Colligative Properties Notes
Colligative Properties
The presence of solutes cause solution to behave differently than their pure solvent.
These new properties are called colligative properties.
Colligative properties depend on the amount of particles dissolved, NOT the type of
particles.
Vapor Pressure Depression
The presence of a solute often causes the vapor pressure to drop.
Thus is because the solute particles fill positions that are normally occupied by solvent
molecules. Thus fewer solvent molecules escape the solution.
Boiling Point Elevation
The presence of solutes often raise the boiling point of the solutions. 1 mole of particles
in 1 kg of water elevates the boiling point of water by 0.512 degrees C. The more
concentrated a solution is, the greater the boiling point elevation will be.
Freezing Point Depression.
Solutions freeze at lower temperatures than their pure solvents. A 1 molal solution of
sugar in water freezes at -1.86 deg. C. The more concentrated the solution, the more the
freezing point will be lowered.
Antifreeze
Antifreeze affects both the boiling and freezing points of water. It prevents water in the
radiator from freezing in the winter by lowering the freezing point and it prevents water
from boiling in the summer because it also raises the boiling point of water.
Osmotic Pressure
Osmosis is the process in which water moves across a semi-permeable membrane.
It occurs because the water molecules are trying to reach equilibrium on either side of the
membrane. Osmotic pressure is the amount of pressure required to prevent osmosis from
occuring. The more solute particle there are in a solution, the higher the osmotic pressure
will be.
Colloids
Solutions are homogeneous mixtures that do not separate. Homogeneous means that the
particles are evenly distributed in the solution. A supsension contains larger particles that
eventually settle out of the mixture (Ex. silt). A colloid is a mixture that contains small
particles dispersed in a medium. (Ex. fog)
Colloids can be solid, liquid, or gas.
Foams, aerosols, and gels are common colloids.
Tyndall Effect
Named for John Tyndall, the physicist who demonstrated that the particle in colloids
were large enough to scatter light waves. A beam of light will pass through a solution
without being seen. The same beam will be visible when it passes through a colloid
because the particles are larger and disturb the light waves.