If a substance can exist but is not thermodynamically stable (for example, a supercooled liquid), it is called a metastable state. It has no tendency to transform into any other physical state. The most common physical standard state is one that is stable thermodynamically (i.e., the normal one). A physical standard state is one that exists for a time sufficient to allow measurements of its properties. However, since any non-normal condition could be chosen as a standard state, it must be defined in the context of use. The normal standard state is commonly defined as the most stable physical form of the substance at the specified temperature and a pressure of 1 bar or 1 atm. The state of aggregation for thermodynamic purposes is the standard state, sometimes called the reference state, and defined by specifying certain conditions. Function values depend on the state of aggregation of the substance, which must be defined for the value to have any meaning. Tables and datafiles are usually presented at a standard pressure of 1 bar or 1 atm, but in the case of steam and other industrially important gases, pressure may be included as a variable. A thermodynamic datafile is a set of equation parameters from which the numerical data values can be calculated. Thermodynamic data is usually presented as a table or chart of function values for one mole of a substance (or in the case of the steam tables, one kg). Both of these definitions for the standard condition for pressure are in use. Data is expressed as temperature-dependent values for one mole of substance at the standard pressure of 101.325 kPa (1 atm), or 100 kPa (1 bar). Numerical values of these thermodynamic properties are collected as tables or are calculated from thermodynamic datafiles. Thermodynamic databases contain information about thermodynamic properties for substances, the most important being enthalpy, entropy, and Gibbs free energy.
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