Pretend you have an indestructible tube that cannot leak, inside which is water. Imagine that in each side of the tube, you have very powerful pistons
What would happen if you compress the water inside?
Would it turn into heat and escape the tube?
Would the water turn into solid because the water molecules are so close to each other?
Would the water turn into a black hole?
What would happen?
Best Answer
What you're asking about is usually shown in a phase diagram. The diagram shows how the "phase", i.e. liquid, gas, or one of various solid phases, exists at different temperatures and pressures:
If your cylinder starts at say $20{}^{\circ}\mathrm{C}$ and atmospheric pressure, it'll be in $\color{green}{\textbf{Liquid}}$ right near the center of the diagram. If you raise the pressure keeping the temperature constant, it'll switch to $\color{blue}{\textbf{Ice VI}}$ at about 1GPa, or about 10,000 atmospheres of pressure: it's hard to turn water to ice by compressing it; the water at the bottom of the ocean is still water.
As you keep raising the pressure further, keeping the temperature constant, it'll go through more and more compact forms of solid ice (the diagram doesn't show "black hole", as that would be many, many orders of magnitude off the top, and can't be physically reached).
I stress "keeping the temperature constant" because (a) that's something your experiment will have to choose to do or not do and (b) because it makes it much easier to read the diagram. The compression is adding energy to the water, from the work done by the pistons. If you go slow, and the cylinder isn't insulated, etc, that energy will conduct away as the cylinder naturally stays the temperature of its environment. If you go fast, or the cylinder is insulated, the temperature will rise and the water will tend to go up-and-right in the diagram: You'll hit the transitions at different points.