Phase Change
Energy can change the microscopic organization of matter without changing temperature. During a phase transition, transferred energy can go into changing phase instead of raising temperature.
How can energy enter a substance while its temperature stays flat?
Within a single phase, added energy often raises temperature. At a phase boundary, energy can instead change intermolecular arrangement and potential energy while the temperature remains nearly constant.
The first changes temperature within a phase. The second changes phase at a transition.
Particles vibrate around relatively fixed arrangements. Heating increases thermal motion until the melting transition is reached.
Particles remain close but can rearrange. Heating raises temperature until the vaporization transition is reached.
Particles are widely separated and move freely through the available volume.
During melting or boiling, energy is still entering. It is being used to reorganize the microscopic state rather than raise temperature.
Different transitions and substances require different energy per unit mass, represented by the latent heat L.
Transition temperatures depend on pressure. A boiling point is not a universal fixed temperature independent of environment.