One key difference between iron sulfide and a mixture is that iron sulfide is a chemical compound, while a mixture is a physical combination of substances.

Here's why:

* Iron sulfide (FeS) is formed by a chemical reaction between iron and sulfur. The atoms of iron and sulfur bond together, creating a new substance with different properties than either iron or sulfur alone.

* A mixture is a combination of substances where each component retains its own properties. For example, sand and salt mixed together are still sand and salt.

In short, iron sulfide is a new substance formed by a chemical change, while a mixture is a combination of substances that retain their individual identities.