Here's why:
* Phase changes require energy: Changing state involves breaking or forming bonds between molecules. This requires energy input (for melting, boiling, sublimation) or energy release (for freezing, condensing, deposition).
* Energy goes into changing state, not temperature: Instead of increasing the kinetic energy of molecules (which would raise temperature), the energy is used to overcome the forces holding the molecules together in the current state.
* Constant temperature during phase change: The added or released energy is used entirely to change the state of the substance, so the temperature remains constant during the phase change.
Example: When ice melts, the energy supplied goes into breaking the bonds between water molecules, allowing them to move more freely as a liquid. The temperature remains at 0°C until all the ice has melted.
Note: There are some nuances in this explanation, such as the concept of "latent heat" and the idea that very small temperature changes can sometimes occur during a phase change. However, for most practical purposes, the temperature of a substance remains constant during a phase change.
