Table of Contents
What is Boiling?
When a liquid is heated to the boiling point, it vaporizes, which is known as boiling. The boiling point is a characteristic of a liquid and is defined as the temperature at which it starts to change into its vapor form.
When a liquid is heated, its kinetic energy increases because its molecules can absorb more thermal energy. In consequence, more water will evaporate. It happens as far as the air is not completely saturated.
However, this process is referred to as “boiling” when it occurs at a temperature and pressure where the liquid’s internal pressure equals the external pressure (ambient pressure).
This transformation occurs at a steady temperature until the whole liquid body becomes vapor. In boiling, the whole liquid body is involved, while in evaporation, just the surface area of the liquid is involved.
What is Evaporation?
It’s a natural phenomenon wherein high-energy liquid molecules escape into the air at normal temperature. Each liquid molecule has its unique quantity of kinetic energy and range of motion.
As a result, molecules with a larger concentration of kinetic energy tend to break free from the solid and liquid phases and enter the gaseous state, despite the forces that may try to prevent this. Since just a small amount of energy is needed for molecules at the liquid’s surface to leave the liquid phase, they are the ones that tend to evaporate.
However, molecules that are farther from the liquid’s surface may still evaporate if they have enough kinetic energy to escape the liquid without colliding with other molecules.
As long as there is unsaturated air above the liquid, evaporation will occur.
Because of this, the liquid’s surrounding air must be able to hold the molecules of liquid vapor that are about to enter it. These molecules gain momentum by taking in the liquid’s heat energy. In this way, when molecules evaporate from a liquid, the temperature of the remaining liquid drops.
Difference Between Boiling and Evaporation
- When liquids are heated to high enough temperatures, they vaporize via processes like evaporation and boiling.
- Both the speed at which they vaporize and their state of vaporization set them apart from one another.
- The term “boiling” is used to describe the vaporization of a liquid when it reaches its boiling point, whereas “evaporation” describes the vaporization of a liquid at ambient temperature. It is the key distinction between boiling and evaporation.
Comparison Between Boiling and Evaporation
|Parameter of Comparison||Boiling||Evaporation|
|Definition||When a liquid reaches its boiling point, it evaporates, a process known as boiling.||The process through which a liquid turns into a gas at ambient temperature is called evaporation.|
|Temperature Contribution||The boiling point of a liquid is the only temperature at which boiling may occur, and this temperature is maintained during the whole boiling process.||The evaporation process occurs at any temperature.|
|The behavior of Liquid Molecules||The process of boiling happens uniformly across the whole liquid.||A liquid’s surface is the primary site of evaporation. As a result, the surface molecules are the ones that participate in this activity the vast majority of the time.|
|Effect on Bulk Liquid Body||The temperature does not change at any point throughout the process of boiling the liquid.||As long as there is unsaturated air above the liquid, evaporation will occur.|
|Quality of the air above the liquid phase||For a liquid to boil, its internal pressure must equal its external pressure (ambient pressure).||As long as there is unsaturated air above the liquid, evaporation will occur.|
- Bergles, A. E. (1988). Fundamentals of boiling and evaporation. In Two-Phase Flow Heat Exchangers. Springer, Dordrecht.
- Zhao, S., Zhang, J., & Ni, M. J. (2022). Boiling and evaporation model for liquid-gas flows: A sharp and conservative method based on the geometrical VOF approach. Journal of Computational Physics, 452, 110908.