Critical Temperature
Definition and meaning of Critical Temperature in chemistry.
Critical temperature is the highest temperature at which a substance can still be liquefied by applying pressure alone; above it, no amount of pressure will condense the gas into a liquid.
In more detail
At the critical temperature (Tc), the kinetic energy of molecules becomes so large that intermolecular attractive forces can no longer hold them together as a distinct liquid phase, no matter how much the gas is compressed. As temperature rises toward Tc, the densities of the coexisting liquid and vapor converge, the interface between them becomes fainter, and the enthalpy of vaporization falls to zero. Beyond Tc, at or above the corresponding critical pressure (Pc), the substance exists as a single supercritical fluid with properties intermediate between gas and liquid. Tc is unique to each substance and increases with the strength of intermolecular forces.
Key facts
| Symbol | Tc |
|---|---|
| CO2 critical temperature | 31.1 °C (304.2 K) |
| Associated quantity | Critical pressure (Pc) |
| Field | Physical Chemistry |
Carbon dioxide has a critical temperature of 31.1°C (304.2 K) and a critical pressure of 72.8 atm. Below 31.1°C, CO2 gas can be compressed into liquid CO2, but above this temperature it remains a gas at any pressure; instead, it becomes supercritical CO2, which is widely used as a nonpolar solvent for decaffeinating coffee.
Frequently asked questions
What happens to a substance above its critical temperature?
It becomes a supercritical fluid: a single phase with no distinct liquid or vapor region, since compression alone can no longer condense it.
Why does a stronger intermolecular attraction raise the critical temperature?
Stronger attractive forces require more thermal (kinetic) energy to overcome, so a higher temperature is needed before those forces fail to hold molecules together as a liquid under any pressure.