Is Boiling Endothermic or Exothermic? (And Why?)

Boiling is an endothermic process. It requires an input of heat energy to convert a liquid into a gas. ¹ During boiling, the liquid molecules gain energy, overcome intermolecular forces, and transition into the gaseous phase, absorbing heat from the surroundings in the process. ^{2 3}

Well, this was just a simple answer. But there are few more things to know about this topic which will make your concept super clear.

So let’s dive right into it.

Why is boiling an endothermic process?

Boiling is an endothermic process because it requires energy input to overcome the intermolecular forces between the liquid molecules. When a liquid is heated to its boiling point, the added energy breaks the bonds holding the molecules together, allowing them to escape into the gas phase. ^{4 5}

In a liquid, the molecules are in constant motion and are attracted to each other by intermolecular forces, such as hydrogen bonding or London dispersion forces. These forces create a cohesive effect, keeping the liquid molecules close together.

When heat is applied to the liquid, its temperature increases, and the average kinetic energy of the molecules also increases. However, the intermolecular forces must be overcome to transition from the liquid to the gas phase.

During boiling, the heat energy supplied is used to break these intermolecular forces rather than increasing the temperature of the liquid.

As the liquid molecules gain enough energy, they overcome the attractive forces and transition into the gas phase. This absorption of energy from the surroundings, in the form of heat, makes boiling an endothermic process.

In summary, boiling is endothermic because it requires an input of energy to overcome intermolecular forces and transition the liquid molecules into the gas phase.

Why is boiling not an exothermic process?

Boiling is not an exothermic process because it involves the absorption of energy from the surroundings rather than releasing energy. When a liquid reaches its boiling point, heat is required to break the intermolecular forces and facilitate the transition of molecules from the liquid phase to the gas phase. ⁶

Exothermic processes involve the release of energy, usually in the form of heat, to the surroundings. For example, when a substance undergoes combustion, it releases heat energy. ⁷

In the case of boiling, however, energy is absorbed from the surroundings. This energy is used to overcome the attractive forces between the liquid molecules, allowing them to escape into the gas phase.

When heat is applied to a liquid, it increases the kinetic energy of the molecules. As the temperature rises, the molecules move more vigorously, and their average speed increases. ⁸

However, the added energy is primarily used to overcome the intermolecular forces, rather than being released as heat. This absorption of energy from the surroundings makes boiling an endothermic process, distinguishing it from exothermic processes.

Further reading

Is Photosynthesis Endothermic or Exothermic?
Is Combustion Endothermic or Exothermic?
Is Melting Ice Endothermic or Exothermic?
Is Sublimation Endothermic or Exothermic?
Is Melting Endothermic or Exothermic?

1. What is an exothermic reaction? (n.d.). Scientific American. https://www.scientificamerican.com/article/what-is-an-exothermic-rea/
2. Wisc.edu https://www2.chem.wisc.edu/deptfiles/genchem/netorial/rottosen/tutorial/modules/intermolecular_forces/02imf/imf2.htm
3. 11.2: Intermolecular Forces. (2014, November 18). Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/11%3A_Liquids_and_Intermolecular_Forces/11.02%3A_Intermolecular_Forces
4. Boiling Water Demonstration: Heating Curve | Chemdemos. (n.d.). Boiling Water Demonstration: Heating Curve | Chemdemos. https://chemdemos.uoregon.edu/demos/Boiling-Water-Demonstration-Heating-Curve
5. Specific heat, heat of vaporization, and density of water (article) | Khan Academy. (n.d.). Khan Academy. https://www.khanacademy.org/science/ap-biology/chemistry-of-life/structure-of-water-and-hydrogen-bonding/a/specific-heat-heat-of-vaporization-and-freezing-of-water
6. Ball, D. W., & Key, J. A. (n.d.). Phase Transitions: Melting, Boiling, and Subliming – Introductory Chemistry – 1st Canadian Edition. Phase Transitions: Melting, Boiling, and Subliming – Introductory Chemistry – 1st Canadian Edition. https://opentextbc.ca/introductorychemistry/chapter/phase-transitions-melting-boiling-and-subliming/
7. The Chemistry of Combustion. (n.d.). The Chemistry of Combustion. https://www.chem.fsu.edu/chemlab/chm1020c/Lecture%207/01.php
8. Cooper, M. M., & Klymkowsky, M. W. (n.d.). Systems Thinking – CLUE: Chemistry, Life, the Universe and Everything. Systems Thinking – CLUE: Chemistry, Life, the Universe and Everything. https://openbooks.lib.msu.edu/clue/chapter/chapter-5-systems-thinking/