* Endothermic processes absorb heat from the surroundings.
* Phase changes require energy. To go from solid to gas, the water molecules need to overcome the strong attractive forces holding them in the solid state. This requires an input of energy.
* Frost to water vapor involves two endothermic steps:
* Melting: Frost (solid) turns into liquid water.
* Evaporation: Liquid water turns into water vapor.
Think of it this way: If you put frost in a closed container, it will eventually disappear as it turns into water vapor. But the container will feel colder than before because it lost heat to the process.
