Condensation

Condensation, transition of a gas into a liquid or a solid, generally upon a surface that is cooler than the adjacent gas. A substance condenses when the pressure exerted by its vapor exceeds the vapor pressure of the liquid or solid phase of the substance at the temperature of the surface where condensation occurs. Heat is released when a vapor condenses. Unless this heat is removed, the surface temperature will increase until it is equal to that of the surrounding vapor.

The phase change from gas to solid is more commonly called deposition or desublimation. An example of desublimation is the formation of snow from water vapor in the air.

Heat of condensation

Condensation is an exothermic process, meaning that heat is released when a vapor is converted to liquid or solid. The heat released by a substance undergoing condensation is usually expressed as its molar heat of condensation. It is defined as the amount of heat released when one mole of the substance changes from a gas to a liquid. It is expressed as ΔH_cond and is equal in magnitude to, but bearing the opposite sign of, the molar heat of vaporization (ΔH_vap) of the substance: Thus, ΔH_vap = −ΔH_cond.

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Water and its Varying Forms

Condensation in the atmosphere

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In the atmosphere condensation results when the air temperature is reduced to the dew point or when sufficient water vapor is added to saturate the air. In the latter scenario abundant tiny particles called aerosols present in the atmosphere serve as nuclei, called condensation nuclei, on which water vapor condenses. Some of these nuclei are hygroscopic (moisture attracting), and condensation begins on them when the relative humidity is less than 100 percent. Other nuclei, however, require some supersaturation before condensation begins.

Examples of condensation are everywhere. A drop in atmospheric temperature to the dew point causes water vapor in the air to condense into dew on grass (and water droplets to form on the surface of cold soda cans). In addition to dew, condensation accounts for the formation of fog and clouds. For rain to occur, other physical processes are required. By returning atmospheric moisture to Earth’s surface, condensation helps to complete Earth’s water cycle.

Commercial applications of condensation

The energy exchange associated with condensation is utilized in several industrial and household processes that involve heat transfer. For example, air conditioners condense refrigerant vapor to help control the temperature and humidity in homes. Condensation also helps dissipate heat and keep electronic devices cool. Different fluids, particularly hydrocarbons, are isolated during distillation and processing of natural gas through condensation.