Critical Point
Definition and meaning of Critical Point in chemistry.
Critical point is the unique temperature and pressure at which the liquid and gas phases of a substance become identical, merging into a single supercritical fluid phase with no distinct interface between them.
In more detail
On a phase diagram, the liquid-vapor coexistence curve ends at the critical point; beyond it, no increase in pressure can condense the gas back into a liquid, because the two phases are no longer distinguishable. At the critical point, the densities of the liquid and vapor phases become equal, and the heat of vaporization drops to zero. This behavior underlies supercritical fluid technology, where a substance held above both its critical temperature and critical pressure exhibits properties intermediate between a liquid and a gas, such as liquid-like density with gas-like diffusivity.
Key facts
| Field | Physical Chemistry |
|---|---|
| Water's critical point | 374 °C (647 K), 22.1 MPa (218 atm) |
| CO2's critical point | 31.1 °C (304.25 K), 7.38 MPa (73.8 atm) |
| Located on | Terminus of the liquid-vapor coexistence curve on a phase diagram |
Carbon dioxide has a critical point at 31.1°C (304.25 K) and 73.8 atm (7.38 MPa); above these conditions, supercritical CO2 is widely used as a nontoxic solvent to decaffeinate coffee and extract essential oils.
Frequently asked questions
What happens to a substance above its critical point?
It exists as a supercritical fluid, a single phase that has no distinct liquid or gas region and cannot be liquefied by pressure alone.
Is the critical point the same as the triple point?
No. The triple point is where solid, liquid, and gas coexist at low pressure and temperature, while the critical point is where the liquid and gas phases merge at high pressure and temperature.