1127. Why does water freeze at very low pressure? I saw an experiment in which a small amount of water first boiled and then froze solid when exposed to a vacuum. — BLG, Old Bridge, NJ
Water molecules are always leaving the surface of liquid water and when they do, they carry away more than their fair share of the water's thermal energy. Placing the water in a vacuum speeds this process because (1) it prevents those gaseous water molecules from returning to the liquid water, in which case they would return the thermal energy, and (2) it makes it possible for bubbles of water vapor to remain stable inside the liquid water even at low temperature, so that the water can boil. Overall, the main effect of putting the water in a vacuum is that its molecules leave rapidly and don't return. Since each leaving water molecule takes away more than its fair share of thermal energy, the water molecules that remain behind become cooler and cooler. You experience this effect when evaporating water from your skin makes you feel cold. In the present case, this cooling is so effective that the remaining water cools all the way to water's freezing point and the water begins to crystallize into ice. Water molecules continue to leave the surface of ice, a process called sublimation, so that even the ice gradually gets colder in the vacuum.