Is “Water is H2O” a posteriori necessary truth? Let me try another way to show the claim that “water is H2O” is a posteriori necessary truth is problematic. First, it seems to me, that if we learn to point to the glass if liquid and call it “water” one time and call it “H2O” another time, it is using the word the same same way to refer to the same thing. No problems here. The words mean the same so by definition it is necessary. Whether a prior or a posteriori, you pick.
We also learn the word “H2O” another away. And to learn how to use it appropriately within the scientific community may take a lot education. Such things as atomic theory, elemental analysis, particle physics to name a few. To learn this concept is not done by pointing to a glass of water, though some do per the first example. In first example, I am referring to the glass of liquid, in the second example I am not referring to the liquid, if anything I am tentatively referring to an elemental formula. The next step to establish the identity of the liquid is by conducting an experiment to determine the elemental formula. The scientist comes back with “H2O”. This was established by experience, a posteriori. However, this could have been wrong, the experiment was performed incorrectly or worse our scientific theory incomplete. This is contingent. How can the term which can potentially be revised to refer to other elements establish an identity as necessary in this particular case? If you say to me, it does not matter you were always referring to the same thing anyway, I would say maybe you are confusing the first example with the second.