ChemistryICSE Class 10 · Nitric Acid

Nitric Acid as an Oxidising Agent: Copper, Carbon and Sulphur (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Nitric acid is a strong oxidising agent because it readily gives up oxygen. Concentrated nitric acid oxidises copper to copper(II) nitrate with reddish-brown nitrogen dioxide, carbon to carbon dioxide and sulphur to sulphuric acid. Dilute nitric acid oxidises copper with colourless nitric oxide. It is itself reduced to NO₂ or NO.

Nitric acid oxidises each substance and is itself reduced: to brown NO₂ when concentrated, to colourless NO with dilute acid and copper.

Illustration drawn for QuestionBuddy.

Understand Nitric Acid as an Oxidising Agent

Nitric acid decomposes easily to give oxygen: $4 H N O_{3} \to 4 N O_{2} + 2 H_{2} O + O_{2}$ . This 'spare' oxygen makes it a powerful oxidising agent. When it oxidises something, nitric acid is itself reduced, to nitrogen dioxide (NO₂, reddish-brown) when concentrated, or to nitric oxide (NO, colourless) when dilute.

Copper. Copper is below hydrogen in the activity series, so it does not give hydrogen with dilute HCl or dilute H₂SO₄. Nitric acid still dissolves it, because the acid oxidises copper to Cu²⁺.

Concentrated nitric acid: $C u + 4 H N O_{3} \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O$ . Reddish-brown fumes of NO₂ are seen at once and the solution turns blue (greenish when very concentrated).
Dilute nitric acid: $3 C u + 8 H N O_{3} \to 3 C u (N O_{3})_{2} + 2 N O + 4 H_{2} O$ . A colourless gas, nitric oxide, forms; it turns reddish-brown at the mouth of the test tube as it meets air ( $2 N O + O_{2} \to 2 N O_{2}$ ). The solution turns blue.

In both, copper is oxidised (Cu → Cu²⁺, it loses electrons) and nitric acid is the oxidising agent.

Non-metals. Hot concentrated nitric acid oxidises non-metals to their highest oxide or oxyacid and gives reddish-brown NO₂:

Carbon → carbon dioxide: $C + 4 H N O_{3} \to C O_{2} + 4 N O_{2} + 2 H_{2} O$ .
Sulphur → sulphuric acid (not sulphur dioxide): $S + 6 H N O_{3} \to H_{2} S O_{4} + 6 N O_{2} + 2 H_{2} O$ . The sulphuric acid can be shown by adding barium chloride solution: a white precipitate of barium sulphate forms.

Why no hydrogen with most metals? Dilute nitric acid is an acid, but any hydrogen that might form is immediately oxidised to water by the nitric acid. So metals normally give oxides of nitrogen, not hydrogen. Only very dilute (about 1%) nitric acid gives hydrogen, and only with magnesium and manganese: $M g + 2 H N O_{3} \to M g (N O_{3})_{2} + H_{2}$ .

Identifying the oxidising agent. In a reaction, the substance that gives oxygen (or removes electrons) is the oxidising agent and is itself reduced; the substance that gains oxygen (or loses electrons) is oxidised. In $C + 4 H N O_{3} \to C O_{2} + \dots$ , carbon gains oxygen, so carbon is oxidised and HNO₃ is the oxidising agent.

Nitric Acid as an Oxidising Agent: formulas and key facts

Copper with concentrated nitric acid: $C u + 4 H N O_{3} \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O$
Copper with dilute nitric acid: $3 C u + 8 H N O_{3} \to 3 C u (N O_{3})_{2} + 2 N O + 4 H_{2} O$
Carbon with hot concentrated nitric acid: $C + 4 H N O_{3} \to C O_{2} + 4 N O_{2} + 2 H_{2} O$
Sulphur with hot concentrated nitric acid: $S + 6 H N O_{3} \to H_{2} S O_{4} + 6 N O_{2} + 2 H_{2} O$
Nitric oxide meets air: $2 N O + O_{2} \to 2 N O_{2}$
Products and observations: Copper and concentrated acid reddish-brown NO₂, blue solution
Copper and dilute acid colourless NO turning brown in air, blue solution
Carbon and concentrated acid CO₂ and reddish-brown NO₂
Sulphur and concentrated acid sulphuric acid and reddish-brown NO₂

Copper and concentrated acid	reddish-brown NO₂, blue solution
Copper and dilute acid	colourless NO turning brown in air, blue solution
Carbon and concentrated acid	CO₂ and reddish-brown NO₂
Sulphur and concentrated acid	sulphuric acid and reddish-brown NO₂

How to solve Nitric Acid as an Oxidising Agent questions

Decide two things before writing anything: is the acid concentrated or dilute, and is the other reactant a metal or a non-metal?

