Here's why:

* Chemical Composition: Acetone has a specific and consistent chemical formula (CH₃COCH₃). This means every molecule of acetone is identical.

* Fixed Properties: Pure substances have fixed and unchanging physical and chemical properties like melting point, boiling point, and density.

* Uniformity: Acetone is uniform throughout, meaning its composition is the same regardless of where you take a sample from.

While you might find acetone in a solution, like nail polish remover, that solution is a mixture. Acetone itself, however, is a pure substance.