Which of the following metals produce hydrogen gas on reaction with dilute nitric acid?

Question

Which of the following metals produce hydrogen gas on reaction with dilute nitric acid?

1. A. Copper (Cu)
2. B. Zinc (Zn)
3. C. Magnesium (Mg) (Correct answer)
4. D. All of the above

Correct Answer

Option C — Magnesium (Mg)

Detailed Solution & Explanation

The correct answer is Magnesium (Mg).

Key Points

• Nitric acid is a strong oxidising agent. When a metal reacts with it, the hydrogen initially released is immediately oxidised to water, while the nitric acid itself is reduced to oxides of nitrogen (NO, NO₂ or N₂O). For this reason, metals generally do not liberate hydrogen gas with nitric acid.
• Magnesium (and manganese) are the exceptions. With very dilute nitric acid, magnesium does evolve hydrogen gas:
  • $Mg + 2HNO_3 \text{ (very dilute)} \rightarrow Mg(NO_3)_2 + H_2 \uparrow$
• Copper lies below hydrogen in the reactivity series and cannot displace it from any acid; with nitric acid it produces nitrogen oxides instead.
• Zinc reacts with dilute nitric acid to give nitrous oxide (N₂O) or ammonium nitrate rather than hydrogen.

Additional Information

• The general rule and its exception:
  • $\text{Metal} + \text{Dilute acid} \rightarrow \text{Salt} + \text{Hydrogen gas}$
  • This holds for dilute HCl and dilute H₂SO₄ with all metals above hydrogen in the reactivity series, but fails for HNO₃ because of its oxidising nature — except for Mg and Mn with very dilute HNO₃.
• Reactivity series (most to least reactive): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au. Only metals above hydrogen can displace it from acids.
• Aqua regia — a 3:1 mixture of concentrated HCl and concentrated HNO₃ — dissolves even gold and platinum, which no single acid can.
• Test for hydrogen gas: it burns with a pop sound when a burning splint is brought near it.