Gas
A
gas is one of the four main
phases of matter (after
solid and
liquid, and followed by
plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. Thus, as energy in the form of
heat is added, a solid (e.g. ice) will first melt to become a liquid (e.g. water), which will then
boil or
evaporate to become a gas (e.g. water vapor). In some circumstances, a solid (e.g. "
dry ice") can directly turn into a gas: this is called
sublimation. If the gas is further heated, its atoms or molecules can become (wholly or partially) ionized, turning the gas into a plasma.
In the gas phase, the
atoms or
molecules constituting the matter basically move independently, with no forces keeping them together or pushing them apart. Their only
interactions are rare and
random collisions. The particles move in random directions, at high speeds, whose range is dependent on the temperature and defined by the
Maxwell-Boltzmann distribution. Therefore, the gas phase is a completely
disordered state. Following the
second law of thermodynamics, gas particles will immediately
diffuse to homogeneously fill any shape or volume of space that is made available to them.
The thermodynamic state of a gas is characterized by its
volume, its
temperature, which is determined by the average velocity or kinetic energy of the molecules, and its
pressure, which is determined by the average velocity and density or number of molecules. These variables are related by the fundamental
gas laws, which state that the pressure in an
ideal gas is proportional to its temperature and number of molecules, but inversely proportional to its volume.
Like
liquids and
plasmas, gases are
fluids: they have the ability to flow and do not tend to return to their former configuration after deformation, although they do have
viscosity. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but expand to fill whatever space they can occupy. The
kinetic energy per molecule in a gas is the second greatest of the states of matter (after
plasma). Because of this high kinetic energy, gas atoms and
molecules tend to bounce off of any containing surface and off one another, the more powerfully as the kinetic energy is increased. A common misconception is that the collisions of the molecules with each other is essential to explain gas
pressure, but in fact their random velocities are sufficient to define that quantity. Mutual collisions are important only for establishing the
Maxwell-Boltzmann distribution.
Gas particles are normally well separated, as opposed to liquid particles, which are in contact. A material particle (say a dust mote) in a gas moves in
Brownian Motion. Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions as to how they move, but their motion is different from Brownian Motion. The reason is that Brownian Motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule(s) with the particle. The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as we would expect to find if we could examine an individual gas molecule.
*
Ideal gas, in physics
*Various
hydrocarbon gases (
organic gases) used for heating, lighting, and energy transmission:
**
Natural gas**
Liquefied Petroleum Gas (
LPG), also called "cooking gas", including
propane and
butane**
Syngas: various synthetic fuel gases: names include
coal gas,
water gas,
illuminating gas,
wood gas,
producer gas,
holzgas,
air gas,
blue gas,
manufactured gas,
town gas,
hygas *
Gas (chemical warfare), various poison gases used in warfare
*
Inhalational anaesthetic, including
laughing gas (= nitrous oxide)
*
Trace gasThe word "gas" was invented by
Jan Baptist van Helmont as a phonetic spelling of the
Flemish pronunciation of the
Greek word "
chaos".
*
Cooling curve*
Gas (chemical warfare), various poison gases used in warfare
*
Gas chamber*
Gas laws*
Gas metal arc welding*
Ideal gas, in physics
*
Kinetic theory of gases*
Liquefied Petroleum Gas, including
propane and
butane*
List of phases of matter*
Natural gas*
:Category:Pollutants*
Vapour
