Critical Temperature: What Makes Liquification Impossible?

[Pengwuino]

So as i understand it, a critical temperature is the temperature at which a substance must be under in order for it to be liquified. Any temperature above this means it is impossible to liquify no matter how much pressure you apply. So what exactly is happening that makes it impossible?

 

[Gokul43201]

The liquid phase is characterized by the intermolecular interaction (or potential) energy being larger than the molecular KE. While the latter quantity can be increased indefinitely by increasing the temperature, the PE has a maximum (negative) value at some characteristic intermolecular separation (see figure). So increasing the pressure can not increase the PE beyond this extremal value.

In the vicinity of the critical point, the PE and KE are roughly equal (~0.1 eV for water), and the phase loses its liquid character. Just beyond the CP, what usually happens is that the molecules tend to group up into tiny (~ a few nanometers across) clusters moving about at large velocities (or KE). However, the velocities of molecules within the cluster are somewhat smaller. So, in this regime, the substance consists of a gas-like dispersion of tiny, liquid-like clusters.

Increasing the temperature (or decreasing pressure) beyond this point results in a true gas.

Figure (see link) : 2-atom PE as a function of interatomic distance, for H-atoms.
http://www.raunvis.hi.is/~agust/F14_3.GIF

 

[wangasu]

Hi,Gokul, what happens at critical points is similar to (or the same as) that at spinodal points of superheated liquids? By the way, does spinodal point depend on heating rate? Thanks.