Standard state
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In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals). This pressure of 1 bar was recently changed from 1 atm by IUPAC in [1997]. The standard state of a material can be defined at any given temperature. When the standard state is referred to in a chemical reaction, it also includes the condition that all solutions are at 1 mol/L or 1 mol/kg. Pure liquids and solids are in their standard states.
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 a lot of other material standards. The standard enthalpy of formation for an element in its standard state is 0.
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