* Substance: A substance has a fixed composition and cannot be separated by physical means. It has a specific set of properties. Examples include water (H₂O), salt (NaCl), and gold (Au).

* Mixture: A mixture consists of two or more substances that are physically combined but not chemically bonded. The substances in a mixture retain their individual properties and can be separated by physical means.

When you heat sulfur and iron filings, they react chemically to form iron sulfide (FeS). This is a chemical change, and the resulting product is a new substance with properties distinct from sulfur and iron.

Here's a breakdown of the process:

1. Initial state: You have a mixture of sulfur and iron filings. They are physically combined, and you can easily separate them (e.g., using a magnet to remove the iron).

2. Heating: When you heat the mixture, a chemical reaction occurs. The sulfur and iron atoms combine to form iron sulfide.

3. Final state: You now have a new substance, iron sulfide, which has different properties than sulfur or iron. This substance cannot be easily separated back into its original components by physical means.

Therefore, heated sulfur and iron filings initially form a mixture, but the reaction results in the formation of a new substance, iron sulfide.