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Vapors : (gaseous) water vapor is condensed by cooling; the resulting (liquid) water droplets are visible as an aerosol

Vapor referred in science and technology, a chemically pure, gaseous substance when it relative to its liquid or solid aggregate state considered, such as water vapor . Vapor can arise through evaporation of the liquid or sublimation of the solid and can be converted back into it through condensation or into it through resublimation .

In colloquial language, steam usually refers to an admixture of air, which is visible because the substance is partly in the form of tiny droplets, i.e. in liquid form.

Vapor-liquid equilibrium

Over time and if there is no disturbance, a dynamic equilibrium is established in which just as many particles of the liquid or solid phase pass into the gaseous phase as, conversely, return from the gas. The steam is then saturated and is also referred to as vapor , brodem or steam . How many particles change from one phase to the other depends, among other things, on the pressure and temperature of the system under consideration . In technology , the equilibrium between liquid and gaseous phase plays a major role in thermal separation processes .

Superheated steam

If the gas phase is no longer in contact with the associated liquid or solid phase and if heat continues to be supplied to the steam , it is superheated and is present as superheated steam. The stronger this warming is, the further one moves away from the area in which one still speaks of steam, and approaches a behavior that is then described as gaseous .


If you slowly cool down superheated steam, the so-called dew point is reached at some point , at which the steam is saturated again and condenses again to a liquid with further cooling . In the case of a direct transition from the gaseous to the solid state, i.e. with a resublimation , this point is called the frost point .

Critical temperature

Above the critical temperature , a gas can no longer be condensed by increasing the pressure. Therefore, a supercritical substance is no longer referred to as steam, but only as a gas. The term fluid is also used for a supercritical substance.

Meanings of steam in language

While steam (from Middle High German tampf / steam "vapor, water vapor") as a polymer formed from a liquid gas - apart from substance intrinsic stains - is invisible, we speak in everyday life in steam usually of a visible mixture of air and fine liquid drops, as he forms for example in the condensation of water vapor and can be seen escaping from many chimneys, for example. For the same form of finely distributed, small water droplets in air, the expressions contrail (at the exit of an aircraft turbine) or fog (near the ground) or cloud (in the sky) are also used in everyday language . The correct scientific term for a mixture of finely divided liquid droplets in a gas is mist . In the case of extremely small drops of liquid (and / or solid particles), this mixture is also known as an aerosol . In the special case of water, one speaks of wet steam in technology .


  • Burkhard Lohrengel: Introduction to the thermal separation process. Separation of gas, vapor and liquid mixing, 2nd revised edition, Oldenbourg Verlag, Munich 2012, ISBN 978-3-486-70889-9 .
  • Raimund Schenkel: The superheated steam. Spielhagen & Schurich, Vienna 1897.

Web links

Commons : Steam  - collection of images, videos and audio files
Wiktionary: Steam  - explanations of meanings, word origins, synonyms, translations