ChemistryICSE Class 10 · Nitric Acid

Laboratory Preparation of Nitric Acid: Reactants, Apparatus and Precautions (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Nitric acid is prepared in the laboratory by heating potassium nitrate (or sodium nitrate) with concentrated sulphuric acid below 200 °C in an all-glass retort: KNO₃ + H₂SO₄ → KHSO₄ + HNO₃. The nitric acid vapour distils over and condenses in a receiver cooled by running water.

Understand Laboratory Preparation of Nitric Acid

Principle. Concentrated sulphuric acid is non-volatile (high boiling point), while nitric acid is volatile. When a nitrate is heated with concentrated sulphuric acid, the less volatile acid displaces the more volatile one, which distils off as vapour.

Reactants and equations. Potassium nitrate or sodium nitrate with concentrated sulphuric acid, heated gently:

$K N O_{3} + H_{2} S O_{4} \to K H S O_{4} + H N O_{3}$

$N a N O_{3} + H_{2} S O_{4} \to N a H S O_{4} + H N O_{3}$

Apparatus. A glass retort with a ground-glass stopper; its neck leads into a receiver (flask) kept cold under a stream of running water. The apparatus is all glass, because hot nitric acid vapour attacks rubber and cork (it oxidises them), so no rubber stoppers or tubing are used.

Temperature below 200 °C. At this temperature the reaction stops at the acid salt (KHSO₄). Heating more strongly would:

decompose some of the nitric acid ( $4 H N O_{3} \to 4 N O_{2} + 2 H_{2} O + O_{2}$ );
waste fuel and risk cracking the glass retort;
form the normal salt (K₂SO₄), which sticks to the retort as a hard crust that is difficult to remove.

Collection. Nitric acid vapour condenses in the cooled receiver as a liquid. It is not collected as a gas.

Why the acid looks yellow. Some nitric acid decomposes during heating (and on standing in light) into reddish-brown nitrogen dioxide, which dissolves in the acid and colours it yellow. The colour can be removed by bubbling dry air (or carbon dioxide) through the warm acid, which drives out the NO₂, or by adding water. Nitric acid is stored in brown bottles away from light for the same reason.

Test for nitric acid. Warm the acid with copper turnings: reddish-brown fumes of NO₂ are given off and the solution turns blue. Hydrochloric and sulphuric acids do not react with copper in this way. Nitric acid also gives no precipitate with barium chloride or silver nitrate solution, which separates it from sulphuric and hydrochloric acids.

Laboratory Preparation of Nitric Acid: formulas and key facts

From potassium nitrate: $K N O_{3} + H_{2} S O_{4} \to K H S O_{4} + H N O_{3}$; Concentrated H₂SO₄; temperature below 200 °C.
From sodium nitrate: $N a N O_{3} + H_{2} S O_{4} \to N a H S O_{4} + H N O_{3}$
Decomposition (yellow colour): $4 H N O_{3} \to 4 N O_{2} + 2 H_{2} O + O_{2}$
Test with copper: $C u + 4 H N O_{3} \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O$
Preparation at a glance: Reactants potassium or sodium nitrate with concentrated sulphuric acid
Condition heat below 200 °C
Apparatus all-glass retort with glass stopper
Collection receiver cooled by running water
Precaution no rubber or cork; heat gently
Reasons to remember: Concentrated sulphuric acid non-volatile, displaces volatile nitric acid
All glass nitric acid vapour attacks rubber and cork
Below 200 °C nitric acid does not decompose, no hard K₂SO₄ crust, glass safe
Yellow colour dissolved NO₂ from decomposition

Reactants	potassium or sodium nitrate with concentrated sulphuric acid
Condition	heat below 200 °C
Apparatus	all-glass retort with glass stopper
Collection	receiver cooled by running water
Precaution	no rubber or cork; heat gently

Concentrated sulphuric acid	non-volatile, displaces volatile nitric acid
All glass	nitric acid vapour attacks rubber and cork
Below 200 °C	nitric acid does not decompose, no hard K₂SO₄ crust, glass safe
Yellow colour	dissolved NO₂ from decomposition

How to solve Laboratory Preparation of Nitric Acid questions

A preparation question asks for reactants, equation, condition, apparatus, precautions, collection and test. Give a reason with each precaution.

1Name the reactants: KNO₃ (or NaNO₃) and concentrated H₂SO₄.
2Write the equation giving the acid salt (KHSO₄ or NaHSO₄).
3State the temperature (below 200 °C) and give at least one reason.
4Describe the apparatus: all-glass retort, receiver cooled with running water; say why glass only.
5Explain the yellow colour and how to remove it if asked.
6Give the copper test for nitric acid with the observation.

Laboratory Preparation of Nitric Acid 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.

Why stored nitric acid turns yellow

Suppose a school stores a bottle of colourless concentrated nitric acid in a clear glass bottle on a sunny shelf, and after a few weeks the acid has turned yellow.

Explain the colour change with an equation, and suggest how the acid should be stored and how the colour could be removed.

