Answer the question on the basis of passage given below:
Nitrogen forms a large number of oxoacids such as \(H_2N_2O_2\) (hyponitrous acid), \(HNO_2\) (nitrous acid) and \(HNO_3\) (nitric acid). Amongst these, \(HNO_3\) is the most important. It is used in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives. It is also used to prepare many important organo-nitro compounds like nitroglycerine, trinitrotoluene etc. It acts as a strong oxidizing agent and used as an oxidiser in rocket fuels.
Nitric acid is commonly known to act as a strong oxidising agent. Which of the following metals will be easily oxidised by both dilute as well as conc. HNO3?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Zn
Nitric acid is a strong oxidising agent and attacks most metals except the noble metals such as gold and platinum. The product depends on the concentration of the acid. Zinc is oxidised by both dilute and concentrated acid:
\( \ce{4Zn + 10HNO3(dilute) -> 4Zn(NO3)2 + 5H2O + N2O} \)
\( \ce{Zn + 4HNO3(conc.) -> Zn(NO3)2 + 2H2O + 2NO2} \)
Dilute acid gives \( \ce{N2O} \), concentrated acid gives \( \ce{NO2} \); in both cases zinc goes into solution as \( \ce{Zn^{2+}} \).
Why the other options are incorrect
Option (2) Au — Incorrect. Gold is a noble metal and is not attacked by nitric acid of any concentration (it dissolves only in aqua regia).
Option (3) Al — Incorrect. Aluminium dissolves in dilute nitric acid, but in concentrated nitric acid a thin passive film of oxide forms on its surface and stops further reaction.
Option (4) Pt — Incorrect. Platinum is a noble metal; like gold it is not oxidised by nitric acid.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Nitric Acid – “… Concentrated nitric acid is a strong oxidising agent and attacks most metals except noble metals such as gold and platinum. The products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation. \(\ce{3Cu + 8HNO3(dilute) -> 3Cu(NO3)2 + 2NO + 4H2O}\) \(\ce{Cu + 4HNO3(conc.) -> Cu(NO3)2 + 2NO2 + 2H2O}\) Zinc reacts with dilute nitric acid to give N2O and with concentrated acid to give NO2. \(\ce{4Zn + 10HNO3(dilute) -> 4Zn(NO3)2 + 5H2O + N2O}\) \(\ce{Zn + 4HNO3(conc.) -> Zn(NO3)2 + 2H2O + 2NO2}\) Some metals (e.g., Cr, Al) do not dissolve in concentrated nitric acid because of the formation of a passive film of oxide on the surface.”