What is "Vapor Pressure Depression" ???

[Cosmic]

I'm really confused about the word "vapor pressure depression."

Is it referring to the total vapor pressure? Because as seen in one example, the total vapor pressure for a mixture can be higher relative to the pure vapor pressure of some substance: ex.) mixture of ethanol/methanol (130 torr) vs. pure ethanol (120 torr).

Looking at it from a different perspective (one The Princeton Review uses to explain "Vapor Pressure Depression"), each solvent (ethanol or methanol) is a fraction of the total moles of a mixture so vapor pressure is less than what it would be for pure solvent: ie. Ethanol is only 80 torr in a mixture as opposed to 120 torr (pure Ethanol). In this case, "vapor pressure depression" applies to the fact that vapor pressure of ethanol (or methanol) isn't as high as it would be in pure form (since it's now a mole fraction of the total mixture). But then the issue I have with this view is that it has nothing to do with the intermolecular forces.

I can understand how dissolving a salt into a solvent would decrease the vapor pressure since intermolecular forces increases (indirectly leading to the conclusion that boiling point will increase). However, TPR's perspective could lead to some confusion: For instance, a mixture of ethanol and methanol would have a lower boiling point (since the total vapor pressure (mixture) > vapor pressure of pure ethanol) even though the vapor pressure with regard to pure ethanol undergoes "vapor pressure depression" when placed in a mixture.

Depending on the perspective you take: vapor pressure depression may or may not correlate to an increase in boiling point.
I think it's probably best to disregard the whole term all together.

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[GreenRabbit]

Your question isn't a simple one.

When you're talking about a mixture, partial pressure is not the same as vapor pressure depression. Partial pressure is the pressure of one component of the mixture. It is estimated by Raoult's law, P = xP*, where P* is the vapor pressure of pure component, and x is the mole fraction of the component in the mixture. Raoult's law also tells us that the total pressure of the mixture is the sum of the individual partial pressures.

So in an ideal water-ethanol mixture, we expect a linear decrease in vapor pressure as we go from alcohol (P* = 45torr @20C) to water (P* = 17torr @20C). The total vapor pressure decreases because water has a lower vapor pressure than ethanol. This is not vapor pressure depression. It is simply a weighted average of the pure vapor pressures. A water-ethanol mixture will boil at a higher temperature than pure ethanol, which is true for any concentration of ethanol <95% ethanol. The qualifying statement is because of...

Deviations: In a non-ideal (or real) solution, the total vapor pressure is either lower or higher than predicted by Raoult's law. If the attraction between A and B is greater than their attraction to themselves, then a mixture will have a lower vapor pressure than expected. If the attraction between A and B is less than their attraction to themselves, then a mixture will have higher vapor pressure than expected. This is where intermolecular forces plays a role. This is called positive and negative deviation, and where the deviated line plateaus leads to positive and negative azeotropes respectively. Water-ethanol has positive deviation, and at the azeotrope the boiling point is lowest. So although from 100% ethanol --> 95% ethanol the total vapor pressure increases, the overall trend still decrease (again, because water has a lower P*)

"Vapor pressure depression" is an expression used not for mixtures, but for solutions containing nonvolatile solutes. As a colligative property, it refers to the vapor pressure of the solvent, and it is the same as the total vapor pressure (the solute being non-volatile has a vapor pressure of 0).