Here's why:

* Fixed Composition: Carbon monoxide always has the same chemical formula (CO) with one carbon atom and one oxygen atom.

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

* Cannot be Separated by Physical Means: You can't separate carbon monoxide into different components using simple methods like filtration or evaporation.

In contrast, a mixture would have:

* Variable Composition: Different parts of a mixture could have different proportions of components.

* Varying Properties: Properties like boiling point or color could change depending on the mixture's composition.

* Separable by Physical Means: You could use methods like distillation, filtration, or magnetism to separate the components of a mixture.