Difference Between Vapor Pressure and Boiling Point

Main Difference – Vapor Pressure vs Boiling Point

Vaporization and boiling are two terms used to express the phase change of a liquid or a solid. Vaporization is the phase change of a liquid or solid into its vapor. Boiling causes the phase change of a liquid into its vapor. Vaporization gives rise to the vapor pressure of a closed system. The boiling point is the temperature at which a liquid is vaporized. Although these two terms are related to each other, there are differences between them. The main difference between vapor pressure and boiling point is that vapor pressure is a measurement of pressure whereas boiling point is a measurement of temperature.

Key Areas Covered

1. What is Vapor Pressure
      – Definition, Conditions, Characteristics
2. What is Boiling Point
      – Definition, Characteristics
3. What is the difference between Vapor Pressure and Boiling Point
      – Comparison of Key Differences

Key Terms: Vapor, Boiling Point, Vapor Pressure, Kinetic Energy, Temperature, Atmospheric Pressure

What is Vapor Pressure?

Vapor pressure can be defined as the force exerted by the vapor. The vapor should meet the following conditions in order to exert the vapor pressure.

• The vapor should be in equilibrium with its liquid or solid phase.
• Vapor should be at a constant temperature.
• Both vapor and its condensed form should exist in a closed system.

The vapor pressure is related to the desire of molecules to escape the liquid or solid. Hence, substances with high vapor pressure at normal temperature are considered to be volatile. As long as the temperature remain constant, the vapor pressure also remains constant. But once the temperature is increased, the kinesis (kinetic energy) of liquid molecules increase, releasing more and more molecules from the liquid. As a result, the transition of liquid molecules into vapor is increased. Thus, the vapor pressure is also increased. At a certain temperature, the vapor pressure becomes equal to the external pressure exerted on the liquid or solid. This temperature is called the boiling point of the liquid.

Figure 1: Vapor Pressure

What is Boiling Point?

The boiling point is the temperature at which a liquid boils. In other words, it is the temperature at which the vapor pressure of a liquid is equal to the external pressure which is applied on the liquid by the surrounding environment.

The boiling point of a liquid varies with the atmospheric pressure. Hence the value of the boiling point for a certain liquid is not always constant. The atmospheric pressure is varied according to the elevation. For example, water normally boils at 100⁰C when the atmospheric pressure is 1 atm. But at higher altitudes, water boils at lower temperatures. That is because the vapor pressure should be equal to the atmospheric pressure for a liquid to boil. As the pressure at higher altitudes is lower, a low heat energy (temperature) is enough to meet the above criteria.

Even at temperatures below the boiling point, liquid molecules will become vapor due to a process called evaporation. Evaporation is the escaping of liquid molecules which are located on the surface of a liquid. These molecules are only loosely bound to the other molecules in the liquid; thus, they can easily detach from other molecules and escape the liquid as vapor. But in boiling, molecules located anywhere in the liquid are able to escape the liquid.

Figure 02: Water Vapor Coming out of the Opening of a Kettle

Difference Between Vapor Pressure and Boiling Point

Definition

Vapor Pressure: Vapor pressure is the force exerted by the vapor released by a liquid or solid substance in a closed container or space.

Boiling Point: Boiling point is the temperature at which the vapor pressure is equal to the external pressure applied on the liquid.

Specific Conditions

Vapor Pressure: Vapor pressure is defined for a closed system with a constant temperature.

Boiling Point: Boiling point is defined for a system with a constant pressure.

Physical States

Vapor Pressure: Vapor pressure is related to both solid and liquid phases.

Boiling Point: Boiling point is related only to the liquid phase.

Variations

Vapor Pressure: Vapor pressure varies with the temperature.

Boiling Point: Boiling point varies with the atmospheric pressure.

Conclusion

Vapor pressure and boiling point are two related terms often used in physical chemistry. Although vapor pressure is related to the boiling point, they have different characteristics. The main difference between vapor pressure and boiling point is that the vapor pressure is a measurement of pressure whereas boiling point is a measurement of temperature.

References:

“Vapor Pressure.” Vapor Pressure. N.p., n.d. Web. Available here. 09 June 2017. 
Helmenstine, Ph.D. Anne Marie. “At What Temperature Does Water Boil?” ThoughtCo. N.p., n.d. Web. Available here. 09 June 2017. 

Image Courtesy:

1. “Vapor pressure” By HellTchi – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “653673” (Public Domain) via Pixabay