Here's why:

* Fixed composition: Ethanol has a specific chemical formula (C2H5OH) and a consistent ratio of carbon, hydrogen, and oxygen atoms. This fixed composition defines it as a pure substance.

* Unique properties: Ethanol has distinct physical and chemical properties that are consistent throughout the sample. This is another characteristic of a pure substance.

In contrast, a mixture would have:

* Variable composition: The proportions of different components in a mixture can vary.

* No fixed properties: The properties of a mixture are determined by the properties of its components and their proportions.

So, even though ethyl alcohol might be found in different solutions (like alcoholic beverages), the ethanol itself remains a pure substance.