Here's why:

* Fixed Composition: A pure substance has a fixed chemical composition. Hydrogen peroxide always contains two hydrogen atoms and two oxygen atoms per molecule.

* Uniform Properties: A pure substance has uniform properties throughout the sample. No matter where you take a sample of hydrogen peroxide, it will have the same chemical and physical properties.

While hydrogen peroxide can be found in different concentrations (e.g., 3% for household use, 30% for bleaching), this refers to the concentration of the solution, not a change in the substance itself.