Why is it impossible for liquid water to exist on the surface of the Moon

Water boils when the temperature and pressure are sufficient to break the bonds. Temp and pressure are directly related. Thereby, you are able to boil water at room temperature win a partial vacuum. The moon is a near complete vacuum. I'd imagine that any liquid water on the moon would either quickly freeze or boil off.

http://www.newton.dep.anl.gov/askasci/ch…

Actually scientists put a charge on the surface of the moon and blew a tiny bit of it up. What filled the crater created by the charge was water. It was liquid at the time they looked at it because the moon was facing the sun - making it hot.

With no atmosphere and extremely low temperature, any liquid water would quickly either vaporize (from the vacuum) or freeze solid (from the cold).

a form of water does exist on there.. ice.
it is too cold for it to melt into water