Condensation is the phase transition where water vapor changes into liquid water. This change occurs when the energetic vapor molecules cool down and slow sufficiently to form a liquid. The process is responsible for the formation of clouds, the appearance of dew on grass, and the fogging of bathroom mirrors after a hot shower.
Condensation is an Exothermic Process
Condensation is classified as an exothermic process, meaning it releases thermal energy, or heat, into the surrounding environment as it occurs. This is the direct opposite of an endothermic process, which must absorb heat from its surroundings to proceed, such as the melting of ice or the evaporation of water.
When water vapor changes into liquid water, the gaseous molecules lose energy in the form of heat. This release of energy results in a negative change in enthalpy for the water itself. The surrounding air or the surface on which the condensation forms receives this released thermal energy, causing a slight warming effect. This fundamental energy exchange is a key factor in atmospheric science and daily phenomena.
The Physics of Latent Heat Release
The reason condensation releases heat lies in the fundamental physics of molecular motion and intermolecular forces. Water molecules in the gaseous state possess a high amount of kinetic energy and move rapidly and independently of one another. They are too far apart to be held together by the attractive forces between them.
For these highly energetic vapor molecules to transition into the liquid phase, they must slow down and come closer together, allowing hydrogen bonds to form between them. The formation of these bonds lowers the potential energy of the system, and this excess kinetic energy must be liberated to the environment. This energy released during the phase change, which occurs without a change in the water’s temperature, is known as the latent heat of condensation.
The amount of energy released is precisely equal to the amount of energy that was absorbed during the initial, endothermic process of evaporation, known as the latent heat of vaporization. For water, the latent heat of vaporization/condensation is substantial, averaging around 2,260 kilojoules for every kilogram of water that changes state at standard pressure. The released heat is absorbed by the surrounding air or the condensation surface, which can then warm up.
Everyday Examples of Condensation Heat
Latent heat release during condensation drives several phenomena. One of the most dangerous examples is the severity of a steam burn compared to a burn from boiling water. While both are at 100°C (212°F), the steam releases its latent heat of condensation when it touches cooler skin, causing more significant tissue damage than the liquid water alone.
In meteorology, this energy release is a major driver of weather systems. As moist air rises and cools, condensation forms clouds, and the latent heat released warms the surrounding air within the cloud. This warming makes the air less dense, causing it to rise further, which can intensify storms and lead to heavier precipitation. The formation of dew on grass overnight is a direct result of atmospheric water vapor condensing on the cooler surfaces, releasing a small amount of heat that slightly warms the air immediately above the ground.