Phase transitions—like ice melting, water boiling, or steam condensing—happen when matter changes from one state to another. Physical chemistry explains them by looking at energy, disorder, and how molecules behave.
- Energy and stability:
Every phase (solid, liquid, gas) has a certain energy. A substance changes phase when the new phase becomes more stable under the current conditions (temperature and pressure). - Heat and disorder:
- Melting or boiling requires energy to break the forces holding molecules together.
- Freezing or condensation releases energy because molecules settle into a more ordered arrangement.
- During these changes, the disorder of molecules (entropy) also changes: liquids are more disordered than solids, and gases are more disordered than liquids.
- Molecular perspective:
Molecules are constantly moving. A phase transition happens when their motion and interactions reach a point where the molecules can rearrange into a different structure.- In solids, molecules are tightly packed and vibrate in place.
- In liquids, molecules move more freely.
- In gases, molecules move very fast and far apart.
- Phase diagrams:
Physical chemists use diagrams showing which phase is stable at different temperatures and pressures. These diagrams help predict when a substance will melt, boil, or condense. - Types of phase transitions:
- First-order: There’s a clear jump in energy and structure (like melting or boiling).
- Second-order: The change is more subtle, like in certain magnetic or superconducting transitions.
In short: phase transitions happen when energy, molecular movement, and stability combine so that a new phase becomes favorable.