$(a)$ $7\, g$ of iron filings and $4\, g$ of sulphur powder is treated with dilute sulphuric acid at room temperature. Name the gas produced.
$(b)$ The same mixture is heated strongly,cooled and then treated with dilute sulphuric acid. Which gas would be produced?
$(c)$ What is the cause of this difference in results of chemical reaction in different conditions?

(A) At room temperature,the iron filings and sulphur powder exist as a mixture. When treated with dilute sulphuric acid,the iron reacts to produce hydrogen gas $(H_2)$.
$(b)$ When the mixture is heated strongly,iron and sulphur react to form a chemical compound,iron sulphide $(FeS)$. When this compound is treated with dilute sulphuric acid,it produces hydrogen sulphide gas $(H_2S)$,which has a characteristic rotten egg smell.
$(c)$ The difference in results occurs because in the first case,the substances exist as a mixture,retaining their individual properties. In the second case,heating causes a chemical reaction that forms a new compound with entirely different chemical properties compared to its constituent elements.