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Vapor Pressure Lowering
The escaping tendency of a solvent is measured by its vapor pressure.
Vapor pressure measures the concentration of solvent molecules in the gas phase.
Adding a nonvolatile solute lowers the vapor pressure of the solvent since a smaller proportion of the molecules at the surface of the solution are solvent molecules, fewer solvent molecules can escape from the solution compared to the pure solvent.
The quantitative relationship between vapor pressure lowering and concentration in an ideal solution is stated in Raoult's Law.
When two liquids, such as a solvent and a solution, are separated by a semipermeable membrane that allows only solvent molecules to pass through, then there is a net transfer of solvent molecules from the solvent to the solution. This process is called osmosis.
Osmosis can be stopped by applying pressure to compensate for the difference in escaping tendencies. The pressure required to stop osmosis is called osmotic pressure.
In dilute solutions, osmotic pressure is directly proportional to the molarity of the solution and its temperature in Kelvin.