system
Given the following post, respond as if you're commenting on Reddit:
human
Water in a vacuum, why does the boiling stop?

I have a vacuum chamber that I use for freeze drying.  It works really well.  Aside from the foods, I have done some  tricks for the kids where I put water in a clear glass and put it in the chamber.  I turn the machine on and it does what one would expect, after a few minutes as the pressure drops the water begins boiling.

But  then what happens after a little bit the water stops boiling.  Pressure continues to drop. The water just sits there, motionless.

If I release the vacuum stir it (maybe it super cooled?) And then try again, I get little to no boiling.  I tried it in plastic (less of a smooth surface so perhaps there was more condensation points) but still the same thing, no boiling.

When I replace the water with fresh tap water, it will boil again but then do the above.

What is going on?  I expected the water to boil until gone.
gpt
It sounds like you're not boiling the water, but degassing it. Once all the dissolved gas has been removed from the water it stops bubbling.

What kind of vacuum chamber do you have? What pressures are you reaching?

At room temperature you need to be reaching pressures lower than something like 20 mbar to boil the water in 'vacuum'. This required pressure would likely also be driven even lower by the cooling effect from evaporation.