condensation, in physics, change of a substance from the gaseous (vapor) to the liquid state (see states of matter). Condensation is the reverse of vaporization, or change from liquid to gas. It can be brought about by cooling, as in distillation, or by an increase in pressure resulting in a decrease in volume. Certain natural phenomena, such as dew, fog, mist, and clouds, are the result of the condensation of water vapor in the atmosphere; the formation of dew illustrates well the fundamental principles involved in such phenomena. The explanation of condensation can be found in the kinetic-molecular theory of gases. As heat is removed from a gas, the molecules of the gas move more slowly, and as a result, the intermolecular forces are strong enough to pull the molecules together to form droplets of liquid. Similarly, reducing the volume of the gas reduces the average distance between molecules and thus favors the intermolecular forces tending to pull them together.

Formation of a liquid or solid from its vapour. Condensation usually occurs on a surface that is cooler than the adjacent gas. A substance condenses when the pressure exerted by its vapour exceeds the vapour pressure of its liquid or solid phase at the temperature of the surface where the condensation is to occur. The process causes the release of thermal energy. Condensation occurs on a glass of cold water on a warm, humid day when water vapour in the air condenses to form liquid water on the glass's colder surface. Condensation also accounts for the formation of dew, fog, rain, snow, and clouds.

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