Why can't gases be liquefied below the critical temperature
As the substance approaches critical temperature, the properties of its gas and liquid phases converge, resulting in only one phase at a critical point: a homogeneous supercritical fluid.
The heat of vaporization is zero at and beyond this critical point, and so no distinction exists between the two phases.
The critical temperature is the temperature at which the density along with all other properties of the liquid and the vapour become the same.
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