How accurate are space movies?

Chemistry Level pending

What would happen if you opened the door to your space station, and held a cup of water in outer space?

The water will freeze and then sublimate The water will only freeze The water will boil then freeze The water will only boil

1 solution

Calvin Lin Staff
Oct 5, 2015

The actual conditions in space

Space is a vacuum with very low pressure (much lower than the atm of the triple point of water)
Space has a very low temperature of 2.7 K

Let's look at the water phase diagram, and see how the state of water changes:

Source, arrows are my edit .

Firstly, having the pressure vacuum will cause the water to boil almost instantly. This is similar to the effect where it is easier to boil water on a mountaintop, due to the reduced atmospheric pressure.
Secondly, we now have gaseous water molecules that are in a very cold environment, and so they will start to freeze. However, because the pressure is really low, the gas will directly freeze into a solid, through a process called desublimate (which is the opposite of sublimation).

The reason why the water boils first, is that the process of boiling is much much faster than the process of freezing. This is why the answer is "boil and then desublimate" as opposed to just "freeze directly".

This article also explains it nicely. Definitely worth a read.

Prasun Biswas - 5 years, 8 months ago

Dear Calvin,

Beautiful explanation! I have only one question: how does the/your diagram work? Assuming, that we have ~0 K and ~0 Pa out there, on your diagram it would be gas phase. Though I have found an other one on the internet: Phase diagram of water So... where am I wrong?

Dávid Sebők - 5 years, 8 months ago

Ah, that's because the image wasn't accurate at low temperatures / pressure. I've updated the image with one that shows better what happens at low pressure (using a logarithmic scale) and low temperature.

So at $\sim 0$ K and $\sim 0$ Pa, it would be in the solid state.

Calvin Lin Staff - 5 years, 8 months ago

why wouldnt it first freeze and then sublime

Kavyesh Talwar - 5 years, 8 months ago

"The reason why the water boils first, is that the process of boiling is much much faster than the process of freezing." Otherwise: The pressure-alteration spread a lot faster (~speed of sound in water) then heat (its kinda fast here because of the great temperature difference and great contact surface, but still not faster than the other).

Dávid Sebők - 5 years, 8 months ago

After boiling it ll freeze and will remain in solid state?

Uzair Tariq - 5 years, 8 months ago

Yes. I've updated the answer to reflect this.

Calvin Lin Staff - 5 years, 8 months ago

How accurate are space movies?

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