Condensation is the phase transition in which a gas or vapor becomes a liquid. It is the reverse of vaporization and releases latent heat to the surroundings. Although commonly associated with water droplets on cold surfaces and the formation of clouds, condensation occurs in many substances. Its occurrence depends on temperature, pressure, and the conditions governing equilibrium between vapor and liquid. (openstax.org)
Physical basis and equilibrium
In a vapor, molecules move relatively freely and are widely separated compared with those in the liquid. Condensation brings them into a denser phase, where intermolecular attractions become more important. Molecules continually enter and leave a liquid surface; at thermodynamic equilibrium, evaporation and condensation balance, so neither phase gains material overall. Net condensation occurs when transfer into the liquid exceeds escape into the vapor. (openstax.org)
For a pure substance, the saturation vapor pressure specifies the pressure at which liquid and vapor coexist at a given temperature. Cooling a vapor or compressing it under suitable conditions can bring it to this coexistence boundary. Below the critical point, the liquid and gas are distinct phases; above the critical temperature, pressure alone cannot produce an ordinary liquid–gas phase transition. These relationships are represented in a phase diagram. (openstax.org)
In a gas mixture such as air, the relevant quantity is the partial pressure of the condensable component, rather than total pressure alone. For water vapor, saturation over a flat liquid surface occurs when its partial pressure equals the equilibrium saturation vapor pressure at that temperature. Cooling lowers this saturation pressure, allowing condensation without requiring the whole gas mixture to become liquid. (weather.gov)
Heat released during condensation
The heat released during condensation is the counterpart of the energy absorbed during vaporization. For a mass (m) undergoing the phase change under specified conditions, its magnitude is commonly written
[ Q=mL_v, ]
where (L_v) is the specific latent heat of vaporization. The same magnitude applies to the reverse transition at the same temperature and pressure. For water near its normal boiling point, (L_v) is approximately (2.26\times10^6) joules per kilogram. (openstax.org)
For a pure substance condensing at constant pressure, the phase change occurs at the saturation temperature. Removing energy therefore need not immediately lower its temperature: the vapor fraction can decrease while the temperature remains unchanged. Cooling a superheated vapor before condensation, or cooling the resulting liquid afterward, involves additional heat transfer and must be distinguished from the latent-heat contribution. (archive.nptel.ac.in)
Atmospheric condensation
Condensation is an essential part of the water cycle. Water evaporated from oceans, soils, and other surfaces enters the atmosphere as invisible vapor. When atmospheric conditions favor condensation, it forms liquid droplets that contribute to clouds and fog. Condensation alone is not equivalent to precipitation: droplets must grow sufficiently before they fall as rain. (usgs.gov)
The dew point is the temperature to which air must be cooled to reach saturation while pressure and moisture content remain constant. Relative humidity expresses how close the water-vapor pressure is to saturation at the current temperature. A cold window or drinking glass can cool adjacent air sufficiently for droplets to form, even when the surrounding room air is unsaturated. The deposited water comes from atmospheric moisture, not from passage through the glass. (forecast.weather.gov)
Cloud formation often begins when rising air expands as atmospheric pressure decreases. This adiabatic cooling can bring the air to saturation. Vapor then condenses onto cloud condensation nuclei, including particles derived from sea spray, smoke, and soil dust. These particles provide sites for droplet formation; their size and composition, together with supersaturation, influence whether they become activated as cloud droplets. (prod-01-alb-www-noaa.woc.noaa.gov)
Condensation on surfaces
On a cooled solid surface, condensate may form either a continuous film or separate droplets. In filmwise condensation, the liquid wets the surface and creates a layer through which released heat must pass. This layer introduces thermal resistance, and its thickness and drainage affect the heat-transfer rate. Gravity commonly assists drainage on vertical surfaces. (archive.nptel.ac.in)
In dropwise condensation, the liquid does not spread into a continuous film. Droplets grow, merge, and depart, repeatedly exposing portions of the surface to vapor. This can produce higher heat-transfer coefficients than filmwise condensation. Maintaining dropwise behavior over long operating periods is difficult, however, because surface coatings and wetting properties may change. Noncondensable gases can also reduce condensation rates. (archive.nptel.ac.in)
Engineering applications and related processes
A condenser is a heat exchanger designed to remove heat from vapor and collect the resulting liquid. Condensers are important in chemical engineering, where controlling phase change is part of process design. Their performance depends on cooling conditions, condensate drainage, surface wetting, and the presence of noncondensable gases. (archive.nptel.ac.in)
Cooling moist air also produces recoverable condensate. Air-conditioning equipment can collect this water, and atmospheric water-generation systems deliberately extract moisture using cooled surfaces. The quantity available depends on environmental conditions and equipment operation. (energy.gov)
Condensation should be distinguished from deposition, the direct conversion of gas to solid, and from adsorption, the accumulation of material at a surface. Bose–Einstein condensation uses the same terminology for a different phenomenon involving quantum-state occupation rather than ordinary vapor-to-liquid conversion. (en.wikipedia.org)