✔ 最佳答案
The answer is: D. sulphur dioxide.... yellow
Iron reacts with common acid to give iron(II) and hydrogen.
Fe(s) + 2H^+(aq) → Fe^2+(aq) + H2(g)
Hydrogen is given off and the solution becomes pale green due to the presence of iron(II) ions.
However, concentrated sulphuric acid, on heating, is a rather strong oxidizing agent. The above reaction thus does not occur. Instead, a redox reaction occurs.
2Fe(s) + 6H2SO4(conc) → Fe2(SO4)3(aq) + 6H2O(l) + 3SO2(g)
Refer to the above equation. Sulphur dioxide gas is given off, and the solution becomes yellow due to the presence of iron(III) ions.
Therefore, the key is D. A, B and C are incorrect.