... 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.