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Drop zinc into dilute hydrochloric acid and it fizzes with hydrogen. Swap the acid for nitric acid and the gas is no longer hydrogen, and with copper you may even see brown fumes. The reason is one of the most useful ideas in school chemistry: in nitric acid, the nitrate ion is a stronger oxidising agent than the hydrogen ion.
Why don't metals give hydrogen with nitric acid?
Most acids react with a metal because the metal gives electrons to H⁺ ions, which pair up as H₂ gas. Nitric acid has a second, stronger electron-taker in the same solution: the nitrate ion, NO₃⁻.
The NCERT Class 10 science chapter on metals and non-metals puts it in one line: HNO₃ is a strong oxidising agent, so it oxidises the H₂ produced to water and is itself reduced to one of the nitrogen oxides, N₂O, NO or NO₂.
Standard electrode potentials show why. In the LibreTexts table of standard reduction potentials, the nitrate-to-NO couple sits at +0.96 V, while the H⁺/H₂ couple is 0.00 V by definition. A higher reduction potential means a stronger oxidising agent, so nitrate takes the metal's electrons more readily than H⁺ does, and any hydrogen that does form is oxidised to water.
What does nitric acid give instead?
The product depends on how concentrated the acid is. The Wikipedia article on nitric acid gives the two classic copper reactions: dilute acid reacts in a 3 : 8 ratio of copper to acid and gives nitric oxide, while concentrated acid gives nitrogen dioxide, the brown gas.
Dilute: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O
Concentrated: Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O
We go through the conditions and colours in copper and nitric acid: why dilute gives NO, concentrated NO₂.
How do you balance copper with dilute nitric acid?
Split the reaction into two half-equations and match the electrons.
Oxidation: Cu → Cu²⁺ + 2e⁻. Multiply by 3, so 6 electrons are lost.
Reduction: NO₃⁻ + 4H⁺ + 3e⁻ → NO + 2H₂O. Multiply by 2, so 6 electrons are gained.
Add them: 3Cu + 2NO₃⁻ + 8H⁺ → 3Cu²⁺ + 2NO + 4H₂O.
Add 6 spectator nitrate ions to each side to write it with HNO₃: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O.
Check the atoms: 8 nitrogen on each side (6 in the salt, 2 in NO) and 24 oxygen on each side (18 + 2 + 4). Notice that only 2 of the 8 HNO₃ are reduced; the other 6 simply end up in copper nitrate. Spotting that split is what separates a balanced equation from a memorised one.
If half-equations still trip you up, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Which metals give hydrogen with nitric acid?
The NCERT textbook names two: magnesium and manganese react with very dilute HNO₃ to give hydrogen gas. The Wikipedia article writes it as Mg + 2HNO₃ → Mg(NO₃)₂ + H₂, and lists zinc as well; for board answers, stick to the textbook's Mg and Mn.
Both metals sit far below hydrogen: the LibreTexts table gives −2.356 V for Mg²⁺/Mg and −1.17 V for Mn²⁺/Mn. Learn the exception as a pair of conditions, not just two names: these metals, and very dilute acid.
Concentrated nitric acid can do the opposite and almost stop a reaction. With metals such as iron and aluminium it forms a thin protective layer on the surface, which the Wikipedia article calls passivation. We explain it in why aluminium becomes passive in concentrated nitric acid.
How do you write this answer in an exam?
For a two- or three-mark "give reason" question, write three sentences:
Nitric acid is a strong oxidising agent.
It oxidises the hydrogen formed to water and is itself reduced to an oxide of nitrogen such as N₂O, NO or NO₂.
Exception: Mg and Mn give H₂ with very dilute HNO₃. Add the copper equation if the marks allow.
Then use this checklist for any metal-and-acid question:
Dilute HCl or dilute H₂SO₄? A metal above hydrogen gives a salt and H₂; copper gives nothing. Compare the laboratory preparation of hydrogen from zinc and dilute acid.
Nitric acid? Expect a nitrate salt, water and an oxide of nitrogen, not H₂.
Very dilute nitric acid with Mg or Mn? Hydrogen.
Concentrated nitric acid with Al or Fe? Passive, so little or no reaction.
If oxidising agents and electrode potentials feel shaky, start from the ideas themselves in the Fermi Concept Library, which covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson.
