Introduction to Colligative Properties


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Colligative properties are properties of a solution which are dependent only on the number of particles in solution, not their nature.


There are four colligative properties:
Osmosis is the net movement of water in the direction opposite to the diffusion of solute. It typically will involve the use a semipermeable membrane. Osmosis is an important biological function and, when it is forced to run in reverse (called "reverse osmosis" as if that's a suprise) it's an important source of fresh water.
Boiling point elevation is exactly what its name says. When you make a solution, the solution will begin to boil at a temperature ABOVE that of the pure solvent. Freezing point depression is also exactly what its name says. The solution will begin to freeze at a temperature BELOW that of the pure solvent. Anti-freeze, for the car radiator, is often marketed as "winter-summer protection." This isn't because the company is being nice and giving you a two-for-one deal. It is because you get one when you get the other. They cannot be separated. Certain fish in the Artic have anti-freeze proteins in their blood. Many cities sprinkle salt on icy roads to melt the ice. You put salt on the ice when making homemade ice cream. All this is done to lower the freezing point below the zero degrees Celsius of pure water.

Many pure liquids (and even some solids) will evaporate to a slight extent at almost any temperature. This will produce a slight gas pressure above the pure liquid. This is called vapor pressure and was discovered by John Dalton.

Imagine this: a rigid-walled container which is empty, a vacuum. Now, introduce into it some pure liquid water. Some, not all, of the liquid evaporates into the gas state. In a short time, there will come an equilibrium between the gas and the liquid. This pressure can be measured and it is called vapor pressure.

The vapor pressure is dependent on temperature only.

In a solution, say sugar in water, the vapor pressure of the solution is lower than that of the pure solvent at the same temperature.

This lowering is exploited commercially in vapor pressure osmometry for molecular weight determination. Commercial equipment gets by with a few microliters of solvent. Further, boiling point elevation is a direct result of vapor pressure lowering, so your car radiator is an application of vp lowering. You may wish to ponder on the reason why before going to the explanation.

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