Why Carbon Dioxide Never Exists as a Liquid

Carbon dioxide (CO2) is a unique substance that behaves differently from many other gases. One of the most notable peculiarities of CO2 is that it cannot exist as a liquid under normal atmospheric conditions. This article delves into the reasons behind this and explores the conditions under which liquid CO2 can form.

Phase Diagram

The phase diagram of CO2 reveals the boundaries of its solid, liquid, and gaseous states. Under normal atmospheric pressure (1 atmosphere), CO2 only exists in two states: solid and gas. The phase diagram shows that CO2 sublimates directly from solid to gas without passing through a liquid phase. The conditions required to maintain CO2 as a liquid are above its triple point, which occurs at an approximate pressure of 5.1 atmospheres and a temperature of -56.6°C.

Sublimation

At standard atmospheric pressure (1 atm), CO2 transitions between solid and gas at -78.5°C through the process of sublimation. When CO2 is cooled to its solid form (dry ice), it does not melt into a liquid; instead, it turns directly into a gas as it warms up. This is due to the weak intermolecular forces between CO2 molecules, which are mainly dispersive or van der Waals forces. These forces are too weak to hold CO2 molecules in a liquid state at 1 atm.

Critical Point

The critical point of CO2 is defined as the maximum pressure and temperature at which liquid and gas phases can coexist. For CO2, the critical point is at 31.0°C and 73.8 atm. At temperatures and pressures above the critical point, CO2 can exist in a supercritical state, which exhibits properties of both gas and liquid. However, at normal atmospheric pressure, CO2 cannot exist as a liquid due to its weak intermolecular forces and low vapor pressure.

Creating Liquid CO2

Despite the inability of CO2 to form a liquid at 1 atmosphere, there is a way to achieve a liquid form under normal circumstances. This is done by increasing the pressure while maintaining the temperature. By keeping CO2 in a strong container, the pressure can be adjusted to levels above 5.11 atmospheres, causing CO2 to form a liquid state. The increased pressure enhances the intermolecular forces, allowing the molecules to come closer together and favor the liquid state.

Conclusion

In summary, CO2 does not exist as a liquid at atmospheric pressure due to its unique sublimation properties and phase behavior. However, when the pressure is sufficiently high, liquid CO2 can be produced. Understanding these principles is crucial for various applications, including the production of carbonated beverages, fire extinguishers, and chemical processes.

For more in-depth knowledge on the physical properties of CO2 and other gases, consult the links below:

Understanding the Phase Diagram of CO2 The Science Behind Sublimation The Critical Point and Supercritical Fluids