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Vapor Pressure - 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 is the pressure caused by the evaporation of liquids. Three common factors that influence vapor press are surface area, intermolecular forces and temperature. The vapor pressure of a molecule differs at different temperatures.

Alright, so when you're dealing with any sort of liquid substance you'll inevitably get some sort of vapor pressure and what vapor pressure is, is the pressure exerted by vapor over a liquid. Let's take our liquid water for example.

What actually happens is at the surface of the water, we have these water particles and water particles at the surface will, it's constantly escape into the atmosphere creating some sort of pressure over top of the liquid. And go like back and forth, back and forth in equilibrium actually from the gaseous, sorry, from liquid phase to the gaseous phase and back and forth at the surface. So let's say we cupped this container here.

Now, the pressure of the gas inside this chamber right here on top of the water part, water mo- liquid water is called the vapor pressure. So this is our vapor pressure. So whatever we measure in here is what we're going to call our vapor pressure. Okay. So let's say we actually increase the temperature of the liquid, what's going to happen to vapor pressure? Well, we're going to create a little flame underneath here. That's my flame. Okay? That's my candle creating lots of flame and it's getting very very hot. And what's going to happen to these water molecules especially on the surface, they're actually going to escape much easier. They're going to move around. Lots of, this thermal energy is going to create much more kinetic energy within the liquid and they're actually going to escape into the gaseous phase much easier and sending more particles here, so that this vapor pressure increases.

So, as temperature increases, the vapor pressure also increases. Okay? So let's say instead of a flame, let's say I put this on ice. Instead of a flame here I'm putting it on ice. These are my ice cubes, I'm cooling it down. I going to just write the word ice to make it clear. Alright, so what's happening is I'm lowering the energy within the system. So the water molecules are slowing down and so what that means is they're more likely to stay within the liquid phase than go into the gaseous phase. So actually going to, vapor pressure is going to start lowering. There are going to be more in here than there are here. So as temperature decreases, vapor pressure too. Okay?

So any time you're talking about vapor pressure, you're actually talking about the pressure of the gas exerted by the liquid over top of it. So this we have in here is our vapor pressure.