Try it first, then show the solutionSolution

Light decomposes nitric acid: $4 H N O_{3} \to 4 N O_{2} + 2 H_{2} O + O_{2}$ .
The reddish-brown NO₂ dissolves in the remaining acid, colouring it yellow.
Store it in a brown (amber) bottle in a cool, dark place; bubble dry air through the warm acid, or add water, to remove the dissolved NO₂.

Answer: Dissolved NO₂ from decomposition by light; store in brown bottles in the dark; air or water removes the colour.

Memory hook: Light makes nitric acid blush yellow: keep it in the dark.

Tricks to remember Laboratory Preparation of Nitric Acid

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

Non-volatile pushes out volatile
Sulphuric acid 'stays home', nitric acid 'flies off' as vapour.
Use it when: Principle of the preparation.
Glass only, two hundred max
No rubber, no cork; keep it under 200 °C.
Use it when: The two key precautions.
Acid salt at low heat
Below 200 °C you get KHSO₄ (one H left); above it, K₂SO₄ crust.
Use it when: Writing the correct product.

Laboratory Preparation of Nitric Acid: solved examples

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

Example 1: All-glass apparatus · 1 mark · MCQBoard level

An all-glass apparatus is used for the laboratory preparation of nitric acid because:
(a) nitric acid is a strong acid
(b) nitric acid vapour attacks rubber and cork
(c) glass is a good conductor of heat
(d) nitric acid is non-volatile

Try it first, then show the solutionSolution

Hot nitric acid vapour oxidises rubber and cork.
Glass is not attacked.

Answer: (b).

Example 2: Reactants and equation · 2 marks · Short answerBoard level

Name the reactants used to prepare nitric acid in the laboratory and write the equation using sodium nitrate.

Try it first, then show the solutionSolution

Sodium nitrate (or potassium nitrate) and concentrated sulphuric acid.
$N a N O_{3} + H_{2} S O_{4} \to N a H S O_{4} + H N O_{3}$ (below 200 °C).

Answer: NaNO₃ + conc. H₂SO₄ → NaHSO₄ + HNO₃.

Example 3: Reasons for the conditions · 3 marks · Short answerHarder board level

Give reasons: (a) the temperature is kept below 200 °C; (b) concentrated sulphuric acid, not concentrated hydrochloric acid, is used; (c) the receiver is cooled with running water.

Try it first, then show the solutionSolution

(a) Higher temperatures decompose nitric acid, risk cracking the glass, waste fuel and form a hard crust of K₂SO₄.
(b) Sulphuric acid is non-volatile and displaces the volatile nitric acid; hydrochloric acid is itself volatile and would distil over, contaminating the product.
(c) To condense the nitric acid vapour into a liquid.

Answer: (a) Prevent decomposition and crust formation. (b) H₂SO₄ is non-volatile. (c) To condense the vapour.

Example 4: The yellow colour · 3 marks · Short answerHarder board level

Nitric acid prepared in the laboratory is often pale yellow. (a) What causes the colour? (b) Write the equation. (c) How can the colour be removed?

Try it first, then show the solutionSolution

(a) Some nitric acid decomposes and the NO₂ formed dissolves in the acid.
(b) $4 H N O_{3} \to 4 N O_{2} + 2 H_{2} O + O_{2}$ .
(c) Bubble dry air (or CO₂) through the warm acid to expel NO₂, or add water.

Answer: Dissolved NO₂ from decomposition; remove by blowing air through or adding water.

Example 5: Masses in the preparation · 4 marks · Long answerHOTS / competency

50.5 g of potassium nitrate is heated with concentrated sulphuric acid below 200 °C. Calculate (a) the mass of nitric acid formed, (b) the mass of sulphuric acid needed, (c) the mass of potassium hydrogen sulphate left in the retort. (K = 39, N = 14, O = 16, H = 1, S = 32)

Try it first, then show the solutionSolution

Molar mass of KNO₃ $= 101$ ; moles $= 50.5/101 = 0.5$ .
(a) HNO₃ $= 0.5 \times 63 = 31.5$ g.
(b) H₂SO₄ $= 0.5 \times 98 = 49$ g.
(c) KHSO₄ $= 0.5 \times 136 = 68$ g. (Check: 50.5 + 49 = 99.5 = 31.5 + 68.)

Answer: (a) 31.5 g (b) 49 g (c) 68 g.

Example 6: Spot the faulty set-up · 3 marks · Short answerHOTS / competency

A student prepares nitric acid using a flask closed with a rubber cork and a rubber delivery tube, heats it strongly over a full flame, and collects the product in a beaker at room temperature. Identify three faults and correct each.

Try it first, then show the solutionSolution

Rubber cork and tubing are attacked by nitric acid vapour: use an all-glass retort with a glass stopper.
Strong heating (above 200 °C) decomposes nitric acid and leaves a hard K₂SO₄ crust: heat gently, below 200 °C.
An uncooled beaker lets the vapour escape: use a receiver cooled by running water.

Answer: All-glass apparatus; gentle heating below 200 °C; water-cooled receiver.

