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 generally as phase boundaries.

At 5.1 atm and −56.6 °C all three phases will coexist. This is the triple point of carbon dioxide.

At 73 atm and 31 °C, both liquid and vapor phases of carbon dioxide will coalesce into a single-phase supercritical fluid. This is the critical point of carbon dioxide.

In the region beyond the critical point no pressure or temperature change can convert a supercritical fluid to a gas or liquid.

The phase diagram of water has a few notable differences from that of carbon dioxide.

The fusion curve of carbon dioxide has a positive slope, while for water, the slope is negative. This is an atypical feature of water. Increasing the pressure favors a liquid-to- solid transition in carbon dioxide but a solid-to-liquid transition in water.

The higher pressure favors the denser solid form of carbon dioxide. In the case of water, the denser liquid form is favored