# Why is water considered a pure substance instead of a mixture?

Why is water considered a pure substance instead of a mixture?

Wouldn't it, when the hydrogen atoms combine with the oxygen atom, be considered a mixture, because the atoms were combined to form a new molecule?

However, if you heat it up, the iron and the sulfur would combine chemically, and a new compound would be formed, which we call "iron sulfur" ($\ce{FeS}$). This is a new compound and loses the properties of the original compounds. For example, it is not attracted by magnets.