Dilute trioxonitrate (V) acid does not produce hydrogen when it reacts with metals because

Analyze the question: Trioxonitrate(V) acid is HNO₃ (nitric acid). Normally, acids react with reactive metals to give a salt and hydrogen gas. The question asks why HNO₃ does not give hydrogen.

Concept: Nitric acid is a strong oxidizing agent. The nitrate ion (NO₃⁻) in acidic solution is reduced more easily than H⁺ ions. Also, any hydrogen that forms is quickly oxidized to water by the acid.

Solution:

1. A normal acid-metal reaction is: Metal + acid → salt + H₂
2. With HNO₃, the nitrate ion takes the electrons that would have reduced H⁺ to H₂.
3. The nitrogen is reduced to nitrogen oxides instead: • Dilute HNO₃ with Cu: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O • Concentrated HNO₃ with Cu: Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O
4. So the products are water and nitrogen oxides (NO, NO₂, sometimes N₂O or NH₄⁺), not hydrogen gas. • (Only very dilute, cold HNO₃ reacting with magnesium or manganese gives some hydrogen.)

Checking the options:

• A. "It reacts with the product" is vague. The real reason is the oxidizing power of the acid, so this is not the best answer.
• B. Being corrosive does not affect whether H₂ forms.
• C. There is a visible reaction (brown fumes of NO₂, for example), so this is wrong.
• D. It is a strong oxidizing agent. This is correct.

Answer: D

Tip: Remember "HNO₃ = Oxidizer, no H₂." Whenever you see nitric acid with a metal, think of nitrogen oxides and water, not hydrogen. Only HCl and dilute H₂SO₄ give the classic metal + acid → salt + H₂ reaction.