Understanding the Liquid Form of Carbon Dioxide: Conditions and Applications

Carbon dioxide (CO2) is a versatile molecule widely used in various industrial and commercial applications. One of its unique properties is the ability to exist in a liquid form under specific temperature and pressure conditions. This article explores these conditions, the phase diagram of CO2, and its practical applications.

Conditions for Liquid CO2 Formation

Carbon dioxide can exist as a liquid under certain conditions. This occurs through a combination of high pressure and low temperature, typically above its triple point. The triple point of CO2 is defined as the unique combination of temperature and pressure where solid, liquid, and gas phases can coexist in stable equilibrium. For CO2, the triple point is at a pressure of about 5.1 atmospheres (0.51 MPa) and a temperature of -56.6 deg;C (-69.9 deg;F).

The Unique Phase Transition

Notably, CO2 does not have a distinct liquid phase at atmospheric pressure; instead, it transitions directly from the solid state (dry ice) to the gaseous state. This unique phase transition makes it distinct from many other substances that can exist in all three states under varying conditions.

Phase Diagram of Carbon Dioxide

A visual representation of these conditions is provided in the CO2 phase diagram. The diagram typically shows a temperature scale in both Celsius and Fahrenheit and a pressure scale in both bar and atmospheres. The critical point, where CO2 can no longer be solely in a liquid state, is at 31 deg;C (88 deg;F) and a pressure of about 74 atmospheres. This point is where the substance transitions between liquid and gas states without a distinct phase.

Compressing CO2 to Liquid

To compress CO2 to a liquid state, one must either lower the temperature to below the triple point or increase the pressure beyond the triple point. At room temperature (approximately 20 deg;C or 68 deg;F), the required pressure to liquefy CO2 would be in the range of dozens of atmospheres. In practice, achieving this requires specialized equipment, such as a steel pressure container.

For instance, medical oxygen is often stored in pressurized containers, typically bottled at 200 bar in Central Europe, and the process to compress gases like CO2 to a liquid state is similar. By applying high pressure, even at temperatures just above the triple point, CO2 can be transformed into a liquid phase.