Vapor Pressure

By Angela Huang

States of Matter and Phase Changes

A state of matter is a distinct form in which matter exists homogeneously under certain conditions with its unique qualities. Generally, matter has three states: solid, liquid, and gas, which are often called phases. A phase change occurs when one state shifts to another, for example, ice to water. An increase in internal energy is a major cause of a phase change. Matter consists of numerous molecules or atoms, and when it absorbs energy, the molecules or atoms start to move more aggressively. As a result, the distance between molecules or atoms increases, and the matter changes phases. For example, solid to liquid or liquid to gas, as shown below. In contrast, if the molecules lose energy, the matter will change from gas to liquid or from gas to solid.

Phase Change of Matter

Vapor Pressure

There is a spontaneous phase change from liquid to gas or from solid to gas. Molecules are always moving and never stop, even if the matter is solid. Simultaneously, there is a force between molecules, called an intermolecular force (IMF), which constrains molecules so they may not move so freely. At a liquid surface, some molecules with high energy may accidentally escape if their speed can overcome the constraint from the IMF, which makes them turn into gas (often called vapor), as shown below. However, the molecules in the vapor state may fly back into the liquid as their movement is random. If the escaping and turning molecules are equal, it reaches an equilibrium. The pressure of the vapor is called vapor pressure. Vapor pressure depends on several factors, such as the characteristics of a substance, temperature, etc.

Vapor Pressure

Application of vapor pressure

Vapor pressure is important for us to understand many phenomena. For example, such as why water evaporates at a certain boiling point, why ice floats when it freezes, and more. Some of the liquid is at low IMF, so its molecules are easy to escape. As a result, it has a high vapor pressure. By measuring vapor pressure, we can classify liquids into volatile and non-volatile, which should be stored in different ways. We need to be very cautious to handling volatile, hazardous materials because they can easily get into the air and be inhaled by us. Utilizing vapor pressure helps us do this and more. 

Sources

1. Theodore E. Brown, H. Eugene LeMay, Bruce E. Bursten, Catherine Murphy, Patrick Woodward, Matthew E. Stoltzfus (2017). Chemistry: The Central Science, 14th edition, Pearson, 1248p.

2. David Halliday, Robert Resnick, and Jearl Walker (2021) Fundamentals of Physics, Extended, Wiley; 12th edition, 1536

3. McCord, P. (n.d.-a). Phase Changes. Phase changes. http://chembook.org/page.php?chnum=5&sect=8 

4. Wikimedia Foundation. (2024, September 8). Vapor pressure. Wikipedia. https://en.wikipedia.org/wiki/Vapor_pressure#/media/File:Vapor_pressure.svg