Hydrogen
Explanation
The chemical properties of metals and non-metals, specifically their reaction with dilute acids, and distinguishes between the behavior of physical mixtures and chemical compounds.
Detailed Analysis:
- Nature of the Substance: The question specifies a physical mixture of iron filings (Fe) and sulphur powder (S). In a physical mixture, the individual components retain their specific chemical properties. They have not chemically combined to form iron sulphide (FeS).
- Reaction of Iron: Iron is a metal placed above hydrogen in the reactivity series. When dilute sulphuric acid ($H_2SO_4$) is added to iron, a displacement reaction occurs where iron displaces hydrogen to form ferrous sulphate and hydrogen gas.
Reaction: $Fe(s) + H_2SO_4(aq) \rightarrow FeSO_4(aq) + H_2(g)$ - Reaction of Sulphur: Sulphur is a non-metal and does not react with dilute acids to evolve hydrogen gas. Therefore, it remains unaffected in this context.
- Comparison with Compound: If the iron and sulphur had been heated together to form the compound Iron Sulphide (FeS) prior to adding acid, the reaction would have produced Hydrogen Sulphide ($H_2S$) gas, which has a characteristic smell of rotten eggs. Since it is a mixture, only Hydrogen is evolved.
Key Takeaway:
Active metals react with dilute mineral acids to evolve hydrogen gas. In a physical mixture, components react individually according to their own chemical properties, unlike in a compound where the constituents lose their individual identities.