Editing Standard state (section)

{{Short description|Reference point (°) used to calculate the properties of a material under different conditions}}
{{distinguish|Standard conditions for temperature and pressure|Standard sea-level conditions}}

The '''standard state''' of a material (pure [[chemical substance|substance]], mixture or [[Solution (chemistry)|solution]]) is a reference point used to calculate its properties under different conditions. A degree sign (°) or a superscript ⦵ symbol (<sup>⦵</sup>) is used to designate a thermodynamic quantity in the standard state, such as change in [[enthalpy]] (Δ''H''°), change in [[entropy]] (Δ''S''°), or change in [[Gibbs free energy]] (Δ''G''°).<ref name="Toolbox">{{cite web | last = Toolbox | first = Engineering | title = Standard state and enthalpy of formation, Gibbs free energy of formation, entropy and heat capacity| work = Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications! | publisher = www.EngineeringToolBox.com   | date = 2017 | url = https://www.engineeringtoolbox.com/standard-state-enthalpy-formation-definition-value-Gibbs-free-energy-entropy-molar-heat-capacity-d_1978.html  | access-date = 2019-12-27 }}</ref><ref name="Thought">{{cite web | last = Helmenstine, PhD | first = Ann Marie | title = What Are Standard State Conditions? - Standard Temperature and Pressure | work = Science, Tech, Math > Science | publisher = thoughtco.com | date = March 8, 2019 | url = https://www.thoughtco.com/standard-state-conditions-overview-609256  | access-date = 2019-12-27 }}</ref> The degree symbol has become widespread, although the Plimsoll is recommended in standards, see discussion about typesetting [[#Typesetting|below]].

In principle, the choice of standard state is arbitrary, although the [[International Union of Pure and Applied Chemistry]] (IUPAC) recommends a conventional set of standard states for general use.<ref name="GB1">{{GoldBookRef| file=S05925 | title = standard state}}</ref> The standard state should not be confused with [[standard temperature and pressure]] (STP) for gases,<ref>{{GoldBookRef | file = S05910 | title = standard conditions for gases}}</ref> nor with the [[standard solution]]s used in [[analytical chemistry]].<ref>{{GoldBookRef | file=S05924 | title = standard solution}}</ref> STP is commonly used for calculations involving gases that approximate an [[ideal gas]], whereas standard state conditions are used for [[Thermodynamics|thermodynamic]] calculations.<ref name="Thought2">{{cite web | last = Helmenstine, PhD | first = Ann Marie | title = Standard Conditions Versus Standard State | work = Science, Tech, Math > Science | publisher = thoughtco.com | date = July 6, 2019 | url = https://www.thoughtco.com/difference-between-standard-conditions-state-607534  | access-date = 2020-09-06 }}</ref>

For a given material or substance, the standard state is the '''reference state''' for the material's thermodynamic state properties such as [[enthalpy]], [[entropy]], [[Gibbs free energy]], and for many other material standards. The [[standard enthalpy change of formation]] for an element in its standard state is zero, and this convention allows a wide range of other thermodynamic quantities to be calculated and tabulated. The standard state of a substance does not have to exist in nature: for example, it is possible to calculate values for [[steam]] at 298.15&nbsp;K and {{val|e=5|ul=Pa}}, although steam does not exist (as a gas) under these conditions. The advantage of this practice is that tables of thermodynamic properties prepared in this way are self-consistent.