* Liquid to Gas (Evaporation/Boiling): This process is called vaporization. To break the hydrogen bonds holding water molecules together in a liquid state and allow them to escape into the gas phase, a significant amount of energy (heat) is needed.
* Liquid to Solid (Freezing): This process is called freezing. When water freezes, the molecules slow down and arrange themselves into a more ordered, crystalline structure. This process releases heat, meaning it actually *requires* the removal of heat to occur.
In summary:
* Vaporization (liquid to gas): Requires heat input (endothermic)
* Freezing (liquid to solid): Releases heat (exothermic)
This is why you need to boil water to turn it into steam, but it freezes when you put it in the freezer.
