At what pressure will ice sublime

[bigal40]

From looking at the phase diagram for water it looks like for sublimation of ice to occur the pressure must be below .006atm(about .18in hg if i calculated correctly). The question is
1) does the ice have to be below .006atm in order to sublime?
2) If it does have to be below .006 atm wouldn't it be impossible for sublimation to occur out side of a partial vacuum.

My chemistry book doesn't explain it in very much detail and the teacher said that on really cold days the pressure gets to .006 atm but i don't believe it.

 

[BrownTown]

1. if thats what the phase diagram says then i guess thats what it must be

2. on cold days the pressure gets to .006 atm?, thats BS, if it did wed all blow up when it got cold. Also i can't really see any relationship between temperature and air pressure

 

[bigdog1218]

Ice outside, or in a freezer is constanly sublimating. When talking about sublimation you refer to the vapor pressure of the substance, not the pressure of the system. The vapor pressure of ice at 30 F will have molcules tranfering between solid and gas phases at equilibrium, if the gas molecules are removed, they will be replaced by additional sublimation of the solid.

Many solids sublimate, but do so at extremely slow rates at room temperature. Iron sitting on a table is stable, but heating it will greatly increase its vapor pressure, and sublimation, before it begins melting.

 

[BrownTown]

I'm pretty sure that what you are talking about isn't sublimation, just like the fact that water left on a table evaporates isn't boiling.

 

[bigdog1218]

Originally posted by: BrownTown
I'm pretty sure that what you are talking about isn't sublimation, just like the fact that water left on a table evaporates isn't boiling.

Its the same principal, they just have different names, water to gas below boiling temperature is evaporation, ice to gas below freezing temperature is sublimation.

 

[BrownTown]

eh, still thinnking its differnt. I believe that when ice turns to water vapor below freezing you are talking about just the surface of the ice, and the reason it is "subliming" is that radiation from some source like the sun in hitting it and exciting just the outer layer of the ice and this is causing it to fly off. I know this is what happens with evaporation, and im gonna assume its sthe exact same with ice till someone with good chemistry knowledge proves me wrong.

 

[bigdog1218]

Originally posted by: BrownTown
eh, still thinnking its differnt. I believe that when ice turns to water vapor below freezing you are talking about just the surface of the ice, and the reason it is "subliming" is that radiation from some source like the sun in hitting it and exciting just the outer layer of the ice and this is causing it to fly off. I know this is what happens with evaporation, and im gonna assume its sthe exact same with ice till someone with good chemistry knowledge proves me wrong.

Ice to water vapor is sublimation, no matter how it happens. Outside in the sun, in your freezer it doesn't matter. When you have a solid changing directly to a gas it's sublimation.

And for the original poster if he even checks this again the vapor pressure of ice at 0 C is .006 atm and therefore the highest vapor pressure ice can have.

So to explain it again, water in the atmosphere has a certain partial pressure. A piece of ice outside, sun or no sun, has a certain vapor pressure. If the vapor pressure of the ice is higher than the partial pressure of water in the atmosphere the water vapor will enter the atmosphere, and more ice will sublime to try and keep equilibrium. But if the partial pressure of water in the atmosphere is above .006 atm sublimation will not occur. So you got mixed up with partial pressure and total pressure.

 

[pm]

Most of the snow outside our house (5300ft. above sea level) seems to sublime away in the winter. The ambient air temperature won't rise above 20F for a week, and yet the snow steadily disappears in the shade.

Good explanation, Bigdog.

 

[bigal40]

Thanks for the explanation bigdog. I was indeed mixed up with the partial pressure of water pressure and the total atmospheric pressure.
but if the pressures given in a phase diagram refer to the partial pressure of the substance why is the boiling point 100C at 1atm, the partial pressure of water vaper isn't 1atm?

EDIT: this diagram is similar to the one in my book

 

[bigdog1218]

Originally posted by: bigal40
Thanks for the explanation bigdog. I was indeed mixed up with the partial pressure of water pressure and the total atmospheric pressure.
but if the pressures given in a phase diagram refer to the partial pressure of the substance why is the boiling point 100C at 1atm, the partial pressure of water vaper isn't 1atm?

EDIT: this diagram is similar to the one in my book

When vapor pressure = atmospheric pressure liquids boil. So the vapor pressure is 1 atm at 100 C.

 

[CycloWizard]

Originally posted by: BrownTown
eh, still thinnking its differnt. I believe that when ice turns to water vapor below freezing you are talking about just the surface of the ice, and the reason it is "subliming" is that radiation from some source like the sun in hitting it and exciting just the outer layer of the ice and this is causing it to fly off. I know this is what happens with evaporation, and im gonna assume its sthe exact same with ice till someone with good chemistry knowledge proves me wrong.

I think you guys are arguing in circles. The problem is that the phase diagram tells you the equilibrium state of water at a given temperature and pressure. The things you guys are talking about are transient, meaning that the piece of ice or puddle of water has not yet reached equilibrium. Given enough time, it will go to the equilibrium state which, for the case of a puddle in a large open space or a piece of ice in a large open space will be water vapor due to mass transport effects. If the piece of ice were put in a small bottle and sealed, then part of it would evaporate such that all the air in the bottle exhibited the vapor pressure of water at that temperature, but ice would remain (assuming the temperature were below some limit).

As for radiation being the cause of evaporation, that isn't really true. Again, it has to do with the puddle of water trying to get to equilibrium. Everything in nature wants to be in equilibrium, as this is the state of maximum disorder and the most stable state that the system can hold. Water evaporates because it has a certain vapor pressure at a given temperature. This means that some amount of water will evaporate such that the partial pressure of water at the gas-liquid interface is equal to that vapor pressure always. This concentration at the liquid interface induces a concentration gradient, since there is less water in the air far from the puddle. By diffusion and convection (mass transfer mechanisms), the water vapor from the surface is removed and transferred to the bulk air. This forces more water to evaporate ad infinitum until the puddle is gone.