Editing Joule–Thomson effect (section)

==Description==
The ''[[adiabatic process|adiabatic]]'' (no heat exchanged) expansion of a gas may be carried out in a number of ways. The change in temperature experienced by the gas during expansion depends not only on the initial and final pressure, but also on the manner in which the expansion is carried out.

*If the expansion process is [[reversible process (thermodynamics)|reversible]], meaning that the gas is in [[thermodynamic equilibrium]] at all times, it is called an ''[[isentropic]]'' expansion. In this scenario, the gas does positive [[mechanical work|work]] during the expansion, and its temperature decreases.
*In a [[Joule expansion|free expansion]], on the other hand, the gas does no work and absorbs no heat, so the [[internal energy]] is conserved. Expanded in this manner, the temperature of an [[ideal gas]] would remain constant, but the temperature of a real gas decreases, except at very high temperature.<ref name=":Goussard">{{cite journal|doi=10.1119/1.17417|title=Free expansion for real gases|journal=American Journal of Physics|volume=61|issue=9|pages=845–848|year=1993|last1=Goussard|first1=Jacques-Olivier|last2=Roulet|first2=Bernard|bibcode=1993AmJPh..61..845G}}</ref>
*The method of expansion discussed in this article, in which a gas or liquid at pressure ''P''<sub>1</sub> flows into a region of lower pressure ''P''<sub>2</sub> without significant change in kinetic energy, is called the Joule–Thomson expansion. The expansion is inherently irreversible. During this expansion, [[enthalpy]] remains unchanged (see [[#Proof that the specific enthalpy remains constant|proof]] below). Unlike a free expansion, work is done, causing a change in internal energy. Whether the internal energy increases or decreases is determined by whether work is done on or by the fluid; that is determined by the initial and final states of the expansion and the properties of the fluid.

[[File:Joule-Thomson sign.png|thumb|Sign of the Joule–Thomson coefficient, <math>\mu_{\mathrm{JT}}</math> for N<sub>2</sub>. Within the region bounded by the red line, a Joule–Thomson expansion produces cooling (<math>\mu_{\mathrm{JT}} > 0 </math>); outside that region, the expansion produces heating. The gas–liquid coexistence curve is shown by the blue line, terminating at the critical point (the solid blue circle). The dashed lines demarcate the region where N<sub>2</sub> is a supercritical fluid (where properties smoothly transition between liquid-like and gas-like).]]
The temperature change produced during a Joule–Thomson expansion is quantified by the [[#The Joule–Thomson (Kelvin) coefficient|Joule–Thomson coefficient]], <math>\mu_{\mathrm{JT}}</math>. This  coefficient may be either positive (corresponding to cooling) or negative (heating); the regions where each occurs for molecular nitrogen, N<sub>2</sub>, are shown in the figure. Note that most conditions in the figure correspond to N<sub>2</sub> being a [[supercritical fluid]], where it has some properties of a gas and some of a liquid, but can not be really described as being either. The coefficient is negative at both very high and very low temperatures; at very high pressure it is negative at all temperatures. The maximum [[inversion temperature]] (621 K for N<sub>2</sub><ref name=":Atkins1">{{cite book|last=Atkins|first=Peter|date=1997|title=Physical Chemistry|edition=6th|location=New York|publisher=W.H. Freeman and Co.|page=[https://archive.org/details/physicalchemistr00atki/page/930 930]|isbn=978-0-7167-2871-9|url=https://archive.org/details/physicalchemistr00atki/page/930}}</ref>) occurs as zero pressure is approached. For N<sub>2</sub> gas at low pressures, <math>\mu_{\mathrm{JT}}</math> is negative at high temperatures and positive at low temperatures. At temperatures below the gas-liquid [[binodal|coexistence curve]], N<sub>2</sub> condenses to form a liquid and the coefficient again becomes negative. Thus, for N<sub>2</sub> gas below 621 K, a Joule–Thomson expansion can be used to cool the gas until liquid N<sub>2</sub> forms.