1Concentrated acid gives NO₂ (reddish-brown); dilute acid with copper gives NO (colourless).
2With copper, the products are always copper(II) nitrate and water plus the nitrogen oxide.
3With carbon, the product is CO₂; with sulphur, the product is H₂SO₄.
4Balance with the standard coefficients: Cu + 4HNO₃; 3Cu + 8HNO₃; C + 4HNO₃; S + 6HNO₃.
5State the observation: brown fumes, blue solution, or a colourless gas turning brown in air.
6Name the oxidising agent (HNO₃) and the substance oxidised (Cu, C or S).

Nitric Acid as an Oxidising Agent in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Telling acids apart with copper

Suppose a technician finds three unlabelled bottles of colourless dilute acid and must find which one is nitric acid, using only a few copper turnings and a test tube.

How would the copper behave in each acid, and what result identifies nitric acid?

Try it first, then show the solutionSolution

Copper is below hydrogen, so dilute hydrochloric and dilute sulphuric acids do not react with it.
Dilute nitric acid oxidises copper: $3 C u + 8 H N O_{3} \to 3 C u (N O_{3})_{2} + 2 N O + 4 H_{2} O$ .
A colourless gas forms that turns reddish-brown at the mouth of the tube, and the solution turns blue.

Answer: Only the nitric acid reacts: colourless gas turning brown in air and a blue solution.

Memory hook: Copper ignores ordinary acids; only an oxidising acid can dissolve it.

Tricks to remember Nitric Acid as an Oxidising Agent

Short memory aids. Say each one aloud once, then use it on a question the same day.

Concentrated = brown at once, Dilute = brown at the door
Concentrated acid gives NO₂ (brown) straight away; dilute gives NO, which browns only at the mouth of the tube where it meets air.
Use it when: Distinguishing the two copper reactions.
1-4, 3-8
Cu + 4HNO₃ for concentrated; 3Cu + 8HNO₃ for dilute.
Use it when: Balancing the copper equations quickly.
Carbon to gas, sulphur to acid
Carbon ends as CO₂ (a gas); sulphur ends as H₂SO₄ (an acid), never SO₂.
Use it when: Non-metal oxidation products.

Nitric Acid as an Oxidising Agent: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Gas with dilute acid · 1 mark · MCQBoard level

When copper reacts with dilute nitric acid, the gas formed is:
(a) nitrogen dioxide
(b) nitric oxide
(c) hydrogen
(d) nitrogen

Try it first, then show the solutionSolution

Dilute nitric acid is reduced to NO; concentrated to NO₂.
No hydrogen is formed, because nitric acid oxidises it.

Answer: (b) nitric oxide.

Example 2: Copper in concentrated nitric acid · 2 marks · Short answerBoard level

(a) Write the balanced equation for the reaction of copper with concentrated nitric acid. (b) State two observations, and explain why the solution becomes blue.

Try it first, then show the solutionSolution

(a) $C u + 4 H N O_{3} \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O$ .
(b) Reddish-brown fumes of NO₂ are given off and the copper dissolves; the solution turns blue because copper is oxidised to Cu²⁺ ions (copper(II) nitrate).

Answer: (a) As above. (b) Reddish-brown fumes and a blue solution; the blue colour is due to Cu²⁺ ions of copper(II) nitrate.

Example 3: Non-metals oxidised · 3 marks · Short answerHarder board level

Write balanced equations for the action of hot concentrated nitric acid on (a) carbon, (b) sulphur. In each, name the substance oxidised and the oxidising agent, and give one observation.

Try it first, then show the solutionSolution

(a) $C + 4 H N O_{3} \to C O_{2} + 4 N O_{2} + 2 H_{2} O$ ; carbon is oxidised.
(b) $S + 6 H N O_{3} \to H_{2} S O_{4} + 6 N O_{2} + 2 H_{2} O$ ; sulphur is oxidised.
Oxidising agent in both: nitric acid. Observation: reddish-brown fumes of NO₂.

Answer: Carbon → CO₂, sulphur → H₂SO₄; HNO₃ is the oxidising agent; brown fumes.

