Here's why:

* Phase changes involve changes in energy, not composition: When a substance changes phase (like from solid ice to liquid water), the molecules themselves don't change their chemical structure. Only the arrangement and movement of the molecules change.

* Energy input or removal reverses the change: Adding heat energy can cause a substance to melt or evaporate. Removing heat energy causes a substance to freeze or condense.

Examples:

* Melting: Solid ice melts into liquid water when heat is added. This change can be reversed by cooling the water, causing it to freeze back into ice.

* Boiling: Liquid water boils into water vapor (gas) when heat is added. This change can be reversed by cooling the vapor, causing it to condense back into liquid water.

Key Takeaway: Phase changes are temporary shifts in the physical state of matter that can be reversed by changing the temperature or pressure. The substance's chemical composition remains unchanged.