- For other meanings see gas (disambiguation).
A gas is one of the phases of matter. Like liquids, gases are fluids: they have the ability to flow and do not resist deformation, although they do have viscosity. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but instead expand to fill whatever space they occupy. The kinetic energy in a gas is the second greatest of the states of matter (after plasma). Because of this increased 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.
The word "gas" was probably coined by a Flemish chemist as a re-spelling of his pronunciation of the Greek word chaos.
Gas particles are highly spread out, 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 expect to find if we could examine an individual gas molecule.
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