* Compound: A compound is formed when two or more elements chemically combine in a fixed ratio. The resulting substance has properties different from its constituent elements.

* Mixture: A mixture is a combination of two or more substances where each substance retains its own properties. The components can be separated by physical means.

* Solution: A solution is a special type of mixture where one substance (the solute) is dissolved evenly throughout another substance (the solvent).

Iron sulfide (FeS) is formed by a chemical reaction between iron (Fe) and sulfur (S). The resulting compound has distinct properties from both iron and sulfur, and it cannot be separated by physical means. It's a new substance formed through a chemical change.