Phase Change
Definition
When matter switches state — melting, freezing, boiling, condensing — absorbing or releasing heat without changing temperature.
Technical definition
A transition between solid, liquid, gas (or plasma) phases occurring at constant temperature while latent heat is absorbed or released. Governed by the Clausius-Clapeyron relation; examples: fusion (melting), vaporization, sublimation, deposition.
Example
Ice water stays at 0°C until all the ice melts — every joule goes into breaking molecular bonds (latent heat), not raising temperature.
Common confusion
Temperature does not change during a phase change, no matter how much you heat. Students expect boiling water to keep getting hotter — it does not; the extra energy becomes latent heat until the phase change completes.