A phase diagram shows which physical state of a substance is stable at different temperatures and pressures. The horizontal axis is temperature, and the vertical axis is pressure. The colored regions usually represent solid, liquid, and gas, while the lines between them show where two phases can coexist.
Phase diagrams matter because they help predict melting, boiling, sublimation, and unusual behavior such as dry ice turning directly into gas.
The most important landmarks are the triple point, where solid, liquid, and gas coexist, and the critical point, where the liquid and gas boundary ends. Moving across a phase boundary means the substance changes phase because temperature, pressure, or both have changed. Most substances have a solid-liquid boundary that slopes upward, but water is unusual because its solid-liquid line slopes downward.
This happens because ice is less dense than liquid water, so increasing pressure favors the denser liquid phase.
Key Facts
- A phase diagram plots pressure P on the y-axis and temperature T on the x-axis.
- Phase boundaries show conditions where two phases coexist in equilibrium.
- At the triple point, solid, liquid, and gas all coexist at one specific P and T.
- At the critical point, the liquid-gas boundary ends and a supercritical fluid can form.
- Water has a negative solid-liquid slope because liquid water is denser than ice.
- For a phase change at equilibrium, ΔG = 0 between the two phases.
Vocabulary
- Phase diagram
- A graph that shows the stable phase of a substance at different temperatures and pressures.
- Triple point
- The unique temperature and pressure where solid, liquid, and gas phases coexist in equilibrium.
- Critical point
- The end of the liquid-gas boundary beyond which liquid and gas are no longer distinct phases.
- Phase boundary
- A line on a phase diagram where two phases are stable together in equilibrium.
- Sublimation
- The phase change in which a solid changes directly into a gas without becoming a liquid.
Common Mistakes to Avoid
- Swapping the axes: temperature belongs on the horizontal axis and pressure belongs on the vertical axis in the usual phase diagram layout.
- Thinking a boundary line means no phase change occurs: a boundary line is exactly where two phases coexist and a phase change can happen at equilibrium.
- Assuming all substances melt at higher temperature when pressure increases: water is a major exception because its melting point decreases as pressure increases.
- Confusing the triple point with the critical point: the triple point involves three phases coexisting, while the critical point is where the liquid-gas distinction disappears.
Practice Questions
- 1 Carbon dioxide has a triple point at about 5.1 atm and -56.6 °C. At 1.0 atm and -78 °C, which phase change occurs when dry ice warms: melting or sublimation?
- 2 Water boils at 100 °C at 1 atm. If a pressure cooker raises the pressure to about 2 atm, should the boiling temperature be higher or lower than 100 °C? Explain using the liquid-gas boundary.
- 3 A skater presses down on ice with a high pressure blade. Use the unusual slope of water's solid-liquid boundary to explain why pressure can help form a thin liquid layer.