* Multiple Forms: Copper can form two main oxides: copper(I) oxide (Cu₂O) and copper(II) oxide (CuO). These are distinct chemical compounds with different compositions and properties.

* Impurities: Even within a specific copper oxide, there can be impurities. For example, a sample of copper(II) oxide might contain traces of other elements or even remnants of the starting material used to create it.

Therefore, copper oxide is best described as a mixture, specifically a mixture of different copper oxides and potentially other impurities.