Here's why:

* Fixed Composition: Silicon dioxide always has a fixed ratio of silicon atoms to oxygen atoms (1:2).

* Uniform Properties: All samples of silicon dioxide will have the same chemical and physical properties.

* Cannot be Separated by Physical Means: You cannot separate silicon and oxygen from silicon dioxide using simple physical methods like filtration or evaporation.

In contrast, a mixture would have variable composition and properties, and its components could be separated using physical methods.