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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?

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A mixture is when two or more substances combine physically together.

However, in water, two hydrogen atoms combine with one oxygen atom chemically, forming a new substance that has properties different from hydrogen alone or oxygen alone.

For example, if you combine iron powder and sulfur powder physically (just mixing them together without applying heat), you can find that the mixture retains the properties of the original components, i.e. you can still use a magnet to attract the iron inside the mixture.

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.


Therefore, water is not a mixture; it is a compound and it is pure.

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A mixture contains different compounds/elements. Pure water only contains 1 type of molecule/compound - H2O, making it a pure substance

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