Here's why:

* Mixture vs. Compound: A mixture is a combination of two or more substances where each substance retains its own chemical identity. In contrast, a compound is formed when two or more elements chemically combine in a fixed ratio, creating a new substance with different properties.

* Iron and Sulfur: In a mixture of iron and sulfur, the iron particles remain as iron, and the sulfur particles remain as sulfur. They are simply physically combined, not chemically bonded.

Think of it like this:

Imagine you have a bowl of sand and sugar. You can see both the sand and the sugar separately, even though they are mixed together. If you heat the mixture, the sugar will melt and turn brown, but the sand will remain unchanged. This is because the sand and sugar are just mixed together, they haven't formed a new substance.

However, if you heat the mixture of iron and sulfur strongly, they will react chemically to form iron sulfide (FeS), a new compound with different properties than either iron or sulfur.