A bottle of water at 0'C is opened on the surface of moon. What happens and why?
Water is a vapor at 0 deg. C in a vacuum. If the water was initially frozen it will sublimate to steam.It's a somewhat more complicated picture than that. Boiling point rises and lowers with increasing and decreasing atmospheric pressure. Since there is no air pressure to speak of on the moon, the boiling point should be much lower than 100C. However, in order to change from the solid (remember it's at 0 degrees initially) it starts as a chunk of ice, and boils off at a rate that depends upon the radiation flux (which would impart sufficient energy to teh H2O molecules to allow them to escape). The higher the flux, the faster it boils off.
This process will reduce the temperature of the water, however...it won't stay at 0 C. If liquid, it will quickly freeze and boil until it is a mixture of vapor and ice with no remaining liquid. If isolated in the shade, the ice will continue to cool as it sublimates until it gets below around 200K, at which point it'll be stable as a solid. If in the sun or exposed to other sources of heat sufficient to keep it above that temperature, it'll continue to sublimate away. The Apollo astronauts used sublimation units that used this to provide cooling and remove humidity from the air in their suits.Water is a vapor at 0 deg. C in a vacuum. If the water was initially frozen it will sublimate to steam.