Full Idea
Putnam presumes it is correct to say that ice and steam are forms of water, rather than that ice, water and steam are three forms of H2O. If we allow the latter, then 'water is H2O' is not an identity, but elliptical for 'water is H2O in liquid state'.
Gist of Idea
We say ice and steam are different forms of water, but not that they are different forms of H2O
Source
comment on Hilary Putnam (The Meaning of 'Meaning' [1975]) by Graeme Forbes - The Metaphysics of Modality 8.2
Book Reference
Forbes,Graeme: 'The Metaphysics of Modality' [OUP 1985], p.196
A Reaction
This nice observation seems to reveal that the word 'water' is ambiguous. I presume the ambiguity preceded the discovery of its chemical construction. Shakespeare would have hesitated over whether to say 'water is ice'. Context would matter.