I think "phases" is what you're looking for.

phases gets my vote too.

Isotopes applies to elements - different numbers of neutrons in the nucleus, but teh same no. of protons
Isomers applies to molecules - same formula, but different structure (although I don't think they cover left- and right-handedness e.g. a long hydrocarbon chain with distinct ends may have a side shoot on the left or right, and is thus left- or right-handed)
Isobars is in weather
ISAs are something completely different.

Tommo

Thanks to all of you, I didn't expect any answers quite so quickly!

Thanks for 'phases', Richard - not a term I was familiar with - unfortunately I don't think it quite works, since (according to Wikipedia) "two states [of a system] are in the same phase if they can be transformed into one another without any abrupt changes", which if I understand correctly would leave rivers and rain in the same phase. You're quite right that all those things I mentioned could be looked at in terms of phases, but I somehow need a term 'X' where each of the things in my list is a different X, if you see what I mean.

(John, this is also why 'states' isn't sufficient - I need a term that registers the difference between rivers and rain, or between crystals of snow and a block of ice, as well as what they have in common (i.e. all being different X's of the same substance ...).

I did hope that there was something a bit like isomers, but it's a physical rather than a chemical distinction I'm looking for, I suppose, isn't it.

  Thanks for all your help though!!

Yep, I think I may have to stick with 'form'. It's only for an extended metaphor, really - I'm not going to be hauled up before a committee of scientists! - but I often find that discovering a nice specific term can help focus the metaphor. I'd already come up with 'form', and also 'manifestation' ...

Thanks, John. I think you've made me feel better about 'form', by giving such a carefully phrased explanation of how the different forms I'd mentioned come about.

Anyway, if anyone suddenly wakes up in the middle of the night (it's what I've been hoping would happen to me all week) with a very precise scientific term then do let me know 

Cheers
t_i_n

http://www.biochemj.org/bj/299/0171/2990171.pdf

this looks impenetrable but it came up when i googled 'expression of form' since i figured that rain and river were two expressions of the state form of water dk if this helps; a really interesting and difficult question innit!


i also think this is worth looking at but not quite what you want but 'fugacity' is such a delightful word!
http://en.wikipedia.org/wiki/Fugacity

Ah, well the different forms of carbon do have a name: they're allotropes. But (someone correct me if I'm wrong, please) they're chemically different, because they have different types of bond, whereas water (liquid) and ice/snow are only physically different (states of matter). And as for rivers and rain ... well, yes, I think your explanation of how those different manifestations come about works better than any single word we've come up with so far.

I do like 'fugacity', though, calum - and it's quite a musical term too! 

I think 'allotrope' is only used in the case of elements, isn't it? So you can have (as people have said) allotropes of carbon and sulphur (atoms of the same element combined in different spatial structures: diamond, graphite and 'buckyballs' comes to mind from somewhere or other in the distant past, as in 'buckminsterfullerenes' in the case of carbon) but you couldn't have different allotropes in the case of molecules such as water.

And allotropes are allotropes only within the same physical state (allotropes of solid carbon, for example). They are the same chemical substance but have different physical properties because of the different way that the atomic bonds line themselves up spatially. They are the same bonds, just configured differently. It's the fact that, in the case of a few elements such as carbon and sulphur, they can do this within the same physical state that makes them allotropes. I suppose there comes a point when what you call these things comes down as much to linguistic choice as much as to chemistry or physics.

I think in terms of their physics and chemistry the difference between liquid water as in a river and liquid water as in a raindrop is probably as mundane as different 'shape'  .

I am very taken with 'fugacity' though. Lovely word. I think I can hear a stampede of composers in the distance all jostling to be first with 'Fugacity IV: fughetta per quatro fagotti'.