Vapor Pressure

Vapor pressure is an ever-confusing topic. I think this occurs because the idea of vapor pressure although fairly simple is often ill-explained using technical terminology. To get around this problem let’s begin by defining a simple and workable definition.

What is Vapor Pressure?

In a nutshell, vapor pressure represents the ability of molecules to escape their liquidy prison and turn into a gas. Therefore molecules with high vapor pressures have the ability to escape and readily transition into their gaseous phase. While molecules with low vapor pressures aren’t able to escape their current form and are stuck as liquids.

The vapor pressure of a substance isn’t static and differs from substance to substance because different molecules have different abilities to escape. What determines this escape ability though? In short, intermolecular forces and temperature but dissolved solutes also matter which we will look at later in this article.

Intermolecular Forces

We can imagine the surface layer of our substance is like a long chain of kids playing Red Rover. They have their arms linked together and are trying to prevent other kids, which stand-in for our escaping molecules, from breaking through.

The stronger our molecules’ intermolecular forces (IMF) the stronger the Red Rover chain. So substances with stronger intermolecular forces (i.e stronger Red Rover chains) will have a much harder time escaping their liquid phase and becoming gaseous. Here molecules will have a low ability to escape and thus low vapor pressures.

In summary, the stronger the intermolecular forces the lower the substance’s vapor pressure and vice versa.

So a substance with weak intermolecular forces will have a comparatively high vapor pressure and its molecules will have a much easier time escaping. Meaning vapor pressure and intermolecular forces are inversely proportional to one another.

IMF and Vapor Pressure


In our analogy temperature alters the kid who trying to break through the chain. Since temperature is a proxy for the kinetic energy of a molecule, higher temperatures means more kinetic energy and thus more energy to break through and escape. In our analogy as we increase the temperature of the substance, we start trading out our kids for professional linebackers, who have no issue rushing through our Red Rover chain.

This means that as we increase the temperature of a substance we increase its vapor pressure and vice versa. This makes intuitive sense as well and if you have ever boiled water you have experienced this first hand. When you heat your water to make pasta, bubbles and water vapor eventually form this occurred because the stove increased the water’s temperature and with it the vapor pressure until the water molecules were able to escape.

Therefore as the temperature of a substance increases so does its vapor pressure making the two variable directly proportional to one another.

Vapor Pressure and Temperature


Solutes and the colligative properties they produce are a bit confusing there will be a whole summary article devoted to them because of this, but for now, let’s focus on looking at how adding solutes affects the vapor pressure of a substance. In our analogy solutes add additional Red Rover players who block the weak links in the chains.

In doing so they strengthen the Red Rover chain making it more difficult for our intrepid molecule to escape. As a result, adding a solute to any pure solvent will always lower the vapor pressure. This is why solutes increase the boiling point of a substance.

Connection to Boiling Point

Which is a great transition into the link between boiling point and vapor pressure. Before diving into this connection, let’s briefly discuss how something boils. Boiling or more technically vaporization occurs when a liquid transitions into its gas phase. In order for this to occur molecules have to escape their liquid phase into the surrounding air, which is where vapor pressure comes into play.

Another factor, ambient pressure, also dictates the phase in which a substance exists. Since ambient pressure presses downwards on the surface of a liquid higher ambient pressures combats the vapor pressure of a substance forcing it to stay in liquid form.

When the vapor pressure is greater than the ambient pressure gas water vapor will be released into the air as it flows from high to lower pressure. When both the ambient and the vapor pressure are equal the boiling points has been reached and vapor starts to form. If the vapor pressure continues to rise the gaseous water will escape. However when the ambient pressure is greater than the vapor pressure the water will remain in its liquid state.

As a result boiling point represents the balance between these two pressures and more specifically defines the temperature at which the two pressures are equal.

This is why water boils at a lower temperature when at higher altitudes. It is also why when astronauts pee into their suits and expel the waste into the vacuum of space their urine boils. It then rapidly freezes via deposition since space is really, really cold. Enough about pee though until we talk about kidneys that is.