What happens to water droplets and ice cubes at zero pressure?

[quantum123]

What happens to water droplets and ice cubes at zero pressure?
Does it depends on the phase diagrams of water?
What if at the same time, the temperature is absolute zero or very close to it?

 

[Borek]

Phase diagram is all you need.

 

[quantum123]

So what is the answer?
Do we need to invoke the concept of vapor pressure?

 

[Borek]

Vapor pressure plays a crucial role.

It occurred to me that question is not precise. Initially I assumed by 'zero pressure' you mean zero pressure from the beginning of the experiment to the end of the experiment. But if it is that we just start with zero pressure and we proceed in some closed volume, whatever evaporates will increase the pressure - so it won't be zero.

 

[quantum123]

How about throwing some ice cubes and spraying your garden hose out of the space shuttle?

 

[Borek]

That's close to assumption that volume is infinite, so the pressure is constant - and for all practical purposes equals zero.

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methods

 

[rcgldr]

At a low enough presure, you don't have water droplets, only solid (ice) and vapor (steam), similar to frozen CO₂ at atmospheric pressure. The evaporative process of solid to gas is called sublimation.

 

[Borek]

Depends. There is nothing wrong with situation when you have a zero pressure and liquid water - it won't last long, as it is not an equilibrium, but it is not impossible.

And the question - as posted - asked "what happens", so my understanding is that we not necesarilly start at equilibrium, but we observe system as it goes to equilibrium.

 

[quantum123]

So if we let ice cubes float around in space-walks and there will be sublimation - in other words, they will all vaporize? But I thought there are attractive intermolecular forces to hold the solid together?

 

[Borek]

It is all in the phase diagram. Forces that you mentioned are temperature dependent and what we see is effect of dynamic equilibrium. Molecules on the surface always evaporate (sublimate), molecules close to the surface always condense. Depending on the pressure and tepmerature speed of these processes change and either evaporation or condensation dominates, moving mass from solid or liquid to gas phase or back.

 

[quantum123]

Do you have a link to a the water phase diagram? (an accurate one)

 

[Mapes]

So if we let ice cubes float around in space-walks and there will be sublimation - in other words, they will all vaporize? But I thought there are attractive intermolecular forces to hold the solid together?

The solid is a low-energy configuration compared to the gas, but remember that atoms can have a distribution of energies as they jiggle randomly against each other. All it takes is for an atom on the surface to gain enough energy to detach, and it's gone, into the vacuum, never to return. In the end (equilibrium), all the atoms are in the gas phase.

 

[quantum123]

Will water boil in space?
If I hold a beaker of water during a space-walk, will I see bubbles coming out?

 

[Borek]

Yes.

You don't need a space walk for that. Take a syringe, fill it with hot water (hot tap water should do). Block the exit and pull the piston so that there is a vacuum created over the water. You will see water boiling for a few seconds, till it reaches equibrium between temperature of water and pressure above.

Similar effect can be seen during vacuum filitration.

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