H2O

Kripke argued in Naming and Necessity, a name does not secure its reference via any process of description fitting. Rather, a name determines its reference via a historical-causal link tracing back to the process of naming. And thus, Kripke thinks that name does not have sense, or, at least, does not have sense which is rich enough to play the reference-determining role. Moreover, name, in Kripke's view, is a rigid designator, which refers to the same object in all possible worlds. Following this line of thought, Kripke suggests that all the scientific identity statement like "Water is H2O" is also a necessary statement, i.e. true in all possible worlds. Kripke thinks that this is a phenomenon that the descriptivist cannot explain.
And, as also proposed by Hilary Putnam and Kripke himself, Kripke's view on name can also be applied to the reference of natural kind term. The kind of theory of reference that is advocated by Kripke and Putnam is called the direct reference theory.
However, Chalmers disagrees with Kripke, and all the direct reference theorists in general. He thinks that there are two kinds of intension of a natural kind term, a stance which is now called two dimensionalism. For example, the words, "Water is H2O" are taken to express two distinct propositions, often referred to as a primary intension and a secondary intension, which together compose its meaning.
The primary intension of a word or sentence is its sense, i.e., is the idea or method by which we find its referent. The primary intension of "water" might be a description, such as watery stuff. The thing picked out by the primary intension of "water" could have been otherwise. For example, on some other world where the inhabitants take "water" to mean watery stuff, but where the chemical make-up of watery stuff is not H2O, it is not the case that water is H2O for that world.
The secondary intension of "water" is whatever thing "water" happens to pick out in this world, whatever that world happens to be. So if we assign "water" the primary intension watery stuff then the secondary intension of "water" is H2O, since H2O is watery stuff in this world. The secondary intension of "water" in our world is H2O, and is H2O in every world because unlike watery stuff it is impossible for H2O to be other than H2O. When considered according to its secondary intension, water means H2O in every world. Via this secondary intension, Chalmers proposes a way to explain the necessity of the identity statement on one hand, and reserve the role of intension/sense in determining the reference, in the other hand.