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Vapor Pressure Lowering - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Vapor pressure lowering is a colligative property of solutions. The vapor pressure of a pure solvent is greater than the vapor pressure of a solution containing a non volatile liquid. This lowered vapor pressure leads to boiling point elevation.

Alright so one of the gas properties that we're going to be discussing is vapor pressure lowering. So and we're talking about a pure solvent like water, at the surface of water it will evaporate and go into the vaporous state. So the water molecules at the surface are going to go back and forth, back and forth in equilibrium into the vapor around it and back to liquid state. So what we call vapor pressure is going to have most of pressure of gas on top of it. But let's say we put something in it and make it a solution, okay so I put some sort of like particle in this, that made of that will be able to dissolve and what happens when something dissolves is that the water molecules are attracted with whatever it is that's being dissolved and they would rather be around that particular particle than escape into the vaporous state. So actually the number of particles that are going to escape into the vapor is actually going to be lowered. Because depending on how many particles of solution I have, so the vapor pressure this pure solvent is greater than the vapor pressure of a solution containing a non-volatile liquid. So why did I say non-volatile liquid? When something is volatile is actually able to evaporate very quickly. So if I have something that's volatile in here that will actually make it escape a lot easier.

But the fact that these are non-volatile meaning that they like to be in liquid rather than gaseous state, then they're going to make the vapor pressure within this solution lower that it was originally. Okay, so let's talk about what the different particles we could put in there. That actually makes a difference, so we have sodium chloride, we have calcium chloride and then we have sugar molecules. Now the main difference between these guys is, these guys are electrolytes meaning they'll separate in solution and this guy is not, it's not electrolyte meaning it'll stay together. So when I say 1 molar of sugar I mean 1 molar of particle. So here when I have this, this actually will end up being 1 molar okay fine. So when I put this in solution, this will break up into 3 different ions, 1 calcium and 3 chloride ions. So when I put 1 molar in it actually ends up being 3 molar of particles.

Here I have sodium chloride this is 2 particles, 1 sodium ion and 1 chloride ion so when I put 1 molar in there it's actually going to break up into 2 [moles] so this one it's going to actually affect the vapor pressure than most, then this guy and lastly this guy. So it actually does make a difference of what you put in solution and how much you put in there and how much the vapor pressure is actually going to lower.