State function

<p>In the <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics of equilibrium</a>, a state function, function of state, or point function for a <a href="/facts/Thermodynamic_system/VUr0kpWu">thermodynamic system</a> is a <a href="/facts/Function_(mathematics)/CdReEKCh">mathematical function</a> relating several <a href="/facts/State_variables/HNF1dCXu">state variables</a> or state quantities (that describe <a href="/facts/Thermodynamic_equilibrium/1D7nGm0d">equilibrium states</a> of a system) that depend only on the current equilibrium <a href="/facts/Thermodynamic_state/Rv6m11Pf">thermodynamic state</a> of the system (e.g. gas, liquid, solid, crystal, or <a href="/facts/Emulsion/YMSqSCuH">emulsion</a>), not the <a href="/facts/Thermodynamic_process_path/mCYaDcK1">path</a> 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 <a href="/facts/Ideal_gas_law/Pphokwhf">ideal gas law</a> 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 <a href="/facts/Heterogeneous_mixture/tms8rbmn">heterogeneous</a> or <a href="/facts/Homogeneous_mixture/tms8rbmn">homogeneous mixture</a>, or the amount of energy required to create such a system or change the system into a different equilibrium state.
</p><p><a href="/facts/Internal_energy/IYdsoxhR">Internal energy</a>, <a href="/facts/Enthalpy/bBou8lCE">enthalpy</a>, and <a href="/facts/Entropy/s52IXeFz">entropy</a> are examples of state quantities or state functions because they quantitatively describe an equilibrium state of a <a href="/facts/Thermodynamic_system/VUr0kpWu">thermodynamic system</a>, regardless of how the system has arrived in that state. In contrast, <a href="/facts/Mechanical_work/mbclFPBs">mechanical work</a> and <a href="/facts/Heat/FjDGssCs">heat</a> are <a href="/facts/Process_quantities/HPM8sd0o">process quantities</a> 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. 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 <a href="/facts/Temperature/QyGe4gmj">temperature</a>. The description breaks down for quantities exhibiting <a href="/facts/Hysteresis/9M2pwR62">hysteresis</a>.
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State function open-in-new

State function