The supercritical phase

At normal temperatures and pressures liquids turn to vapour when they are heated up. Water is the best-known example of this process. However, when temperature and pressure are raised to a certain level, liquids enter what is known as a supercritical phase. In the case of water, this means a pressure of 221 bars and a temperature of 374oC. Carbon dioxide is not so demanding; it requires 73 bars and 31oC to become supercritical. The concept of supercriticality means that we are no longer talking about liquids or gases, but a phase with properties of both these phases.