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Why is heat released when water vapour condenses?

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argon phase diagram — liquid Figure 1: Phase diagram of argon.

When water vapour condenses into a liquid, it releases heat because condensation is an exothermic process.12

To understand why this occurs, consider the energy required for phase changes. In a gaseous state, molecules move independently with high energy. To transition into a liquid, these molecules must overcome the energy state of the gas and settle into a more ordered, cohesive structure where intermolecular forces can hold them together.34

The energy that was originally absorbed to vaporize the liquid—known as the latent heat of vaporization—must be released when the vapour returns to a liquid state. This released energy is referred to as the molar heat of condensation. Because this process releases a significant amount of thermal energy, it is a fundamental principle utilized in various industrial and household applications, such as air conditioning and heat engines.321