How long would it take to turn ice back to water?

If you flung an ice cube into space how long would it take for the ice to turn into water?

• Answer:

  it would never turn into liquid water, it would gradually sublime to gas. Whether water is a solid, liquid or gas, depends on both the temperature of the water, and the pressure. It's  well-known that water boils at 100 degrees C and freezes at 0 degrees  C.  This is no coincidence, because the Celsius degrees scale is defined  by these properties of water. As  you may know, the pressure will affect the temperature at which water  will boil.  This is why if you are up a mountain where the pressure is  lower, water boils at a lower temperature than 100 degrees C.  When  you're in space, where the pressure is very near zero, any liquid water  will instantly boil to gas. This  is also why water will boil at a higher temperature than 100 degrees C  if the pressure is high.  This is how a pressure-cooker works, it allows  you to cook food in water at a higher temperature than 100 degrees C by  putting it under a lot of pressure so that the water stays liquid,  cooking food faster than using steam, and doing it without having to use  oil. This is summed up in a phase diagram like this: As  you can see, water exists as a liquid (the green region) in only a  small part of the diagram. You can see that water does not even form a  liquid below 0.01 degrees. Because  space is at almost zero pressure, and deep space is only a few kelvin  in temperature (the bottom-left of the chart), water will remain as ice. But  even though the water stays as ice, the random movement of atoms can  sometimes cause individual atoms to break free of the ice lattice, and  escape.  This is exactly the same process as evaporation (where liquid  water turns to gas even at temperatures below its boiling point).   Although this process is very slow due to the very low temperatures of  space, over time it gradually causes the sublimation of the ice.