system
Given the following post, respond as if you're commenting on Reddit:
human
If water boils in a vacuum, what happens if there is not much space for the water to expand into (e.g. the airless volume inside a container is only twice the size of the volume of water as a liquid)?

I was trying to image what would happen if I had some water (say a liter) inside an airtight container and it filled the space. If I could draw a plunger and expand the space to 2 liters, but no air could get in, and the container would resist the pressure, what would happen to the water. Would it become a basically a less dense liquid spread throughout the 2 liter space)?
gpt
If you have a vacuum that gets fills with water vapour, it's no longer a vacuum. As water starts to boil due to the vacuum, the pressure increases and eventually the vapour pressure would be in thermodynamic  equilibrium with the water and it would stop boiling.