Here's why:

* Substance: A substance is a pure form of matter with a fixed composition and distinct properties.

* Carbon: Charcoal is primarily composed of carbon, with varying amounts of other elements like hydrogen, oxygen, and nitrogen depending on its source and production method.

While charcoal might contain small amounts of other elements, it is primarily a form of pure carbon.

Contrast this with a mixture: A mixture is a combination of two or more substances where each substance retains its individual properties.