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
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.
Nothing. Ice is less dense than water, so when the ice melts it will take up less space in the box, leaving you with a box that appears just a bit less full than when you shut it (assuming the ice is fully submerged when it’s shut).
If you do the reverse of this (which I presume is what you actually want the answer to), then you’d be freezing the water, which will make it take up more space. The pressure from the walls of the box, however, will prevent the water from freezing as there isn’t enough space for the crystal structure of ice to form.
Ice is bigger then water. So when ice melts into water it shrinks. This lowers the pressure inside the container. A regular container might get crushed by the atmospheric pressure outside it when this happens, either just deforming the container or it might damage the container. However your almost indestructible box will handle this just fine. In fact any sturdy container will handle a bit of underpressure without even deforming. Even if you fill it all the way with ice and let it melt. You might have some problems opening the lid though, this is what often happens to screw lids in the fridge that can make it hard to open.
If you put the indestructible, incompressible box in a special make-believe place, where no heat can warm it from the outside, and the pressure outside the box is exactly the same as inside the box:
The temperature of the water would decrease.
The temperature of the ice would increase.
Some water molecules would freeze when they got to 0°C and some ice molecules would melt when they got to 0°C.
When you measure temperature, you measure the average of all molecules. The final temperature inside the box will settle somewhere in the middle between what the ice started at and what the water started at.
If more ice melts than water freezes (likely):
Ice takes up more space than water, which means when it melts, there is now less pressure inside the box.
Like if you heat up a glass and press the mouth to your skin. The negative pressure — created as the air inside cools — pulls your skin inside until the forces are equal (“equilibrium”).
The water is also cooling down, which also reduces the pressure (cold liquids shrink aka get more dense).
So the box’s sides are being pulled inward with a strength that depends on the starting conditions. But the box is indestructible and incompressible, so it’s fine.
If you opened the box, it would make a sound like opening a sealed jar.
If more water freezes than ice melts (unlikely):
For the opposite reasons as above, there is now more pressure inside the box.
So the box’s sides are being pushed outward with a strength that depends on the starting conditions. But the box is indestructible and incompressible, so it’s fine.
If you opened the box, it would make a sound like opening a soda container.
If you put the indestructible, incompressible box on Earth at room temperature, where heat can warm it from the outside:
You get a black hole. Everyone dies.
Just kidding. Eventually the inside of the box and the outside of the box will be the same temperature.
The water that used to be ice shrunk (remember, ice = bigger than water).
The water that used to be…colder water…grew (remember, hot liquids expand aka get less dense).
But we started with more water, so we grew more than we shrunk (also, the water…that used to be cold water…that used to be ice…also grew).
So same as scenario #2. And the box is still fine.
The thing to know is that the state of matter (liquid, gas, solid) is NOT only determined by temperature. We like to say things like water freezes at 0C and boils at 100C but that is a simplified statement because it is only true at normal atmospheric pressure.
So in any kind of experiment where pressure (ie something contained in a fixed volume container) and temperature varies, the outcome will be determined by what is called a phase diagram.
“Put something in a hard shell and then increase the volume of its contents” is how bombs work.
In case of bombs the volume expansion is generated by chemistry.
Melting ice decreases in volume. The reverse – putting water into a container and freezing it – is a sure way to break things though. Do you have winters? We do. And people keep damaging their plumbing or water systems by leaving the water in them for the winter. The pipes and tanks freeze over and eventually burst.
If you’re interested in the different phases of ice, you should read Kurt Vonnegut’s *Cat’s Cradle*.
Even if you’re not, you should still read it as it’s an outstanding book.
Latest Answers