# Why so few fluid phases?

Taking a look at the phase diagram of water

By Cmglee - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14939155

I see ice phases as much as up to 'Ice XI', but only one 'liquid' and one 'vapor' phase. Why is this? Either there are many ice phases, or too few liquid phases relatively.

The latter seems to be the case as it seems to be consistent with other substances. No matter what number of solid phases you get, there is usually only one liquid and one vapor phase.

Why are there so few common$^1$ fluid phases? What is the physical explanation for this?

1: For the sake of not complicating the answer too much, please ignore more exotic phases such as mesophases etc. If, though, your answer benefits from relating to them without confusing the reader too much, by all means $\ldots$

• To me, there are too many fluid phases. Molecules in a solid are ordered, like soldiers in a formation - no wonder you can arrange them in a multitude of ways. But when there is a complete chaos, when they are moving, bouncing, turning this way and that, never standing still - how come it's possible to have two different manners of doing that? Come to think of it, they are not all that different, because critical point and stuff. But still they are different, and that bothers me. – Ivan Neretin Apr 14 '16 at 20:32
• The difference between liquid and gaseous is short range order, in the case of water resulting from hydrogen bonding. Nothing to worry about. ;-) – Karl Apr 14 '16 at 23:36
• And if the the intermolecular potential is purely repulsive (i.e. no short range attraction) you only get 1 fluid phase. – Ian Bush Apr 15 '16 at 7:48