Entropy is a measure of the disorder or randomness of a system. During phase transitions, the entropy of a substance changes depending on how ordered or disordered the molecules are in each phase. Here’s a breakdown:
- Solid → Liquid (Melting/Fusion)
- In a solid, molecules are tightly packed in an ordered structure.
- When it melts into a liquid, molecules can move more freely.
- Entropy increases because the disorder of the system increases.
- Liquid → Gas (Vaporization/Boiling)
- In a liquid, molecules are close but can move around.
- In a gas, molecules are far apart and move randomly.
- Entropy increases significantly because gas has much more disorder than a liquid.
- Solid → Gas (Sublimation)
- Molecules go directly from an ordered solid to a very disordered gas.
- Entropy increases dramatically.
- Gas → Liquid (Condensation)
- Molecules go from highly disordered gas to more ordered liquid.
- Entropy decreases.
- Liquid → Solid (Freezing/Solidification)
- Molecules become fixed in a regular lattice.
- Entropy decreases because the system becomes more ordered.
- Gas → Solid (Deposition)
- Gas molecules directly form an ordered solid structure.
- Entropy decreases significantly.
Key idea:
- Entropy increases when a phase becomes more disordered (solid → liquid → gas).
- Entropy decreases when a phase becomes more ordered (gas → liquid → solid).