Critical Pressure
Definition and meaning of Critical Pressure in chemistry.
Critical pressure is the vapor pressure of a substance at its critical temperature, the minimum pressure needed to liquefy a gas when it is exactly at that temperature, and no amount of additional pressure will condense it above this point.
In more detail
Critical pressure marks, together with critical temperature, the critical point on a phase diagram, where the densities of the liquid and vapor phases converge and become identical. Beyond this point the liquid-vapor boundary (meniscus) disappears, and the substance exists as a single supercritical fluid with properties intermediate between a gas and a liquid. Critical pressure and temperature are essential inputs for the principle of corresponding states, for van der Waals equation constants, and for designing supercritical fluid processes such as extraction and chromatography.
Key facts
| Symbol | Pc |
|---|---|
| Water's critical pressure | 22.06 MPa (217.7 atm) |
| CO2 critical pressure | 7.38 MPa (72.8 atm) |
| Field | Physical Chemistry |
Carbon dioxide has a critical pressure of 72.8 atm (7.38 MPa) and a critical temperature of 31.1 degrees C. Above both values, CO2 becomes a supercritical fluid, a state exploited in supercritical CO2 extraction to decaffeinate coffee without organic solvents.
Frequently asked questions
What happens to a substance above its critical pressure and temperature?
It becomes a supercritical fluid: a single phase with no distinct liquid or gas region and no visible meniscus, showing gas-like diffusivity but liquid-like density.
How is critical pressure measured?
Experimentally, a sealed sample is heated and pressurized along its vapor pressure curve until the meniscus between liquid and vapor vanishes; the pressure at that point is the critical pressure.