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Particle model of a gas

In addition to solid and liquid , gaseous is one of the three classic physical states . A substance is a gas if its particles move freely at a large distance from one another and evenly fill the available space. Compared to a solid or a liquid , the same mass as a gas takes about a thousand to two thousand times the space under normal conditions .

Together with liquids, gases are among the fluids .


The origin of the word gas was unclear for a long time. Although it was more or less known that the word was introduced as a technical term in the 17th century by the Flemish doctor and naturalist Johan Baptista van Helmont († 1644), there was still uncertainty about the etymology , and its origin and a. from Hebrew , from Dutch geest ("spirit"), from Dutch. Gisten ("ferment") or from German Gäsen ( Paracelsus for "ferment"), gäscht ("foam" on fermenting liquid) suspected. The clarification was brought about in 1859 by the linguist Matthias de Vries , who made a statement from van Helmont's Ortus Medicinae (Amsterdam 1648), according to which he deliberately recreated the word specifically for the haze of the water caused by the cold and was based on the Greek , im Dutch very similarly pronounced word χάος (" chaos ") had intended: " ideo paradoxi licentia, in nominis egestate, halitum illum gas vocavi, non longe a chao veterum secretum " ("In the absence of a name I have taken the liberty of the unusual, to call this breath of gas , because it differs little from the chaos of the ancients. ").

The physical state gaseous

The aggregate state "gaseous" arises from the "solid" or "liquid" form through the addition of energy ( heat ). For some elements and compounds, the standard conditions ( temperature 20  ° C , pressure 101325 Pa ) are sufficient to be present as a gas; at sufficiently high temperatures, however, all matter is converted into a gaseous state. The energy supplied becomes the kinetic energy of the individual particles (depending on the temperature with speeds in the range of 500 m / s), which causes the gaseous state to completely fill the given space with a statistical uniform distribution of the gas particles. Here the overall system strives for the state of highest entropy (second law of thermodynamics ). That this is the most probable state can be made clear in the following way: If you mentally subdivide the volume available to a gas into space cells about the size of a molecule, then there are many more possibilities to apply the molecules to the many cells of the entire volume to distribute than to a small fraction. The macrostate of the space-filling distribution has the most possible arrangements (microstates) for the particles and thus also the highest entropy. The number of micro-states, the statistical weight, can be calculated. More details under Entropy (Thermodynamics) / Examples.


Boiling nitrogen in a metal cup (−196 ° C)

With ideal gases the free mobility of the individual particles is perfect according to the kinetic gas theory ; this state is only reached at high temperatures compared to the boiling point (which, for example, applies to hydrogen and helium at room temperature).

If any ideal gas is filled into a given volume, at the same pressure and temperature there will always be the same number of particles (atoms or molecules), i.e. H. independent of the mass of each particle and therefore independent of the type of gas. Expressed quantitatively, one mole (according to Avogadro, 6.022 × 10 23 particles) of any gas takes up a space of 22.4 liters under normal conditions (see also molar volume and Loschmidt constant ).

With real gases , more or less strong forces of attraction between the particles are effective ( Van der Waals forces ). The difference is noticeable when compressing: gases are compressible , the volume of ideal gases is inversely proportional to the pressure ( equation of state ). Real gases deviate more or less from the principles described above.

Gases also have the properties of liquids: they do not flow and do not withstand deformation , although they are viscous .

State transitions

Gas boiler from Stadtwerke Esslingen

The transition from the liquid to the gaseous state of aggregation is called evaporation (above the boiling point ) or evaporation (below the boiling point), the reverse transition from the gaseous to the liquid state of aggregation is called condensation . The direct transition from the solid to the gaseous state of aggregation is sublimation , the reverse transition from the gaseous to the solid state of aggregation is called resublimation .


In order to store the largest possible amount of gas in a container, i.e. to obtain a high density , the gas is strongly compressed (see also compressed gas ). Cylindrical or spherical pressure vessels (e.g. gas bottles ) are mostly used to ensure that the gas containers can withstand high pressure . Gas boilers or gasometers are low-pressure storage tanks with a large geometric volume. Due to the low pressure (<1 bar), the amount stored is insignificant. Gas suppliers usually store the gas in the pipeline network ( gas network ) by using high-pressure pipes with a large nominal diameter.

Related topics

Web links

Commons : Gases  - collection of images, videos and audio files
Wiktionary: Gas  - explanations of meanings, word origins, synonyms, translations
Wikiquote: Gas  Quotes

Individual evidence

  1. gas. In: Adelung: Grammatical-Critical Dictionary of High German Dialect. II, 1811 ( ).
  2. ^ A b Matthias De Vries: Woordafleidingen. In: De Taalgids. 1, 1859, pp. 247–282, here pp. 262–265 ( ( Memento of the original from October 28, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and Archive link according to instructions and then remove this note. ). @1@ 2Template: Webachiv / IABot /
  3. gas. In: Jacob Grimm , Wilhelm Grimm (Hrsg.): German dictionary . tape 4 : Forschel – retainer - (IV, 1st section, part 1). S. Hirzel, Leipzig 1878, Sp. 1428 ( ).