1. Solid to Liquid (Melting):
* Molecular Motion: Molecules in a solid are tightly packed and vibrate in fixed positions. As energy (heat) is added, these vibrations become more vigorous. Eventually, the vibrations overcome the attractive forces holding the molecules in a rigid structure, allowing them to move more freely.
* Spacing: The molecules move further apart as the rigid structure breaks down, leading to a less dense state.
2. Liquid to Gas (Boiling or Evaporation):
* Molecular Motion: In a liquid, molecules can move around more freely, but they are still relatively close together. As more energy is added, the molecules move faster and have enough energy to overcome the attractive forces holding them together. They escape into the gaseous phase.
* Spacing: The molecules become widely spaced apart as they enter the gaseous phase.
3. Gas to Liquid (Condensation):
* Molecular Motion: Gas molecules move very fast and are far apart. When they lose energy (cool down), they slow down and the attractive forces between them become more significant. This allows them to clump together as a liquid.
* Spacing: The molecules become more closely packed.
4. Liquid to Solid (Freezing):
* Molecular Motion: As the liquid cools, molecules slow down and their vibrations decrease. At the freezing point, the molecules arrange themselves into a regular, ordered pattern (crystal lattice).
* Spacing: The molecules become tightly packed together, forming a solid structure.
Key Points:
* Energy: Phase changes are driven by changes in the energy (heat) of the molecules.
* Attractive Forces: The strength of the attractive forces between molecules determines the phase at a given temperature and pressure.
* Density: The density of a substance changes as it transitions between phases due to the changes in the spacing between molecules.
Let me know if you'd like more detail on any specific phase change!
