temperature must be.