Here's why:

* Substance: A substance is a pure form of matter with a definite and consistent composition. It has specific chemical properties and a fixed chemical formula.

* Pure water: Pure water consists solely of hydrogen and oxygen atoms in a fixed ratio (H₂O). Its composition is always the same, regardless of its source.

Therefore, pure water fits the definition of a substance. It's important to note that "pure water" is a theoretical concept, as it's difficult to find water completely free of impurities in nature. However, for scientific purposes, we often consider water with negligible impurities to be pure.