Where students lose marks in Laboratory Preparation of Nitric Acid

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

Mistake: Writing K₂SO₄ as the product
Students balance with 2KNO₃ and K₂SO₄, which is the high-temperature product.
Wrong working
2KNO₃ + H₂SO₄ → K₂SO₄ + 2HNO₃
Right working
KNO₃ + H₂SO₄ → KHSO₄ + HNO₃
Do this: Below 200 °C the acid salt forms.
Usually costs: 1 mark.
Mistake: Leaving out 'concentrated'
Dilute sulphuric acid does not displace nitric acid; examiners expect 'concentrated'.
Do this: Always write concentrated sulphuric acid.
Usually costs: ½ to 1 mark.
Mistake: Reason for all-glass apparatus missing
Answers say 'glass is used' without stating that nitric acid vapour attacks rubber and cork.
Do this: Give the reason in the same sentence.
Usually costs: 1 mark.
Mistake: Confusing the copper test products
In 2025 (Q7(i)) examiners noted copper oxide written for copper nitrate and NO for NO₂ in the copper and concentrated nitric acid equation.
Do this: Concentrated nitric acid with copper: Cu(NO₃)₂, NO₂ and H₂O.
Usually costs: 1 mark.

Exam tips

Practise a labelled diagram: retort with glass stopper, burner, receiver flask under running water.
Use 'below 200 °C' exactly; it is the number examiners look for.
If the question says 'state one precaution', give one with its reason.
Remember that the preparation is studied with reactants, products, conditions, equations, apparatus, precautions, collection and identification.

Practice questions on Laboratory Preparation of Nitric Acid

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 salt left in the retort when potassium nitrate is heated with concentrated sulphuric acid below 200 °C is:
(a) K₂SO₄
(b) KHSO₄
(c) KNO₂
(d) K₂O
Hint
The acid salt forms at low temperature.
Answer
(b) KHSO₄.
Q2 · 1 mark · Very short answerBoard level
Name the property of sulphuric acid used in the preparation of nitric acid.
Hint
Volatility.
Answer
Its non-volatile nature.
Q3 · 2 marks · Short answerBoard level
Give the test for nitric acid with the observation.
Hint
Copper turnings.
Answer
Warm with copper turnings: reddish-brown fumes of NO₂ are given off and the solution turns blue. $C u + 4 H N O_{3} \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O$ .
Q4 · 1 mark · Very short answerHarder board level
A laboratory keeps concentrated nitric acid in amber glass bottles fitted with glass stoppers, not corks. Give one reason for each choice.
Hint
Light, and what the acid does to cork.
Answer
Amber glass cuts out light, which would decompose the acid to NO₂, water and oxygen; a glass stopper is used because nitric acid attacks (oxidises) cork.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Nitric acid is collected in a receiver cooled by running water.
Reason (R): Nitric acid is formed as a vapour in the retort.
(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
Vapour must be condensed.
Answer
(a).
Q6 · 2 marks · Short answerHarder board level
Give two reasons why the temperature in the preparation of nitric acid is kept below 200 °C.
Hint
Product and apparatus.
Answer
Any two: nitric acid decomposes at higher temperature; the normal salt K₂SO₄ forms a hard crust that is difficult to remove; the glass retort may crack; fuel is wasted.
Q7 · 2 marks · Short answerHarder board level
What mass of nitric acid can be obtained from 17 g of sodium nitrate? (Na = 23, N = 14, O = 16, H = 1)
Hint
1 : 1 ratio.
Answer
Moles of NaNO₃ $= 17/85 = 0.2$ ; HNO₃ $= 0.2 \times 63 = 12.6$ g.
Q8 · 3 marks · Short answerHOTS / competency
Suppose a student uses sodium chloride by mistake instead of sodium nitrate, with concentrated sulphuric acid, in the same apparatus. What would distil over, and how could a quick test with silver nitrate solution show that it is not nitric acid?
Hint
Same principle, different volatile acid.
Answer
Hydrogen chloride (giving hydrochloric acid in the receiver): $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$ . With silver nitrate solution it gives a white precipitate of AgCl, insoluble in nitric acid; nitric acid gives no precipitate.

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.Why is the acid salt formed?Show answer
The temperature is kept below 200 °C.
2.What attacks rubber and cork?Show answer
Nitric acid vapour.
3.Colour of NO₂?Show answer
Reddish-brown.
4.How is the receiver kept cold?Show answer
With running water.
5.Name the other nitrate that can replace KNO₃.Show answer
Sodium nitrate (NaNO₃).

Laboratory Preparation of Nitric Acid: frequently asked questions

How is nitric acid prepared in the laboratory?

By heating potassium or sodium nitrate with concentrated sulphuric acid below 200 °C in an all-glass retort and condensing the vapour in a water-cooled receiver.

Why is concentrated sulphuric acid used?

It is non-volatile and displaces the more volatile nitric acid from the nitrate.

Why is laboratory nitric acid yellow?

Some of it decomposes to reddish-brown nitrogen dioxide, which dissolves in the acid.

How do you test for nitric acid?

Warm it with copper turnings: reddish-brown NO₂ fumes and a blue solution confirm nitric acid.