<p>In a vacuum, there is no pressure. This is critical for most liquids to remain in a liquid state because with no pressure, the temperature at which they start to boil drops. Water boils when there is no pressure (and as morbid as it sounds, so does blood</p><p>Yes, we do tend to think of water existing as a solid below zero degrees, as a liquid up to 100 degrees and then boil into steam. But, in fact, those are three different states that water can take at any temperature. It's just that, energetically, it prefers to be in particular states at different temperatures.</p><p>For example, you can have ice at 4 degrees C and it will be melting, but it hasn't melted yet despite being somewhat warmer than the nominal temperature we expect it to melt at.</p><p>Now, the states that water likes to be in at different temperatures change depending what the pressure is. So those temperatures of zero degrees C and 100 degrees C are only what you have at atmospheric pressure - the pressure of the air around us.</p><p>But if you take that air away, then, because you haven't got the pressure of the air pushing in on it, water molecules inside that material will naturally want a bit more spaced apart.</p><p>That means they're more likely to want to be a gas or liquid than a solid. And so ice melts, and water boils, at a lower temperature</p>