Wikipedia says that Carbon Dioxide cannot be liquid under atmospheric pressure.

I thought the phase of an element was related to both pressure and temperature.

If that is so, then should you not be able to set the temperature conditions such that you could have a cup of liquid Carbon Dioxide?


This is the phase diagram from a better source:

Carbon dioxide phase diagram

The pressure axis is the vertical one at the left. One bar (which equals 0.986923 atmospheres) is near the bottom, as shown on the diagram. So carbon dioxide is a gas at all temperatures above −78.5 °C when the pressure is one bar. Solid carbon dioxide (dry ice) sublimes at one bar (about one atmosphere), i.e., goes directly from solid to gas.

If you want liquid carbon dioxide, you need a temperature, pressure pair in the liquid region, e.g., −40.0 °C and 20 bar.

  • $\begingroup$ OK, so the idea is that it goes from gas right to solid? It skips the liquid phase at 1 atm? $\endgroup$
    – Vaccano
    Feb 19 '20 at 23:25
  • 2
    $\begingroup$ Below -78.5 degrees Celcius, at one atmosphere, carbon dioxide is solid. Above that temperature, still at one atmosphere, it is gas. There is no liquid phase at one atmosphere. One bar equals 0.986923 atmospheres. $\endgroup$
    – Ed V
    Feb 19 '20 at 23:34
  • 4
    $\begingroup$ @Vaccano All stable enough substances have a phase diagram for 3 phases. If a line of a given pressure or temperature does not cross the area of the phase of interest, that phase is not at given p/T thermodynamically stable. It can occur temporarily at sudden condition changes, until it transforms itself to other phase. $\endgroup$
    – Poutnik
    Feb 20 '20 at 2:49

The other answer already shows that this is a solid fact. Carbon dioxide is not (actually cannot stay) liquid at 100 kPa, period.

Why that is the case is much less obvious. I cannot give you a clear, rigorous, quantitative explanation.

But the general reasoning about this and similar observations is that once the crystal structure has broken up under increasing temperature, there is very little that can keep the molecules together.

The linear CO2 molecules are unpolar from a distance, but each bond is quite polar. They have to align carefully to have enough attraction, in the solid. Now if the crystal structure breaks up, all the outer oxygen atoms see are other oxygen atoms, with a negative partial charge. So the condensed phase just blows up.

Only with a significant outside pressure, the molecules in the liquid can jump from one attractive position to another without boiling away.

Naphtalene is a similar case: The molecule is rigid and has a high aspect ratio. It needs a lot of energy to be able to move in the condensed phase at all, but once it is able to turn a bit, it looses 90% of its contact area (for vdW bonding) with its neighbours, and takes off.

Liquid water is kept together by hydrogens bonds, which can break and reform extremely fast. Nitrogen molecules don´t care how their neighbours are aligned, neither does methane or ethane. Alkanes are flexible additionally. They all can move in a liquid phase and still keep up enough interaction.


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