Gas

Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape and without a definite volume. Compared to the solid and liquid states of matter a gas has lower density and a lower viscosity. more...

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The volume of a gas will change with changes in temperature or pressure, as described by the ideal gas law. A gas also has the characteristic that it will diffuse readily, spreading apart in order to uniformly fill the space of any container.

Physics

In a gas phase the atoms or molecules constituting the matter basically move independently, (more freely than those in a solid or liquid) 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 speed. The range in speed 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, when no work is being done on or by a gas, the 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, and its pressure. 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 flowing and free moving 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, unconstrained gases in a vacuum environment do not occupy a fixed volume, but expand to fill the entire space. Note that this is true in the case of empty, vacuum environments. If one sprays carbon dioxide from a fire extinguisher, for example, the gas will not expand to fill the room. Instead, the gas will pour out like a fluid and pool on the floor. This is due to the fact that it is more dense than the air surrounding it.

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 substrate 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.

Read more at Wikipedia.org