Colligative Properties of
Solutions
How do you get
from this…
…to this?
Add an ionic compound!
Colligative Properties
• Properties determined by the number of particles
in solution (So the concentration of solute
particles).
• NOT the type of solute particles (the chemical
identity or name of substance).
4 Colligative Properties:
1. Vapor Pressure Reduction (lowering)
2. Boiling Point Elevation
3. Freezing Point Depression
4. Osmotic Pressure
1. Vapor Pressure Reduction
Vapor Pressure
• Pressure exerted by a vapor (gas)
• http://www.youtube.com/watch?v=re9r0kzQp_M Vapor
Pressure (2 min clip)
• Ex. Cooking with a pressure cooker
Vapor Pressure Reduction
• Solute particles take up space in a solution.
• Solute particles on surface decrease # of solvent particles on
the surface.
• Less solvent particles can evaporate which lowers the vapor
pressure of a liquid.
2. Boiling point (BP) Elevation
• When a solute is dissolved in a solvent, the boiling point of
the solvent will elevate (become higher) than the normal
boiling point of the pure solute. (will need more heat to
boil)
•
1.
2.
3.
The amount the BP will be lowered depends on:
The concentration of the solution
concentration=  BP elevation
Whether or not the solution is an electrolyte
Electrolytes =  BP elevation
The identity of the solvent can change the amount of FP
depression
Example – BP Elevation
• Why do cooks add salt (solute) to their water (solvent)
when boiling water?
▫ To cook the food faster
• Explanation:
• Adding salt increases boiling point of water to higher
temperature. Adding salt to pasta water means it takes
longer to bring it to a boil, but the pasta will cook more
quickly due to the higher temperature.
More Examples – BP Elevation
• Antifreeze =a liquid added to the cooling system of a
car to increases the BP of the water in it so it doesn’t
overheat as fast in summer.
3. Freezing Point Depression (lowering)
• When a solute is dissolved in a solvent, the freezing
point of the solvent will depress (become lower) than
the normal freezing point of the pure solute
•
1.
The amount the FP will be lowered depends on:
The concentration of the solution
concentration=  FP depression
2. Whether or not the solution is an electrolyte
Electrolytes =  FP depression
3. The identity of the solvent can change the amount
of FP depression
Example – FP Depression
• Salt (solute) is added to melt ice (water = solvent)
• Works by lowering the freezing point of water.
Solid to liquid
More Examples – FP Depression
• Antifreeze =a liquid added to the cooling system of a
car to lower the FP of the water in it so it doesn’t freeze
solid.
• Some animals are able to survive freezing
temperatures by producing natural antifreezes.
▫ Natural antifreezes are just higher levels of alcohols (ex.
glycerol) and sugars (ex. glucose) in the cells.
• Examples are box turtles, painted turtles and garter
snakes, sugar maples and white spruce.
•
http://videos.howstuffworks.com/discovery/52100-frozen-planet-caterpillar-survives-frozen-death-video.htm
Electrolytes
• Electrolytes are substances that dissolve and break
apart into ions to create a solution that conducts an
electric current
▫ Sports drinks and salt water
▫ Ionic compounds are usually strong electrolytes because
they separate completely in water
• Each type of ion has an different effect on boiling
point and freezing points. Plus, if a solution has more
ions = a greater change
4. Osmotic Pressure
• Osmosis – a flow of solvent molecules (Ex. Water)
from a less concentrated to a more concentrated area
through a semi permeable material. The larger solute
(ex. Salt) molecules cannot pass.
• Ex. Plants – put in glass
filled with water. Water will
enter the plant. When water
gone, plant shrivels. Water
Leaves the plant back to the
glass.
Examples of Osmosis
• Becoming bloated after eating salty foods?
▫ Cells absorb water because of all the salt from the foods.
• Pickles are cucumbers that have been soaked in salt
solution. Cucumbers lose water and shrivel.
• Grapes to raisins… any dried fruit really.
• As a result bacteria tend to die so salt is used as a
preservative.
4. Osmotic Pressure
• Osmotic pressure – pressure required to prevent
osmosis.
• Ex. Hospitals give fluids intravenously that have same
osmotic pressure as what is in your cells to prevent
cells from bursting or shriveling up.