[thedoc (OP)]

David and Judith Coley  asked the Naked Scientists:
   
There was a BBC 4 science programme about making 5 common gases into liquid and the history of the scientist that did it. They began with the easiest gas, I think it may have been Hydrogen, and cooled through a coiled heat exchanger and also expanded it through a nozzle to cool it. Then they compressed it and did this a nunber of times until it was liquefied.
 
Then they used this as the coolant to do the same to the next difficult gas, ie the one with next the lower temperature of changing from gas to liquid, and so on until they had liquefied the gas with the lowest liquefying temperature.
 
They explained that there was a certain critical temperature above which no amount of compression of the gas would turn it into a liquid. This puzzled me and I want to know what the mechanism is that dictates this critical temperature.  Is it something to do with the atomic structure of the molecule - how they are bonded together etc. or the balance or equilibrium of the electrons orbiting the nucleus, or what please?
 
 Very Curious      
 
Dave Coley
What do you think?

[Bored chemist]

Imagine that you get a strong glass container and put some dry ice (solid CO2) it it) and then pump the air out  and seal the container.
That gives you a container that has CO2 and nothing else which make sit easier to consider.
Let it warm up a bit and you will have (near room temperature) liquid CO2 at the bottom, and CO2 gas above it.
Warm it some more and some of the liquid will evaporate- there is now more CO2 in the gas, and the volume hasn't changed much, so the gas gets denser.
However the liquid expands slightly as it warms up so its density falls.
If you think about it, there must be a point where, as the density of the gas rises and the density of the liquid falls, they must reach the same value.
So you have a container where the density (and thus the concentration) of CO2 is the same throughout and you can't tell if it's a gas or a liquid.
That's the critical temperature.

[evan_au]

The verbal description provided by Bored Chemist is often expressed in a phase diagram.

The one for Carbon Dioxide is here, although it is similar for other stable substances:
https://en.wikipedia.org/wiki/File:Carbon_dioxide_pressure-temperature_phase_diagram.svg

This shows that the critical temperature of CO₂ is about 300K, which is around room temperature.
CO₂ can't exist as a liquid above this temperature (to the right of the dotted line).
It is a bit more complex in unstable chemicals, because they may decompose before they change state.