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In chemistry, the standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. IUPAC recommends using a standard pressure $p^\circ=10^5\ \mathrm{Pa}$. Strictly speaking, temperature is not part of the definition of a standard state. For example, the standard state of a gas is conventionally chosen to be unit pressure (usually in bar) ideal gas, regardless of the temperature.

Source: Wikipedia

Now, as we can see the definition for the standard state of gas is independent of its temperature as well as volume. But the properties of gas depend on those and standard state is a reference point for calculating properties. So shouldn’t the definition of standard state for an ideal gas include temperature or volume?

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The IUPAC definition of standard state is:

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 standard pressure p = p°, 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 standard pressure p = p°. For a solute in solution it is the (hypothetical) state of solute at the standard molality , standard pressure or standard concentration and exhibiting infinitely dilute solution 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 standard pressure.

Specifying that the standard state is a specific temperature and pressure, would mean that the standard state is not even remotely realizable for many gases. For example, how could you have pure water vapor at standard temperature and pressure?

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  • $\begingroup$ Well, there is some water vapor in equilibrium with liquid water at standard temperature and pressure. $\endgroup$ – ralk912 Mar 15 '18 at 17:30
  • $\begingroup$ @ralk912 but you don't have pure gas phase water at STP $\endgroup$ – DavePhD Mar 15 '18 at 17:44
  • $\begingroup$ Well no, but you implied that there is no water vapor at STP at all. $\endgroup$ – ralk912 Mar 15 '18 at 17:45
  • $\begingroup$ @ralk912 I'll add the word "pure" to the last sentence to clarify. $\endgroup$ – DavePhD Mar 15 '18 at 17:57
  • $\begingroup$ So you’re saying all gases can’t exist in gaseous state at a fixed temperature, but that is not the case with pressure? @DavePhD $\endgroup$ – MollyCooL Mar 16 '18 at 11:53

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