... it
explodes.
Now this isn't the explosion that most people think of, the kind that is in
Rambo, but an explosion nonetheless. The liquid immediately
bursts into gaseous form and then begins to form an
equilibrium between its
vapor and
liquid form. (In a pure
vacuum, there would probably be a lot more
gas than
liquid at equilibrium)
The
simplest way to see this occurence is the following:
1. Fill a long
tube, closed at only one end, with mercury.
2. While holding the open
end shut, place the open end into a
bath of mercury.
3. You have now basically formed a
barometer... and barometers have a vacuum in the
top space above the mercury
level.
4. Now is the
tricky part... Have a dropper or something like that filled with
water.
5. Stick the
dropper into the
mercury bath under the tube's opening.
6. Squeeze out the water.
7. The water, being less
dense than mercury, will float up the tube towards the vacuum. When the water meets with the vacuum (not a pure vacuum, but then again, a pure vacuum
can't really exist) it will explode into it.