* Color Change: The starting materials, iron (grey) and sulfur (yellow), have distinct colors. The resulting iron sulfide has a dark, almost black color. This significant color change suggests a new substance has been formed.
* Formation of a New Substance: The iron sulfide has different chemical and physical properties than either iron or sulfur alone. It has a different melting point, density, and magnetic properties. This confirms the creation of a new substance.
* Heat and Light Emission: The reaction between iron and sulfur releases heat and light, indicating an energy change and the formation of new bonds. This is a common sign of a chemical reaction.
* Irreversibility: While iron sulfide can be broken down into iron and sulfur again, this requires additional energy and a different chemical process. This indicates that the original reaction was not simply a physical mixing but a chemical transformation.
In summary: The color change, formation of a new substance with unique properties, heat and light emission, and the irreversibility of the process all point to a chemical change occurring when iron and sulfur combine to form iron sulfide.
