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Does sublimation occur when a solid turns into a gas and THEN vapor pressure is produced or does the solid first have a vapor pressue and then when the vapor pressure of the solid equals the atmospheric pressure then it sublimates? Please explain.

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Vapor pressure is, by definition, the pressure that a gaseous phase exerts after its reaction (some phase -> gas) rate equalled the rate of the inverse reaction (gas -> some phase) of phase transition. Therefore, the solid is all the time sublimating off the substrate, as well as the vapour is sublimating back to solid phase (though in grains, not back to the substrate). IUPAC mentions the vapour pressure at equilibrium as 'saturation vapour pressure', indicating that the terminology is also used out of equilibrium.

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  • $\begingroup$ Vapor pressure does not require a reaction $\endgroup$ – iad22agp Nov 22 '18 at 15:14
  • $\begingroup$ @iad22agp , as a state function, vapor pressure does not require a reaction per se, but by defining it on phase transition equilibrium, it is properly defined in terms of at least two reactions, in comparison to a non-equilibrium state. $\endgroup$ – Aquiles Sep 15 '19 at 22:01

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