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Why is heat released when a gas condenses into a liquid?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

When a gas condenses into a liquid, it undergoes an exothermic process that releases latent heat. This release occurs because the transition from a gaseous state to a liquid state involves a significant change in the molecular arrangement of the substance.1

In a gas, particles move independently with high thermal energy, occupying a large volume. To transition into a liquid, these particles must overcome their independent motion and form a more ordered, cohesive state where intermolecular forces can hold them together. The energy that was previously required to break these forces during vaporization is released as the particles settle into the liquid phase.12

This energy, known as the heat of vaporization (or the molar heat of condensation when released), is the amount of energy per unit mass or mole that must be removed from the substance to accomplish the phase change. Because this process releases a substantial amount of energy, it is effectively utilized in various technologies, such as air conditioners and heat engines.13