Editing Kinetic theory of gases (section)

== Assumptions ==
The application of kinetic theory to ideal gases makes the following assumptions:
* The gas consists of very small particles. This smallness of their size is such that the sum of the [[volume]] of the individual gas molecules is negligible compared to the volume of the container of the gas. This is equivalent to stating that the average distance separating the gas particles is large compared to their [[atomic radius|size]], and that the elapsed time during a collision between particles and the container's wall is negligible when compared to the time between successive collisions.
* The number of particles is so large that a statistical treatment of the problem is well justified. This assumption is sometimes referred to as the [[thermodynamic limit]].
* The rapidly moving particles constantly collide among themselves and with the walls of the container, and all these collisions are perfectly elastic.
* Interactions (i.e. collisions) between particles are strictly binary and [[Uncorrelatedness (probability theory)|uncorrelated]], meaning that there are no three-body (or higher) interactions, and the particles have no memory.
* Except during collisions, the interactions among molecules are negligible. They exert no other [[force]]s on one another.

Thus, the dynamics of particle motion can be treated classically, and the equations of motion are time-reversible.

As a simplifying assumption, the particles are usually assumed to have the same [[mass]] as one another; however, the theory can be generalized to a mass distribution, with each mass type contributing to the gas properties independently of one another in agreement with [[Dalton's law|Dalton's law of partial pressures]]. Many of the model's predictions are the same whether or not collisions between particles are included, so they are often neglected as a simplifying assumption in derivations (see below).<ref>{{cite book |last1=Chang |first1=Raymond | last2=Thoman | first2=John W. Jr. |title=Physical Chemistry for the Chemical Sciences |date=2014 |publisher=University Science Books |location=New York, NY |page=37}}</ref>

More modern developments, such as the [[revised Enskog theory]] and the extended [[Bhatnagar–Gross–Krook operator|Bhatnagar–Gross–Krook]] model,<ref>{{Cite journal |last1=van Enk |first1=Steven J. |author-link=Steven J. van Enk |last2=Nienhuis |first2=Gerard |date=1991-12-01 |title=Inelastic collisions and gas-kinetic effects of light |url=https://link.aps.org/doi/10.1103/PhysRevA.44.7615 |journal=Physical Review A |volume=44 |issue=11 |pages=7615–7625 |doi=10.1103/PhysRevA.44.7615|pmid=9905900 |bibcode=1991PhRvA..44.7615V |url-access=subscription }}</ref> relax one or more of the above assumptions. These can accurately describe the properties of dense gases, and gases with [[Molecular vibration|internal degrees of freedom]], because they include the volume of the particles as well as contributions from intermolecular and intramolecular forces as well as quantized molecular rotations, quantum rotational-vibrational symmetry effects, and electronic excitation.<ref>{{cite book |last1=McQuarrie |first1=Donald A. |title=Statistical Mechanics |date=1976 |publisher=University Science Press |location=New York, NY}}</ref> While theories relaxing the assumptions that the gas particles occupy negligible volume and that collisions are strictly elastic have been successful, it has been shown that relaxing the requirement of interactions being binary and uncorrelated will eventually lead to divergent results.<ref>{{Cite journal |last=Cohen |first=E. G. D. |date=1993-03-15 |title=Fifty years of kinetic theory |url=https://dx.doi.org/10.1016/0378-4371%2893%2990357-A |journal=Physica A: Statistical Mechanics and Its Applications |volume=194 |issue=1 |pages=229–257 |doi=10.1016/0378-4371(93)90357-A |bibcode=1993PhyA..194..229C |issn=0378-4371|url-access=subscription }}</ref>