VAPOR PRESSURE: The pressure of the vapor over a liquid (and some solids) at equilibrium.
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. 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 energy, but none have sufficient energy to break the mutual attractive force molecules have for each other. 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?
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.
Vapor pressure is directly related to temperature. If there is a higher temperature, what does that tell us about the kinetic energy?
How does that affect how many liquid particles will be able to escape and become gas?
If there are more gas particles what does that do to the pressure?
Table H in your reference is a graph of vapor pressure vs. temperature for a few common substances. Another name for propanone is acetone.
At room temp. (25°C) what is the vapor pressure of propanone?
How about water?
What does that say about the difference in intermolecular forces between those two substances?
Vapor pressure is also related to the boiling point of a substance. As the temperature of a liquid rises, the vapor pressure of the liquid rises until it becomes equal to the pressure on the liquid from the atmosphere. At that point any particle in the substance can change from the liquid phase to the gas (or vapor) phase. That's called boiling, and the temperature at which this occurs is called the boiling point. The normal boiling point of a liquid is the temperature at which the vapor pressure reaches standard atmospheric pressure (1 atm, or 101.3 kPa ).