Is water a mixure? Bridging the distinction between physical and chemical properties

Two inter-linked theses are defended in this paper. One is the Duhemian theme that a rigid distinction between physical and chemical properties cannot be upheld. Duhem maintained this view not because the latter are reducible to the former, but because if physics is to remain consistent with chemistry it must prove possible to expand it to accommodate new features, and a rigid distinction would be a barrier to this process. The second theme is that naturally occurring isotopic variants of water are in fact distinct substances, and naturally occurring samples of water are mixtures of these substances. For most practical purposes it is convenient to treat protium oxide, deuterium oxide, and so on, as the same chemical substance, but to insist on this as a matter of principle would stand in conflict with the first thesis.