Iron sulphide in hydrochloric acid. Image 4 of 4. Iron sulphide reacts with hydrochloric acid to form iron chloride and the poisonous gas hydrogen sulphide, which bubbles off. This is part of a demonstration showing the differences between a chemical and a physical change. A mixture of iron and sulphur (a physical change) has different properties from iron sulphide, the compound formed when they react together (a chemical change). This can be demonstrated by their reactivity with hydrochloric acid. Images A500/596, A500/597 and A500/598 respectively show iron, sulphur and an iron/sulphur mixture in hydrochloric acid.