Phase Diagrams

Carbon dioxide can exist as a solid, liquid, or gas under specific temperatures and pressures. This dependence is mapped into a phase diagram, which includes three general features: regions, lines, and points.

Regions represent the temperature and pressure conditions for a single phase. At standard pressure, carbon dioxide can be solid or gaseous, depending on the temperature.

At pressures below 5.1 atm, if the temperature of solid carbon dioxide is raised, it will directly transition to the gaseous phase without passing through the liquid form.

A transition through all three phases — solid to liquid and liquid to gas — will occur at pressures above 5.1 atm.

The lines or curves that separate the regions denote the temperatures and pressures at which the phases on either side of the curve are in equilibrium.

For example, the point at 1 atm and −78.5 °C, lies on the curve separating the solid and vapor phases, so a solid–vapor equilibrium exists under those conditions. Accordingly, this is called the sublimation curve.

Similarly, the liquid-gas equilibrium exists on the vaporization curve, and the solid-liquid equilibrium exists on the fusion curve. These curves are known more genera