What is a phase diagram?
A phase diagram is a map of a substance's phases (solid, liquid, gas) as a function of pressure and temperature at equilibrium. It shows phase boundaries, the triple point, and the critical point.
Phase diagrams show three equilibrium curves—melting, boiling, and sublimation—where phases coexist. The triple point is where all three phases meet; the critical point is where liquid–gas distinction vanishes.
Step-by-step worked examples
Water has a triple point at 0.01°C and 0.006 atm. What does this mean physically?
At this unique point, solid ice, liquid water, and water vapor coexist in equilibrium No other combination of P and T allows all three phases together Small changes in P or T shift the system to one dominant phase
The boiling point of water is 100°C at 1 atm but 98°C at 0.9 atm. Why?
Boiling point depends on external pressure (follows the liquid-gas curve on phase diagram) Lower pressure allows vapor pressure to reach equilibrium at lower temperature This is why water boils at lower temperatures at high altitudes
At very high pressure, water remains liquid even above 100°C. Where on the phase diagram does this occur?
Above 1 atm on the phase diagram, along the liquid-gas curve Water can exist as liquid up to the critical point (~374°C, ~218 atm) Above critical pressure, there's no phase boundary—supercritical fluid forms
Flashcards
Quick quiz
Q1.At the triple point of CO₂ (−56.6°C, 5.1 atm), what phases are present?
Q2.If pressure increases along a vertical line on a phase diagram at constant temperature, what happens?
Q3.The critical point on a phase diagram marks…
Q4.Why does dry ice (solid CO₂) sublimate at room temperature and 1 atm?
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Common mistakes
Thinking the melting and boiling points are fixed regardless of pressure. — Correct: Both melting and boiling points vary with pressure and are shown as curves on phase diagrams.
Confusing the critical point with the triple point. — Correct: Triple point: all 3 phases coexist (lower P, lower T). Critical point: highest P,T where liquid-gas boundary exists.
Assuming sublimation occurs at all pressures. — Correct: Sublimation only occurs below the triple point pressure; at higher pressures, melting occurs first.
Thinking supercritical fluids have no density. — Correct: Supercritical fluids have density and viscosity intermediate between liquid and gas.
FAQ
How are phase diagrams determined experimentally?
Heating/cooling curves and measurements of phase transition temperatures at various pressures, then plotting equilibrium boundaries.
Why is the slope of the melting curve usually steep?
Melting involves small volume changes; pressure has a large effect. The Clausius-Clapeyron equation shows dP/dT ∝ ΔH/ΔV.
What is a supercritical fluid and why is it useful?
A fluid above the critical point with liquid-like density and gas-like viscosity; useful in extraction and synthesis (e.g., supercritical CO₂).
Can all substances have a triple point?
Yes, all pure substances have a triple point where all three phases coexist, though it may be at extreme pressures.