Good question! I will assume that you
manage to trap pure steam in the container
(no air is left in the container), and that the
container is completely airtight with no
leaks. I also will assume the container is
rigid and won't crumple up when the pressure
Here's the simple answer:
When you put the container in the fridge, the
steam, which was over 100°C cools down. This means
it will try and condense into liquid water.
However, as some droplets of liquid water start to
form, there will be less gas in the container and
so the pressure in the container will drop.
When the pressure in the container drops, the
boiling point of water will also drop. If too much
steam condenses into water, then the boiling point
of water will drop below fridge temperature (lets
say 4°C), and some water will turn back into
steam, raising the pressure of the container.
Then, if the pressure is higher, the steam will
want to condense again... and so on, and so on.
The result of this process (which happens very
quickly), is that a steady equilibrium will be
established between the liquid water and the water
vapor (steam). Most of the steam will have turned
into water, but just enough of the steam will
remain as a gas so that the pressure in the
container is high enough to keep the boiling point
of the water right at your refrigerator
temperature. This equilibrium pressure
will be about 1% of the atmosphere pressure (see
Opening the container will be difficult due to the
low pressure. But, if you manage to get it open,
then air from outside will immediately rush in.
The pressure will rise up to atmospheric pressure,
and the boiling point of water will go back up to
100°C. Since all of your remaining water vapor is
refrigerator temperature, pretty much all of it
will immediately condense into liquid, with maybe
a little entering the air.
If you want to know more, you can use the (
phase diagram of water ). This tells you
(among other things), how the boiling point of
water (line between "Liquid" and "Vapor")
changes with temperature and pressure. At
1 atmosphere (10,000 Pa or 1 bar), the boiling
point is at 100°C. But, at 10 mbar (0.01
atmospheres), the boiling point drops to about
4°C, which is refrigerator temperature. This is
how we know that the pressure in the sealed
container will be 1% of the atmospheric pressure
when the container reaches equilibrium in the
fridge. You can go further and use equations like
P V = n R T to figure out what percentage
of water molecules will end up in the liquid vs
vapor at equilibrium- I got that about 1.3% of
water molecules will end up in the vapor phase,
and the rest will be liquid water.
There are four phases of matter that we are
currently aware of: solid, liquid, gas, and
plasma. What state something is in depends on
pressure (P), temperature (T), composition (X),
and oxygen fugacity (fO2). Let's
take a look at
your question from a thermodynamic perspective.
I've put a
phase diagram (a graph
that depicts the
different states of matter according to varying
P-T-X-fO2 conditions) for water below
for you to follow along with.
We are at 1 atm pressure, so first find where that
is on the y-axis. Steam is the gaseous phase of
water, and if P = 1 atm, we can see from the phase
diagram that the temperature of the steam must be
between 100 degrees C and 374 degrees C (Tcrit for
H2O). We know this because there is a
line there (we call this a phase equilibria
along which two states (in this case, gas +
liquid) coexist in perfect thermodynamic
equilibrium. So, at the point where you have
collected the steam, we must be in the regime of
the P-T diagram that I've indicated using the
green rectangle. Once you put the container in the
fridge, the temperature of the container will
decrease (in the direction of the black arrow)
until it comes to thermal equilibrium with the air
inside of the fridge (usually set ~2 degrees C).
How long this takes is a function of many
different parameters (you can look up Fourier's
law of thermal conduction if you're really
interested). I'd guess that two hours is more than
long enough for the container to cool to a lower
T, making a phase transition from gas to liquid.
You'd likely find water at some temperature
between 0 C and 100 C (I've outlined it in the
I hope this has really helped to explain WHY phase
transitions happen. Thermodynamics is some pretty
The steam collected from the boiling water is
going to be the pure gas form of the water
The water molecules are really hot, so they are
moving around really fast, which is how they
escaped the surface of the water into the air.
Once they’re cooled in the fridge, they’ll
condense back into liquid form and you’ll find a
little cup of purified water!