What happens if you put water and ice in an (almost) indestructible box?

Question

If I filled a box TO THE BRIM with 75% water and 25% ice, sealed the box shut in a way that it was as if it was never openable, and then waited for the ice to melt, what would happen? Would it manage to destroy the box? Would I accidentally start a black hole that would suck up the Earth and kill us all?

In: Physics

Answer 1

I’m not knowledgeable enough to do the math to know exactly which would happen, but either:

* The water will just be at a slightly lower pressure, or;

* The water will evaporate into a gas.

Phase of matter depends on more than merely temperature, it also depends on pressure. [Here is a phase diagram for water](https://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Phase_diagram_of_water.svg/700px-Phase_diagram_of_water.svg.png) which shows what phase you get depending on both temperature and pressure.

To understand, think about what’s going on with the molecules. There is an attractive force between molecules which tries to stick them together. If there’s pressure, it also keeps the molecules close so that the intermolecular forces can hold them. If you add energy, the molecules are bouncing around too hard for the force to hold them tightly.

So, if you increase the pressure, you can force the molecules to stick together and form a solid, held together more by the pressure than by the molecules themselves. If you release the pressure and put it in a vacuum, it’s *only* the intermolecular forces holding it together and that may not be enough, so it’ll boil even at a low temperature.

Your hypothetical box is maintaining the same volume, but as the ice melts, it takes up less space. That means the pressure inside will drop as the fluid tries to expand to fill the space evenly.

More than likely, you’ll just get water at a lower pressure. But if the ice takes up enough space and the difference when it melts is great enough, you’ll get a low enough pressure for water to boil off the surface. Without doing the math because I can’t, I’d guess that you’ll end up with a very small empty space at the top of the water which will allow water to evaporate off until it fills that space with enough pressure to stop any more from evaporating off.