iron sulfide

Iron sulfide is the compound formed when iron and sulfur are heated together — proof that a genuinely new substance, not just a mixture, has been created.

• Before heating, iron filings and sulfur powder can be mixed and still separated with a magnet, since the iron keeps its own magnetic property; once heated into iron sulfide, the magnet has no effect at all.
• Its colour and texture become uniform throughout, unlike the mixture it came from, where yellow sulfur and dark iron grains were still visible side by side.
• Iron sulfide reacts with dilute hydrochloric acid to release hydrogen sulfide gas, easy to recognise by its distinctive rotten-egg smell — quite different from the odourless hydrogen gas that plain iron filings would produce with the same acid.
• This contrast between Sample A (the mixture) and Sample B (iron sulfide) is exactly why chemists insist a compound's properties are new, not just "iron plus sulfur added up."