Editing State function (section)

{{Short description|Function describing equilibrium states of a system}}
{{Thermodynamics|expanded=sysprop}}
In the [[Thermodynamics#Equilibrium thermodynamics|thermodynamics of equilibrium]], a '''state function''', '''function of state''', or '''point function''' for a [[thermodynamic system]] is a [[Function (mathematics)|mathematical function]] relating several [[state variables]] or state quantities (that describe [[Thermodynamic equilibrium|equilibrium states]] of a system) that depend only on the current equilibrium [[thermodynamic state]] of the system<ref>{{harvnb|Callen|1985|pages=5,37}}</ref> (e.g. gas, liquid, solid, crystal, or [[emulsion]]), not the [[Thermodynamic process path|path]] which the system has taken to reach that state. A state function describes equilibrium states of a system, thus also describing the type of system. A state variable is typically a state function so the determination of other state variable values at an equilibrium state also determines the value of the state variable as the state function at that state. The [[ideal gas law]] is a good example. In this law, one state variable (e.g., pressure, volume, temperature, or the amount of substance in a gaseous equilibrium system) is a function of other state variables so is regarded as a state function. A state function could also describe the number of a certain type of atoms or molecules in a gaseous, liquid, or solid form in a [[Heterogeneous mixture|heterogeneous]] or [[homogeneous mixture]], or the amount of energy required to create such a system or change the system into a different equilibrium state.

[[Internal energy]], [[enthalpy]], and [[entropy]] are examples of state quantities or state functions because they quantitatively describe an equilibrium state of a [[thermodynamic system]], regardless of how the system has arrived in that state. They are expressed by [[exact differential]]s. In contrast, [[mechanical work]] and [[heat]] are [[process quantities]] or path functions because their values depend on a specific "transition" (or "path") between two equilibrium states that a system has taken to reach the final equilibrium state, being expressed by [[inexact differential]]s. Exchanged heat (in certain discrete amounts) can be associated with changes of state function such as enthalpy. The description of the system heat exchange is done by a state function, and thus enthalpy changes point to an amount of heat. This can also apply to entropy when heat is compared to [[temperature]]. The description breaks down for quantities exhibiting [[hysteresis]].<ref>{{harvnb|Mandl|1988|p=7}}</ref>