Example 4: Why no hydrogen? · 3 marks · Short answerHarder board level

(a) Explain why zinc does not normally give hydrogen with dilute nitric acid. (b) Under what condition, and with which metals, is hydrogen obtained? Write one equation.

Try it first, then show the solutionSolution

(a) Nitric acid is a strong oxidising agent; any hydrogen formed is at once oxidised to water, and the acid is reduced to oxides of nitrogen.
(b) With very dilute (about 1%) nitric acid and magnesium or manganese.
$M g + 2 H N O_{3} \to M g (N O_{3})_{2} + H_{2}$ .

Answer: (a) H₂ is oxidised to water. (b) Very dilute acid with Mg or Mn.

Example 5: Volumes from copper · 4 marks · Long answerHOTS / competency

Calculate the volume of gas at S.T.P. formed when (a) 1.28 g of copper reacts with excess concentrated nitric acid, (b) 1.92 g of copper reacts with excess dilute nitric acid. Also find the mass of nitric acid used in (a). (Cu = 64, H = 1, N = 14, O = 16)

Try it first, then show the solutionSolution

(a) Moles of Cu $= 1.28/64 = 0.02$ ; Cu : NO₂ = 1 : 2, so 0.04 mol NO₂ $= 0.04 \times 22400 = 896$ cm³.
HNO₃ used $= 4 \times 0.02 = 0.08$ mol $= 0.08 \times 63 = 5.04$ g.
(b) Moles of Cu $= 1.92/64 = 0.03$ ; Cu : NO = 3 : 2, so 0.02 mol NO $= 448$ cm³.

Answer: (a) 896 cm³ NO₂; 5.04 g HNO₃. (b) 448 cm³ NO.

Example 6: Proving sulphuric acid forms · 3 marks · Short answerHOTS / competency

1.6 g of sulphur is heated with excess concentrated nitric acid. (a) What mass of sulphuric acid forms? (b) What volume of NO₂ is released at S.T.P.? (c) Suggest a test to show that sulphuric acid, not sulphur dioxide, has formed. (S = 32, H = 1, O = 16)

Try it first, then show the solutionSolution

Moles of S $= 1.6/32 = 0.05$ .
(a) H₂SO₄ $= 0.05 \times 98 = 4.9$ g. (b) NO₂ $= 6 \times 0.05 = 0.3$ mol $= 6.72$ L.
(c) Dilute the cooled mixture and add barium chloride solution: a white precipitate (BaSO₄), insoluble in dilute acid, shows sulphate ions.

Answer: (a) 4.9 g (b) 6.72 L (c) white precipitate with BaCl₂ solution.

Where students lose marks in Nitric Acid as an Oxidising Agent

The slips examiners see most often, and what to write instead.

Mistake: Writing dilute-acid products for concentrated acid
In 2024 (Q8(ii)) examiners noted NO written instead of NO₂ for copper with concentrated nitric acid.
Wrong working
Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2NO + 2H₂O
Right working
Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O
Do this: Concentrated gives NO₂; dilute gives NO.
Usually costs: 1 to 2 marks.
Mistake: Confusing NO₂, NO and hydrogen
In 2025 (Q4(iii)) candidates confused nitrogen dioxide with nitric oxide or wrote hydrogen.
Do this: Brown gas = NO₂; colourless gas turning brown in air = NO; hydrogen only from very dilute acid with Mg or Mn.
Usually costs: 1 mark.
Mistake: Naming the acid without 'concentrated'
In 2025 (Q7(i)) candidates wrote HCl, H₂SO₄ or just 'nitric acid' when asked to identify concentrated nitric acid.
Do this: Write 'concentrated nitric acid' when the gas is reddish-brown.
Usually costs: 1 mark.
Mistake: Copper oxide instead of copper nitrate
The same 2025 report found copper oxide given as a product and the equation left unbalanced.
Do this: Copper always ends as copper(II) nitrate, Cu(NO₃)₂.
Usually costs: 1 mark.

Exam tips

Learn the four equations (Cu conc., Cu dilute, C, S) as a set; one of them appears in most years.
Write 'hot, concentrated' for carbon and sulphur; cold dilute acid does not oxidise them.
Always describe the colour of the gas: reddish-brown NO₂ is an observation mark.
For 'oxidising agent' MCQs, choose the substance that is reduced (HNO₃), not the one oxidised.

