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Author Topic: Anti-Sublimation?  (Read 7811 times)

Offline Quantum_Vaccuum

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Anti-Sublimation?
« on: 09/10/2007 23:16:31 »
There is defiantly the change from solid to gas, the process of sublimation, but is it possible, to have the gas state of mater, turn straight to solid, skipping the liquid stage? If sublimation occurs by surface area gaining enough energy to make it vaporize, could the vapor of the object loose so much energy that it anti-sublimates?


 

another_someone

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Anti-Sublimation?
« Reply #1 on: 09/10/2007 23:25:55 »
It is what happens to CO2 at normal atmospheric pressure.
 

Offline Quantum_Vaccuum

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« Reply #2 on: 09/10/2007 23:27:25 »
No, thats normal sublimation isn't it, it doesn't go from gas striaght to a solid.
 

another_someone

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« Reply #3 on: 09/10/2007 23:35:00 »
No, thats normal sublimation isn't it, it doesn't go from gas striaght to a solid.

http://en.wikipedia.org/wiki/Carbon_dioxide
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Carbon dioxide has no liquid state at pressures below 5.1 atm.

If it has no liquid state, then what do you think happens to it as you cool it below its freezing point?
 

Offline Quantum_Vaccuum

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« Reply #4 on: 10/10/2007 01:10:26 »
i suppose so, but is that the only liquid, and does that mean that the melting and freezing piont would be the same?
 

another_someone

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« Reply #5 on: 10/10/2007 01:29:26 »
http://en.wikipedia.org/wiki/Deposition_%28physics%29
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Deposition is a process in which gas transforms into solid (also known as desublimation). The reverse of deposition is sublimation.

One example of deposition is the process by which, in sub-freezing air, water vapor changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground.

Another example of physical deposition is the artificial process of physical vapor deposition, used to deposit thin films of various materials onto various surfaces.
 

Offline Quantum_Vaccuum

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Anti-Sublimation?
« Reply #6 on: 10/10/2007 01:33:37 »
ok i'm starting to understand, but how could it occur without the melting and freezing point being the same?
 

another_someone

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Anti-Sublimation?
« Reply #7 on: 10/10/2007 01:50:24 »
There are two different issues.

You can desublimate simply by bringing the temperature below the freezing point.  This simply short circuits the formation of liquid, but the liquid form may still possibly be formed at intermediate temperatures, depending on the pressure.

With CO2, for liquid CO2 to exist, the pressure needs to be 5.2 times normal atmospheric pressure.  For water, at pressures below 0.0060373057 atmospheres, liquid water cannot exist, just as liquid CO2 cannot exist at ordinary atmospheric pressure.
 

another_someone

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Anti-Sublimation?
« Reply #8 on: 10/10/2007 02:01:48 »
http://en.wikipedia.org/wiki/Triple_point
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In physics and chemistry, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium.

For example, the triple point temperature of mercury is at −38.8344 C, at a pressure of 0.2 mPa.

The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.
 

Offline lightarrow

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Anti-Sublimation?
« Reply #9 on: 10/10/2007 12:55:57 »
There is defiantly the change from solid to gas, the process of sublimation, but is it possible, to have the gas state of mater, turn straight to solid, skipping the liquid stage? If sublimation occurs by surface area gaining enough energy to make it vaporize, could the vapor of the object loose so much energy that it anti-sublimates?

"Sublimation" means: solid --> gas or gas --> solid.
 

another_someone

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Anti-Sublimation?
« Reply #10 on: 10/10/2007 13:42:01 »
"Sublimation" means: solid --> gas or gas --> solid.

Sorry, but this is not the case:

http://en.wikipedia.org/wiki/Sublimation_%28physics%29
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Sublimation of an element or compound is a transition from the solid to gas phase so rapidly that the liquid phase cannot be observed. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).

The opposite of sublimation is deposition.
 

Offline lightarrow

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« Reply #11 on: 10/10/2007 18:32:18 »
"Sublimation" means: solid --> gas or gas --> solid.

Sorry, but this is not the case:

http://en.wikipedia.org/wiki/Sublimation_%28physics%29
Quote
Sublimation of an element or compound is a transition from the solid to gas phase so rapidly that the liquid phase cannot be observed. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).

The opposite of sublimation is deposition.
In italian, some authors call "sublimazione" both transitions: solid --> gas and gas --> solid, while others call "brinazione" the transition gas --> solid. I was teached at univ. the first version, so I believed it was the only one, but, actually it's not.  ???
 

Offline Bored chemist

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« Reply #12 on: 10/10/2007 19:16:49 »
I don't often say this but that wiki article is simply wrong.
"Sublimation of an element or compound is a transition from the solid to gas phase so rapidly that the liquid phase cannot be observed. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram)."
It has nothing to do with the phase change being too rapid to form or observe a liquid, it's that there is no possible liquid phase to form.
They may call the formation of solid from the vapour deposition, but I don't know anyone else who does.
 

Offline DrDick

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Anti-Sublimation?
« Reply #13 on: 15/10/2007 20:43:37 »
Deposition is a fairly common term for this process (gas → solid).  It's part of CVD (chemical vapor deposition) that is used in semiconductor manufacture (not exactly the same thing as what we're talking about, but close enough).

Dick
 

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Anti-Sublimation?
« Reply #13 on: 15/10/2007 20:43:37 »

 

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