The Desire to Evaporate
Leave a glass of water on the counter, and it will eventually disappear. Even though the water isn't boiling, some of the molecules at the surface have enough kinetic energy to break free and turn into a gas.
If you put a lid on that glass, those escaped gas molecules will bounce around and create a physical pressure against the lid. This is called the Vapor Pressure ().
- Liquids that evaporate easily (like acetone or rubbing alcohol) have a High Vapor Pressure.
- Liquids that evaporate slowly (like water or motor oil) have a Low Vapor Pressure.
Vapor Pressure Lowering
What happens if you dissolve a non-volatile chemical (like salt or sugar) into the water? The solute particles physically block the surface of the liquid, meaning fewer water molecules have the space to escape.
Because fewer molecules escape into the air, the vapor pressure inside the sealed glass physically drops. This is another Colligative Property, and it explains exactly why boiling points are elevated!
The Lowering Equation (Raoult's Law for Solutes)
To calculate the new vapor pressure of the solution, you simply multiply the pure solvent's original vapor pressure by the mole fraction of the solvent remaining in the mixture.