Explore P-T phase diagrams for water and carbon dioxide.
A phase diagram compresses an enormous amount of information about a substance's behavior into a single pressure-temperature plot, and correctly reading one is a frequently tested AP Chemistry Unit 3 and IChO skill that goes well beyond memorizing "solid, liquid, gas."
Each region of a phase diagram represents the pressure-temperature conditions under which one phase is thermodynamically stable, and the lines separating regions represent conditions where two phases coexist in equilibrium — cross a line and the substance undergoes a phase transition. The triple point is the unique pressure-temperature combination where all three phases (solid, liquid, gas) coexist simultaneously; for water, this occurs at 0.01°C and 0.006 atm, a fixed reference point precise enough that it was historically used to define the Kelvin scale itself.
The critical point marks the end of the liquid-gas boundary line: above the critical temperature and pressure, the distinction between liquid and gas disappears entirely, and the substance becomes a supercritical fluid with properties of both. Beyond the critical point, no amount of additional pressure will condense the gas back into a distinct liquid phase.
Water's phase diagram contains a famous anomaly: the solid-liquid boundary line has a negative slope, meaning increasing pressure on ice at a fixed temperature near the triple point can melt it — a direct consequence of ice being less dense than liquid water. Nearly every other substance shows a positive slope here, because their solid phase is denser than their liquid phase, so increasing pressure favors the more compact solid.
Carbon dioxide's phase diagram explains a familiar phenomenon directly: at normal atmospheric pressure (1 atm), the CO2 triple point pressure (5.1 atm) is never reached, so solid CO2 (dry ice) sublimates straight to gas without ever passing through a liquid phase at ordinary pressure.
This phase diagram explorer lets you trace paths through pressure-temperature space for both water and CO2, directly identifying the triple point, critical point, and phase boundaries you'll be asked to read on exams.