Pressure plays a crucial role in determining the phase of a substance because it affects how tightly particles are packed and the energy required for phase changes.
1. General Principle
- Increasing pressure generally favors the phase with the higher density because particles are forced closer together.
- Example: For most substances, solid is denser than liquid, so higher pressure can promote solid formation.
- Decreasing pressure favors less dense phases, such as gases, because particles have more space to move.
2. Specific Effects
- Solid–Liquid Transition (Melting/Freezing)
- If a solid is denser than its liquid (like most metals), increasing pressure raises the melting point.
- Exception: Water. Ice is less dense than liquid water, so increasing pressure lowers the melting point, which is why ice can melt under high pressure (ice skating works because of this!).
- Liquid–Gas Transition (Boiling/Condensation)
- Boiling occurs when the vapor pressure of the liquid equals the external pressure.
- Higher pressure → higher boiling point (because vapor formation is harder under pressure).
- Lower pressure → lower boiling point, which is why water boils at <100°C on a mountain.
- Solid–Gas Transition (Sublimation/Deposition)
- Sublimation (solid → gas) is easier at low pressure, because gas molecules can escape more readily.
3. Phase Diagrams
On a P–T phase diagram:
- Phase boundaries slope depending on density differences.
- Triple point shifts with pressure changes.
- Critical point marks the limit above which liquid and gas are indistinguishable.
4. Intuitive Takeaway
- High pressure → favors dense phases (solid > liquid > gas, usually).
- Low pressure → favors less dense phases (gas > liquid > solid).
- Phase transitions (melting, boiling, sublimation) depend strongly on pressure.