From organic synthesis to quantum chemical calculation, chemists interact with chemical substances in a wide variety of ways. But what even is a chemical substance? My aim is to propose a notion of chemical substance that is consistent with the way in which chemical substances are individuated in chemistry, addressing gaps in previous conceptions of chemical substance. Water is employed as a case study to develop the account, not only because it is a familiar example of a chemical substance, but (...) also because its structural peculiarities make it an ideal test case for drawing out potential issues and limitations. Examining four distinct views of chemical substance—the microstructural, thermodynamic, purification, and a functional/relational account—I conclude that each has considerable drawbacks when used as a standalone concept. However, these accounts are not rendered obsolete, but are combined into a semi-pluralist conceptual patchwork. My interactive account of chemical substance is consistent with existing substance descriptions and chemical practice. (shrink)