Nitric Acid (HNO₃)

ICSE Class 10 Chemistry • Chapter 10

1. Laboratory Preparation

Method: Heating sodium/potassium nitrate with conc. H₂SO₄

KNO₃ + H₂SO₄ → KHSO₄ + HNO₃↑

NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃↑

Temperature: Below 200°C (to prevent decomposition of HNO₃)

Apparatus: Glass retort (HNO₃ corrodes rubber/cork)

Why below 200°C?

Above 200°C: 4HNO₃ → 4NO₂ + 2H₂O + O₂ (acid decomposes into brown NO₂ fumes)

2. Industrial Manufacture - Ostwald Process

Step 1: Catalytic Oxidation of Ammonia

4NH₃ + 5O₂ → 4NO + 6H₂O

Conditions: 800°C, Pt-Rh catalyst

Step 2: Oxidation of NO

2NO + O₂ → 2NO₂

Step 3: Absorption in Water

4NO₂ + 2H₂O + O₂ → 4HNO₃

3. Physical Properties

Property	Pure HNO₃	Conc. HNO₃
Colour	Colourless	Yellow (due to dissolved NO₂)
Concentration	100%	68%
Nature	Fuming, corrosive, strong acid
Density	1.52 g/cm³

4. Chemical Properties

A. As an Acid

With Bases: HNO₃ + NaOH → NaNO₃ + H₂O

With Carbonates: 2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂↑

Turns blue litmus red

B. As an Oxidising Agent (MOST IMPORTANT)

HNO₃ is a powerful oxidising agent!

It oxidises metals, non-metals, and compounds by providing nascent oxygen.

HNO₃ → H₂O + 2[O] + NO₂ (or NO)

Action on Metals

Metal	Dilute HNO₃	Conc. HNO₃
Copper	3Cu + 8HNO₃(dil) → 3Cu(NO₃)₂ + 4H₂O + 2NO↑ (colourless)	Cu + 4HNO₃(conc) → Cu(NO₃)₂ + 2H₂O + 2NO₂↑ (brown)
Zinc	4Zn + 10HNO₃(very dil) → 4Zn(NO₃)₂ + 5H₂O + N₂O↑	Zn + 4HNO₃(conc) → Zn(NO₃)₂ + 2H₂O + 2NO₂↑

Dilute HNO₃ gives NO (colourless)

Conc. HNO₃ gives NO₂ (brown/reddish-brown)

Very dilute + active metal → N₂O or even NH₄NO₃

Passive Metals

Fe and Al become passive with cold conc. HNO₃

Reason: A thin oxide layer forms on surface, preventing further reaction.

Note: Dilute HNO₃ DOES react with Fe and Al.

Action on Non-Metals

Carbon: C + 4HNO₃ → CO₂ + 4NO₂ + 2H₂O

Sulphur: S + 6HNO₃ → H₂SO₄ + 6NO₂ + 2H₂O

(Non-metals oxidised to their highest oxides/acids)

5. Special Reactions

Brown Ring Test (for Nitrate ions)

Procedure: Add FeSO₄ solution to salt solution, then add conc. H₂SO₄ carefully along the sides.

Observation: Brown ring at junction of two liquids

Reaction: FeSO₄ + NO + 5H₂O → [Fe(H₂O)₅NO]SO₄ (brown complex)

Aqua Regia

Composition: HNO₃ : HCl = 1 : 3 (by volume)

Property: Dissolves noble metals like gold and platinum

Au + 3HCl + HNO₃ → AuCl₃ + 2H₂O + NO↑

6. Uses of Nitric Acid

Manufacture of fertilisers (ammonium nitrate)
Manufacture of explosives (TNT, dynamite, gun cotton)
In dyes and drugs
Pickling of metals (cleaning before electroplating)
As a laboratory reagent

Quick Reference

With	Dilute HNO₃ Product	Conc. HNO₃ Product
Cu	NO (colourless, turns brown)	NO₂ (brown fumes)
Fe, Al	Reacts	Passive (cold conc.)
C, S	—	CO₂/H₂SO₄ + NO₂

Exam Practice (PYQ Trends)

PYQ: 2023

BOARD Why does copper give different products with dilute and concentrated nitric acid? Write equations.

PYQ: 2022

BOARD What is aqua regia? Give one use. Why is it so named?