https://doi.org/10.1351/goldbook.S05925
State of a system chosen as standard for reference by convention. Three standard states are recognized: For a gas phase it is the (hypothetical) state of the pure substance in the gaseous phase at the @S05921@ \(p=p^{\,\unicode{x26ac}}\), assuming ideal behaviour. For a pure phase, or a mixture, or a solvent in the liquid or solid state it is the state of the pure substance in the liquid or solid phase at the @S05921@ \(p=p^{\,\unicode{x26ac}}\). For a solute in solution it is the (hypothetical) state of solute at the standard molality \(m^{\,\unicode{x26ac}}\), @S05921@ \(p^{\,\unicode{x26ac}}\) or @S05909@ \(c^{\,\unicode{x26ac}}\) and exhibiting infinitely @D01739@ behaviour. For a pure substance the concept of standard state applies to the substance in a well defined state of aggregation at a well defined but arbitrarily chosen @S05921@.