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A gas or a gaseous state or a gaseous substance is one of the basic states in which the individual particles of the substance are relatively far from each other, move freely in space and are affected by negligibly small attractive forces. In a narrower sense, a gas is a substance in a gaseous state (air) other than steam.

Compared to liquids and solids, gases have a much lower density and viscosity and are compressible. A gas belongs to the group of "liquids" with a liquid. An ideal gas is used to simplify the investigation of the properties of gases. Real gas or real gas, unlike ideal gas, also has viscosity, i.e. internal mechanical resistance, and cannot be completely compressed.

Gas physics


Distribution of speeds of molecules of different gases at a temperature of 25° C
Gas particles are not held together by any forces (or their action is negligible), they move chaotically (disorganized) at different speeds and are affected only by random collisions with each other. The velocity of particles is varied, the distribution of particle velocities is described by Maxwell-Boltzmann statistics.

Impacts of particles on the walls of the container in which the gas is enclosed act on them with a force that manifests itself as the pressure of the gas in the closed container. If the gas is in a closed container, after opening it, it diffuses into the surroundings and after a certain time fills the entire space evenly. This phenomenon is a practical consequence of the second theorem of thermodynamics.

The instantaneous state of a certain amount of gas is determined by three state quantities - pressure, temperature and volume. These three quantities are mutually dependent and their relationship (changes) is described by the equation of state of an ideal gas:

{\displaystyle p.V=n.R.T\,}


p is pressure (Pa)
V is the volume (m3)
n is the amount of substance (mol)
R is the universal gas constant (8.314472 J K-1 mol-1)
T is Thermodynamic Temperature (K)



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