Phase Change
A phase change is a physical change from one state of matter to another, such as melting, freezing, boiling, condensing, sublimation, or deposition.
Definition
A phase change is a physical change from one state of matter to another, caused by adding or removing energy, and it includes melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), sublimation (solid directly to gas), and deposition (gas directly to solid). During any phase change the temperature holds constant while the energy involved goes into breaking or forming the attractions between particles rather than speeding them up or slowing them down. Formally, a phase change is a first-order transition where the Gibbs free energies of the two phases are equal, so it happens at a fixed temperature for a given pressure and involves a latent heat, the enthalpy of fusion, vaporization, or sublimation, and on a phase diagram the transition lines follow the Clausius-Clapeyron relation, linking their slope to the enthalpy and volume changes involved.
Example
Ice melting into water, water boiling into steam, and steam condensing back into drops on a cold mirror are all phase changes of the same substance, $H_2O$, just rearranged. Heating a pot of ice raises its temperature to $0^\circ\text{C}$, holds it there until every bit has melted, then raises it again to $100^\circ\text{C}$, where it holds again until the water finishes boiling, and those flat stretches on the heating curve mark the phase changes themselves. Water's enthalpy of fusion is $6.01\text{ kJ/mol}$ ($334\text{ J/g}$) while its enthalpy of vaporization is $40.7\text{ kJ/mol}$ ($2260\text{ J/g}$), so boiling a gram of water takes almost seven times the energy of melting the same gram of ice.
Key Insight
Sweating cools you because evaporation is a phase change that absorbs heat, pulling energy from your skin as the water becomes vapor. Ice famously melts under pressure because it is less dense than liquid water, so the solid-liquid line on water's phase diagram slopes backward, an anomaly shared by only a few substances and the reason glaciers flow and ice turns slippery beneath a skate blade.