* New Substance Formed: When iron (Fe) and sulfur (S) are heated, they react to form iron sulfide (FeS). This is a completely new substance with different properties than either iron or sulfur alone.

* Irreversible: The formation of iron sulfide is difficult to reverse. You can't easily separate the iron and sulfur back out without further chemical reactions.

* Energy Changes: Chemical reactions often involve energy changes, such as the release of heat or light. The reaction of iron and sulfur likely releases heat, indicating a chemical change.

In contrast, a physical change alters the form or appearance of a substance but doesn't create a new substance. Examples include:

* Melting ice (water changes state but remains H₂O)

* Crushing a rock (size changes, but it's still the same rock)