Practice questions on Nitric Acid as an Oxidising Agent

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The product formed when sulphur is oxidised by hot concentrated nitric acid is:
(a) sulphur dioxide
(b) sulphur trioxide
(c) sulphuric acid
(d) hydrogen sulphide
Hint
Highest oxyacid.
Answer
(c) sulphuric acid.
Q2 · 1 mark · Very short answerBoard level
Name the gas given off when carbon is heated with concentrated nitric acid, other than carbon dioxide.
Hint
Reddish-brown.
Answer
Nitrogen dioxide (NO₂).
Q3 · 2 marks · Short answerBoard level
Write the balanced equation for the action of dilute nitric acid on copper and give one observation.
Hint
3 : 8.
Answer
$3 C u + 8 H N O_{3} \to 3 C u (N O_{3})_{2} + 2 N O + 4 H_{2} O$ . A colourless gas forms which turns reddish-brown at the mouth of the tube; the solution turns blue.
Q4 · 2 marks · Short answerHarder board level
In $C u + 4 H N O_{3} \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O$ , which substance is oxidised and which is the oxidising agent? Explain in terms of electrons.
Hint
Cu → Cu²⁺.
Answer
Copper is oxidised: each Cu atom loses 2 electrons to become Cu²⁺. Nitric acid is the oxidising agent: it accepts the electrons and is reduced to NO₂.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Copper does not liberate hydrogen with dilute nitric acid.
Reason (R): Copper is below hydrogen in the activity series.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Even zinc, above hydrogen, gives no H₂ with nitric acid.
Answer
(b). Both are true, but the reason no hydrogen forms is the oxidising nature of nitric acid (it does dissolve copper, giving NO).
Q6 · 3 marks · Short answerHarder board level
Name the gas formed in each case: (a) magnesium with very dilute nitric acid; (b) copper with hot concentrated nitric acid; (c) copper with dilute nitric acid.
Hint
Only one gives hydrogen.
Answer
(a) Hydrogen (b) nitrogen dioxide (c) nitric oxide.
Q7 · 3 marks · Short answerHarder board level
0.6 g of carbon is oxidised completely by hot concentrated nitric acid. Calculate the volumes of carbon dioxide and nitrogen dioxide at S.T.P. (C = 12)
Hint
C : CO₂ : NO₂ = 1 : 1 : 4.
Answer
Moles of C $= 0.05$ . CO₂ $= 0.05 \times 22.4 = 1.12$ L; NO₂ $= 0.2 \times 22.4 = 4.48$ L.
Q8 · 3 marks · Short answerHOTS / competency
Suppose two students each add copper to nitric acid. One sees brown fumes at once and a greenish-blue solution; the other sees a colourless gas that turns brown only at the mouth of the tube. Explain the difference and write the equation for each.
Hint
Concentration of the acid.
Answer
The first used concentrated acid, reduced to NO₂ (brown): $C u + 4 H N O_{3} \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O$ . The second used dilute acid, reduced to colourless NO, which turns brown when it meets air: $3 C u + 8 H N O_{3} \to 3 C u (N O_{3})_{2} + 2 N O + 4 H_{2} O$ ; $2 N O + O_{2} \to 2 N O_{2}$ .

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Colour of the solution when copper dissolves in nitric acid?Show answer
Blue (copper(II) nitrate).
2.Which metals give hydrogen with very dilute nitric acid?Show answer
Magnesium and manganese.
3.Product of sulphur with hot conc. HNO₃?Show answer
Sulphuric acid (with NO₂ and water).
4.Moles of HNO₃ per mole of Cu (concentrated)?Show answer
4.
5.Why does NO turn brown at the mouth of the tube?Show answer
It reacts with oxygen of the air to form NO₂.

Nitric Acid as an Oxidising Agent: frequently asked questions

Why is nitric acid called an oxidising agent?

It readily gives oxygen (it decomposes to NO₂, water and oxygen), so it oxidises metals and non-metals and is itself reduced to NO₂ or NO.

What is the difference between copper's reaction with concentrated and dilute nitric acid?

Concentrated acid gives reddish-brown NO₂ at once; dilute acid gives colourless NO, which turns brown in air. Both give a blue solution of copper(II) nitrate.

Why doesn't nitric acid usually give hydrogen with metals?

Any hydrogen formed is oxidised to water by the nitric acid. Only very dilute acid with magnesium or manganese gives hydrogen.

What does sulphur form with concentrated nitric acid?

Sulphuric acid, along with nitrogen dioxide and water.