Re: liquid in vacuum

There are two things that happen to liquids when they are placed in a vacuum.

Evaporation is a dynamic process wherein molecules (or atoms) of a substance are both leaving a surface and joining the surface at the same time (those joining the surface coming from the space "above" the surface), and evaporation is simply the condition when the number (per unit time) of molecules (or atoms) leaving the surface exceeds the number (per unit time) joining the surface. So, in a vacuum, because there are essentially no molecules (or atoms) of the substance flying around in the space next to the exposed surface, there are no molecules (or atoms) to join the surface from the space "above" the surface, and the result is a high net evaporation rate because almost none of the molecules (or atoms) of the substance that leave the surface are replaced.

Second, pressure is a measure of the time rate of momentum transfered to a body by the molecules (or atoms!) of the "stuff" that surrounds the body, by collisions with the surface. In a vacuum there are essentially no surrounding molecules and therefore no pressure on the surface. The lack of surface pressure allows the structure of the liquid to expand, or relax, and in doing so the substance can internally "boil", in that the molecules (or atoms!) can rapidly pull away from each other. The process is much like an explosion, but the energy available for separation comes from the internal motion of the atoms (the "heat" energy) and so the energy is much more limited than what we typically think of as an explosion (That is, each separating atom (or molecule) takes some of the internal energy away and so the liquid cools and then has less tendency to "boil".). The "boiling" is a limiting process because of the self cooling, unless an external supply of energy is available.