Phase Diagrams

Phase diagrams contain three key features: regions representing temperature and pressure conditions for single phases, lines or curves called phase boundaries that separate regions where two phases coexist in equilibrium, and special points like the triple point and critical point. Regions are labeled solid, liquid, or gas, while curves indicate where phase transitions occur at specific conditions.

At pressures below 5.1 atm, solid carbon dioxide undergoes sublimation, transitioning directly to the gaseous phase without passing through the liquid state. This direct solid-to-gas transition occurs because the pressure is too low to stabilize the liquid phase, regardless of temperature changes.

The triple point is the unique temperature and pressure condition where all three phases—solid, liquid, and gas—coexist in equilibrium simultaneously. For carbon dioxide, this occurs at 5.1 atm and −56.6 °C. Below the triple point pressure, a substance cannot exist as a liquid regardless of temperature.

Water's fusion curve has a negative slope, meaning its melting point decreases with increasing pressure—an unusual property. Carbon dioxide's fusion curve has a positive slope, with melting point increasing under higher pressure. This difference reflects that increased pressure favors water's denser liquid form but carbon dioxide's denser solid form.

A supercritical fluid is a unique state of matter with physical properties intermediate between liquid and gas that forms above the critical point. For carbon dioxide, this occurs at 73 atm and 31 °C. Beyond the critical point, no pressure or temperature change can convert the supercritical fluid back to distinct liquid or gaseous phases.

The curves on a phase diagram, called phase boundaries, represent the temperatures and pressures where two phases coexist in equilibrium. The sublimation curve shows solid-vapor equilibrium, the vaporization curve shows liquid-gas equilibrium, and the fusion curve shows solid-liquid equilibrium. Each curve marks the conditions for specific phase transitions.

Carbon dioxide cannot exist as a liquid at 1 atm because this pressure is below the triple point of 5.1 atm. At standard pressure, cooling gaseous carbon dioxide causes it to deposit directly into solid form. The liquid phase only becomes stable at pressures above 5.1 atm, regardless of temperature.