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Author Topic: Do liquids ever evaporate while frozen?  (Read 3313 times)

Offline thedoc

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Do liquids ever evaporate while frozen?
« on: 24/07/2014 01:30:01 »
Hannah asked the Naked Scientists:
   
Do other liquids besides water evaporate while frozen?

What do you think?
« Last Edit: 24/07/2014 01:30:01 by _system »


 

Offline chiralSPO

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Re: Do liquids ever evaporate while frozen?
« Reply #1 on: 24/07/2014 03:04:38 »
Technically, when frozen, a substance is no longer liquid...

But yes, solids do evaporate. It is referred to as sublimation.

Common examples of this are carbon dioxide (dry ice), which is not liquid at normal pressures, and only goes between solid and gas; elemental iodine also is only solid or gaseous at normal pressure, and is particularly good for demonstrating this because it forms a bright purple vapor that crystallizes into shiny black flakes that fall down like snow; menthol smells quite nice, and also forms beautiful needle-like crystals--I have a bottle of menthol crystals, and the under-side of the lid is frosted with little menthol crystals because it evaporates from the bottom of the jar and then recrystallizes on the lid.

Almost all solids have a slight (or not so slight) vapor pressure (think of any solid that you can smell), and can sublime under the right conditions.
 

Offline lightarrow

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Re: Do liquids ever evaporate while frozen?
« Reply #2 on: 28/07/2014 10:16:52 »
I would say that every solid has a non-zero vapour pressure at room temperature and so could vaporize; even Iron, for example. How much vaporizes, it's another story. For a solid like Iron, the speed would be ridiculosly low.

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Offline chiralSPO

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Re: Do liquids ever evaporate while frozen?
« Reply #3 on: 28/07/2014 14:13:29 »
I would say that every solid has a non-zero vapour pressure at room temperature and so could vaporize; even Iron, for example. How much vaporizes, it's another story. For a solid like Iron, the speed would be ridiculosly low.

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At room temperature (300 K), I calculate 1 out of 10122 atoms in a diamond lattice could leave the solid for vapor phase (assuming they are all at the surface). This means that any reasonable sample size (say less than an Earth mass) would actually exert 0 vapor pressure at room temperature... Under sunlight radiation there would be a higher pressure of ejected electrons!
 

Offline lightarrow

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Re: Do liquids ever evaporate while frozen?
« Reply #4 on: 28/07/2014 21:23:55 »
At room temperature (300 K), I calculate 1 out of 10122 atoms in a diamond lattice could leave the solid for vapor phase (assuming they are all at the surface). This means that any reasonable sample size (say less than an Earth mass) would actually exert 0 vapor pressure at room temperature... Under sunlight radiation there would be a higher pressure of ejected electrons!
Diamonds?
 

Offline chiralSPO

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Re: Do liquids ever evaporate while frozen?
« Reply #5 on: 28/07/2014 21:48:07 »
Yes diamond. A counter-example to the claim that every solid has a non-zero vapor pressure.
 

Offline lightarrow

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Re: Do liquids ever evaporate while frozen?
« Reply #6 on: 29/07/2014 12:59:27 »
Yes diamond. A counter-example to the claim that every solid has a non-zero vapor pressure.
I wrote that for Iron the vaporization speed would be ridiculously low.
Certainly for diamond would be even more ridiculous...
You wrote that you computed 1 out of 10122 atoms to leave the solide lattice. That's what I intended with "ridiculously low".
On the other hand, you didn't wrote "exactly zero atoms would live the solid".
Furthermore, you made, I presume, a classical computation, not a quantistic one (there is tunnel effect, for example, to consider, but I couldn't make an estimate of it).
Anyway, if you mean that we couldn't measure that vaporization speed, I certainly agree with you.

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Offline chiralSPO

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Re: Do liquids ever evaporate while frozen?
« Reply #7 on: 29/07/2014 15:20:54 »
Yeah, we are really splitting hairs here. I think we agree in general, but maybe use slightly different words to explain our thoughts. (my calculations were based on the Boltzman/thermal distribution expected for solid vs gaseous states with an energy difference of 700kJ/mol at 300 K, so whether the equilibrium is established by purely classical means, or includes tunneling, I think the ratio still holds, plus or minus a few orders of magnitude)

I guess I would summarize my argument as: because matter comes in discrete units (atoms or molecules), if the probability of a unit leaving the solid phase for the gas phase is sufficiently low that no units actually enter the gas phase, I would define that as having a vapor pressure of zero. (ie there are substances that under reasonable conditions do not have non-zero vapor pressures)

Again, this would be a silly argument to hijack this thread with, but still I maintain that most solids have non-zero vapor pressure.
« Last Edit: 29/07/2014 15:23:34 by chiralSPO »
 

Offline lightarrow

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Re: Do liquids ever evaporate while frozen?
« Reply #8 on: 29/07/2014 16:12:48 »
Fair enough.

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Re: Do liquids ever evaporate while frozen?
« Reply #8 on: 29/07/2014 16:12:48 »

 

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