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I have asked this question recently: Why does evaporation take place? and I got a satisfactory answer, that temperature is macroscopic and is the average temperature of the object.

That said, why does the stone or steel not evaporate like water over time? Some stones have been out in the sun baking for millions or even billions of years yet they haven't really evaporated (although they may have got smaller due to erosion or some other geological effect).

Why is that?

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    At normal conditions of temperature and pressure, metals or solid objects like stones, do not sublime. Only under certain circumstances, especially at very high temperatures(much higher than 100°C ...) and low pressures, they may undergo sublimation. A common example is a normal light bulb. The white hot tungsten filament slowly sublimates, but that's because the temperature reaches around $\approx 3500 K$. – Ellie Sep 13 '14 at 15:47
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    Phonon, nice, but is the rate of sublimation stictly zero for a rock at room temperature, or just approximately so? Is the rate the exponential of a huge negative number? What is it and why is it large? – Luboš Motl Sep 13 '14 at 15:56
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    Crudely, according to the molecular kinetic concept, sublimation is a continuous process of molecular emission from the interface between the gas and the solid, the rate of emission being governed by the thermal motion of molecules. As you guessed, the rate is given by an exponential, obtained here from Clapeyron-Clausius equation, which is approximately $exp[-\Delta Q M/RT]$, with $\Delta Q$ and $M$ being the sublimation heat and vapor molecular mass respectively. Clearly the rate is dominated by $\Delta Q M$, and very hard to overcome just by changing $T.$ – Ellie Sep 13 '14 at 16:29
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    What they said. Everything evaporates but the vapor pressure is so dang small you might as well calculate the statistical probability of the quantum binding forces leading to molecular ejection. – Carl Witthoft Sep 13 '14 at 16:32
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    @LubošMotl for further reading on this sublimation rates and mass flow conditions, check out the works of Knudsen-Langmuir, an OK summary can be found here[http://www.virlab.virginia.edu/nanoscience_class/lecture_notes/Microfabrication_Supporting_materials_files/R_B_Darling%20-%20Washington%20U%20-%20PhysicalVaporDeposition.pdf] – Ellie Sep 13 '14 at 16:37
  • @CarlWitthoft And is really the concept of vapor pressure relevant for all materials? I think stones floating in space (i.e. $P \approx 0$) don't evaporate at a large range of temperatures. But this might be because we are talking equilibrium thermodynamics here - the typical times of reaching such an equilibrium might be of impressive orders, which might also be an important fact for our discussion. – Void Sep 13 '14 at 17:34
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    Oh, I just googled the actual evaporation pressures of some metals and they are all below $10^{-9}\rm Pa$ at the given ranges, so even super-slow evaporation in space is covered. – Void Sep 13 '14 at 17:44
  • Hmm, I know its sci-fi, but A.C.Clarke was considering sub-light interstellar travel over eons, and was concerned about sublimation of the surfaces of the spaceship, to the vacuum. Made me think the effect is so tiny to ignore it in normal usage, but not strictly zero. – Charles Teague Sep 14 '14 at 13:19

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Actually, solids have a pressure equilibrium but this pressure is in most cases very low so this is a process extremely slow.

Pierre Alvarez
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The chance that water will evaporate is greater than that of steel because of boiling point. The chance that a water molecule will get "bumped" to boiling point is higher than that of steel because steel has a higher boiling point. This doesn't mean that metals don't evaporate. They just do so very slowly.

Jimmy360
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