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Why are there sometimes two similar substances in the pure gases and the vapors?

Why are there sometimes two similar substances in the pure gases and the vapors?

Is there any difference between them and if so, what is it?

Answer: They are the same substances, but they are represented with different models. For example, the vapor "water" gives the properties of the water around its liquid-vapor equilibrium, whereas the pure gas "H2O" is modeled as an ideal gas (which never condenses). The gas "H2O" can be a component of any compound gas, as a combustion product for example, whereas the vapor "water" cannot be mixed with other substances. The zero reference for enthalpies of ideal gases is 1 bar and 25 °C, whereas it is specific for each vapor.

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