Brass is classified as a mixture, specifically an alloy. An alloy is a combination of two or more metals, and in the case of brass, it primarily consists of copper and zinc. The proportions of copper and zinc can vary, which leads to different types of brass with varying properties.
The main reason brass is not considered a pure substance or a compound is due to its composition. A pure substance has a uniform and definite composition, while a compound is a substance formed when two or more elements chemically bond in a fixed ratio. Brass, however, does not have a fixed composition; instead, it is a blend of metals that retain their individual properties, which is characteristic of a mixture.
In summary, brass falls under the category of a mixture due to its nature as an alloy made from copper and zinc, allowing for a variety of compositions and properties.