Here's why:

* Fixed Composition: Hydrogen peroxide always has the same chemical formula (H₂O₂), meaning it has a fixed ratio of hydrogen and oxygen atoms.

* Uniform Properties: A pure substance has consistent physical and chemical properties throughout its entire sample.

* Single Component: It's made up of only one type of molecule, the hydrogen peroxide molecule.

Mixtures, on the other hand, are combinations of two or more substances that retain their individual properties and can be separated by physical means.