What Is a Phase Diagram?
A phase diagram is a graphical representation showing the states of matter (solid, liquid, gas) of a substance at different temperatures and pressures. It's one of the most powerful tools in thermodynamics.
The Three Regions
Every phase diagram has three main regions:
- Solid region: Low temperature, high pressure (lower-left)
- Liquid region: Middle area (for most substances)
- Gas region: High temperature, low pressure (lower-right)
The Important Lines
Sublimation Curve
The line between the solid and gas regions. Along this line, solid and gas coexist. Crossing this line means sublimation (solid→gas) or deposition (gas→solid).
Melting Curve
The line between solid and liquid regions. Crossing it means melting or freezing.
Vaporization Curve
The line between liquid and gas regions. It ends at the critical point, beyond which liquid and gas become indistinguishable (supercritical fluid).
Key Points
The triple point is where all three lines meet — the only condition where all three phases coexist. The critical point is where the liquid-gas boundary ends.
💡 Pro Tip
To determine if a substance sublimes at atmospheric pressure, draw a horizontal line at 1 atm on the phase diagram. If it only crosses the sublimation curve (not the melting curve), the substance sublimes. Try our interactive phase diagram tool.