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What are Colligative Properties?
Some physical properties of solutions differ in important ways from those of the pure solvent. For example, pure water freezes at 0°C, but aqueous solutions freeze at lower temperatures. Ethylene glycol is added to the water in radiators of cars as an antifreeze, to lower the freezing point of the solution. It also raises the boiling point of the solution above that of pure water, permitting operation of the engine at a higher temperature.
The lowering of the freezing point and the raising of the boiling point are examples of physical properties of solutions that depend on the quantity (concentration) but not the kind of solute particles. Such properties are called colligative properties. (Colligative means "depending on the collection"; colligative properties depend on the collective effect of the number of solute particles.) In addition to freezing-point lowering and boiling-point elevation, there are two other colligative properties: vapor-pressure reduction and osmotic pressure.
The rate at which molecules leave the liquid surface for the gas phase equals the rate at which gas-phase molecules return to the surface of the liquid. A nonvolatile solute added to the liquid reduces the capacity of the solvent molecules to move from the liquid phase to the vapor phase. At the same time, however, there is no change in the rate at which solvent molecules in the gas phase return to the liquid. The shift in equilibrium due to the solute reduces the vapor pressure over the solution. The vapor pressure over the pure solvent is thus higher than that over the solution.
Now that you understand the mechanism, you can observe how the solvent moves from the beaker containing pure water to the one containing the sugar solution. Compared to the vapor pressure required for the solution, a pure solvent requires a higher vapor pressure to attain equilibrium. Therefore, just as a pure solvent tries to achieve equilibrium by creating vapor, a solution tries to achieve equilibrium by eliminating molecules from the vapor phase. There is a net transfer of solvent molecules from the pure solvent to the solution. Until no more free solvent is present, the process is repeated.
A nonvolatile solute's concentration determines how much it decreases the vapor pressure. The effect of a solute is doubled by doubling its concentration. In actuality, whether the solute particles are neutral or charged, the overall concentration will decrease the vapor pressure in a manner that is essentially proportionate.
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