Phase change is a physical process in which a substance goes from one phase to another. Usually changes occur at a constant temperature and at this constant temperature heat is either given or extracted from the object. And the nature of phase change depends upon the direction of heat transfer.
heat going into the system changes a solid into gas or liquid, whereas heat coming out of the system changes gases to liquid and liquids to solids.
A phase change is occuring; the liquid water is changing to gaseous water, or steam. On a molecular level, the intermolecular forces between the water molecules are decreasing. The heat is providing enough energy for the water molecules to overcome these attractive forces.
