Vapor Pressure

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Vapor pressure is the pressure of the vapor over a liquid (and some solids) at equilibrium.

Now, what does that definition mean? I'm going to go through some explanation steps that, hopefully, give you a correct idea of vapor pressure.

1) Imagine a closed box of several liters in size. It has rigid walls and is totally empty of all substances.

2) Now, I inject some liquid into the box, but the box is not full of liquid. What will happen to the liquid?

3) That's right. Some, maybe all, of the liquid will evaporate into gas, filling the empty space. Now if all the liquid evaporates, we just have a box of gas. That's not what we want. So let's suppose that only some of the liquid evaporated and that there are both the liquid state and the gas state present in the box.

The gas that is above the liquid is called its vapor and it creates a pressure called vapor pressure.

What I mean is this: suppose you put a gas-pressure measuring device into the gas and never touched the liquid. Would a gas pressue be recorded? The answer is YES!

However, here is a key, key point. The vapor must be in contact with the liquid at all times. Remove the liquid and you just have a box of gas, you do not have vapor or vapor pressure.

Let's emphasize this point!!!! For vapor pressure to exist, the vapor (gas phase) MUST be in physical contact with the liquid (or solid) it came from. You CAN'T have vapor pressure without two phases being present and in contact!!!!!!

How is vapor pressure created? Another way to put it - how do molecules of the liquid become molecules of gas?

Each molecule in the liquid has energy, but not the same amount. The energy is distributed according to the Maxwell-Boltzmann distribution. Even if you don't know what that is, the point is that some molecules have a fairly large amount of energy compared to the average. Those are the ones we are interested in.

We are ESPECIALLY interested if one of these high energy molecules happens to be sitting right at the surface of the water. Now, all the molecules are in motion because of their enery, but none have sufficient energy to break the mutual attractive force molecules have for each other. Suppose, just suppose that our surface molecule was moving up away from the surface AND had enough energy to break away from the attractive forces of the molecules around it.

Where would that molecule go? It would continue to move away from the liquid surface AND IT BECOMES A MOLECULE OF GAS. This is great because we are now making some vapor pressure. It happens to another molecule and another and another.

But wait! The vapor pressure stops going up and winds up staying at some fixed value. What's going on? Here's the answer, I hope you can handle it!

As more and more molecules LEAVE the surface, what do some start to do? That's right, some RETURN to the surface and resume their former life as a liquid molecule. Soon the number of molecules in the vapor phase is constant because the rate of returning equals the rate of leaving and so the pressure stays constant.

This image is my attempt to summarize this process. I'll put it here without comment:

Here's a REAL IMPORTANT point about vapor pressure:

the vapor pressure depends ONLY on the temperature

That is true because as the temperature goes up, there are more and more molecules with the right combination of energy and direction to break free of the liquid's surface.

Finally, you might think that, if you were to put more liquid in, the vapor pressure would go up. That's NOT correct. Keep in mind that there are two opposing processes at work: (1) molecules leaving the liquid's surface and (2) molecules returning to the liquid at its surface. Process #1 depends only on temperature and process #2 depends only on how many molecules are in the vapor phase. Adding more liquid affects neither process.

Here is a good homework question about vapor pressure:

If a liquid is sealed in a container, kept at constant temperature how does its vapor pressure change over time?

Advice: reword the below, if you happen to get the above question (or something like it) as part of your homework.

Answer: let's assume there was zero vapor at the instant of sealng the liquid in the container. The pressure in the sealed container would increase up to a certain value, which depends on two things: (1) the identity of the substance and (2) the temperature.

Once the pressure got to that certain value, it would remain there and NOT increase any more. (Keep in mind that this is all happening at constant temperature.)

Also, remember that molecules will continue to move from the liquid into the gas state. However, the rate of that will equal the rate of molecules returning from the gas state to the liquid state.

Lastly, remember that for the above to work, there must be some liquid remaining in the sealed container. Vapor pressure is the behavior of the gas while still in contact with its liquid (or solid).

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