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.
Understanding Chemistry: Phase Diagrams
A phase is chosen by a balance between energy and disorder. Particles in a solid are held in regular positions, so they usually have lower internal energy. Particles in a gas move much more freely, so a gas has greater disorder.
Temperature makes the disorder term more important. Pressure makes volume more important, since squeezing a substance favors the phase that occupies less space. Chemists combine these effects in a quantity called Gibbs free energy.
Under fixed conditions, the stable phase is the one with the lowest Gibbs free energy. On a boundary, the competing phases have equal Gibbs free energy, so neither has an overall advantage.
Energy must be transferred during a phase change. Melting requires energy to loosen particle attractions. Vaporization requires still more energy because particles must separate widely.
This energy is called latent heat. It changes the arrangement and motion of particles rather than raising the temperature. That is why ice can melt in a drink while the mixture stays near the melting temperature.
At equilibrium, particles continue changing in both directions. For example, molecules leave a liquid surface while others return from the vapor. The amounts stay steady when the two rates match.
The liquid to gas boundary is closely linked to vapor pressure. Every liquid has some particles energetic enough to escape from its surface. Their vapor pushes back on the liquid, creating vapor pressure.
A liquid boils when its vapor pressure matches the surrounding pressure. At high altitude, outside pressure is lower, so water boils at a lower temperature and food can take longer to cook. In a pressure cooker, the higher pressure raises the boiling temperature.
Below the pressure of the triple point, heating a solid cannot produce an ordinary liquid. It crosses into vapor instead. This is why dry ice can disappear without leaving a puddle.
The end of the liquid to gas boundary has a special physical meaning. Near this condition, liquid and vapor become increasingly similar in density. Their surface boundary fades away.
Beyond it, there is one fluid phase that can flow through spaces like a gas yet have a density closer to a liquid. Such supercritical fluids are used in some extraction processes and in industrial cleaning. Phase diagrams for pure substances are useful models, but real materials can be more complicated.
Salt dissolved in water changes freezing and boiling conditions. Mixtures can have several components and broader transition ranges.
When reading a diagram, follow the conditions of a real process rather than treating every line as a heating path. Heating at constant pressure is represented by movement across the page. Compression at constant temperature is represented by upward movement.
A diagonal change means both conditions change together. Notice the scale on the pressure axis, since it is often logarithmic and equal distances may represent large pressure changes. Real substances can temporarily remain liquid below their freezing point or vapor above their condensation point.
This metastable behavior does not replace the diagram. It shows that phase changes may need a small starting disturbance, such as a crystal surface or a dust particle.
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.