Nitric Acid (HNO3): Preparation, Properties & Reactions | Chemca

Inorganic Chemistry

Nitric Acid ($HNO_3$): Preparation & Properties

By Chemca Editorial Team • Last Updated: January 2026 • 12 min read

Nitric Acid ($HNO_3$), historically known as Aqua Fortis, is a strong mineral acid and a powerful oxidizing agent. It is a colorless liquid in its pure form but often appears yellow due to the presence of dissolved oxides of nitrogen ($NO_2$).

1. Preparation: The Ostwald Process

Industrially, Nitric Acid is manufactured by the catalytic oxidation of Ammonia.

Step 1: Catalytic Oxidation

Ammonia reacts with atmospheric oxygen over a Platinum/Rhodium gauze catalyst at 500 K and 9 bar pressure.

$$ 4NH_3 + 5O_2 \xrightarrow{Pt/Rh, \ 500 K} 4NO + 6H_2O $$

Step 2: Oxidation of NO

Nitric oxide combines with oxygen to form Nitrogen Dioxide ($NO_2$).

$$ 2NO + O_2 \rightleftharpoons 2NO_2 $$

Step 3: Absorption in Water

Nitrogen dioxide dissolves in water to form Nitric Acid.

$$ 3NO_2 + H_2O \rightarrow 2HNO_3 + NO $$

2. Oxidizing Properties (Crucial for Exams)

$HNO_3$ is a strong oxidizing agent. The reduction product depends on the concentration of the acid and the nature of the metal.

General Rule of Thumb

Concentrated $HNO_3$: Usually gives $NO_2$ (Reddish-brown gas).
Dilute $HNO_3$: Usually gives $NO$ (Colorless gas) with less active metals (Cu, Ag).
Dilute $HNO_3$: Usually gives $N_2O$ (Nitrous oxide) with active metals (Zn, Fe).
Very Dilute $HNO_3$: Gives $NH_4NO_3$ with active metals (Zn, Sn).

A. Reaction with Copper (Less Active Metal)

Dilute: $3Cu + 8HNO_3 \rightarrow 3Cu(NO_3)_2 + 2NO + 4H_2O$
Conc: $Cu + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O$

B. Reaction with Zinc (Active Metal)

Dilute: $4Zn + 10HNO_3 \rightarrow 4Zn(NO_3)_2 + N_2O + 5H_2O$
Conc: $Zn + 4HNO_3 \rightarrow Zn(NO_3)_2 + 2NO_2 + 2H_2O$

C. Reaction with Magnesium & Manganese

Only Mg and Mn react with very dilute $HNO_3$ (~1%) to evolve Hydrogen Gas ($H_2$).

$$ Mg + 2HNO_3 \text{ (v. dil)} \rightarrow Mg(NO_3)_2 + H_2 \uparrow $$

3. Reaction with Non-Metals

Concentrated $HNO_3$ oxidizes non-metals to their corresponding oxo-acids or oxides, while reducing itself to $NO_2$.

Iodine ($I_2$)
$\rightarrow$ Iodic Acid ($HIO_3$)

$I_2 + 10HNO_3 \rightarrow 2HIO_3 + 10NO_2 + 4H_2O$

Carbon ($C$)
$\rightarrow$ Carbon Dioxide ($CO_2$)

$C + 4HNO_3 \rightarrow CO_2 + 2H_2O + 4NO_2$

Sulfur ($S_8$)
$\rightarrow$ Sulfuric Acid ($H_2SO_4$)

$S_8 + 48HNO_3 \rightarrow 8H_2SO_4 + 48NO_2 + 16H_2O$

Phosphorus ($P_4$)
$\rightarrow$ Phosphoric Acid ($H_3PO_4$)

$P_4 + 20HNO_3 \rightarrow 4H_3PO_4 + 20NO_2 + 4H_2O$

4. Brown Ring Test

This is the confirmatory test for Nitrate ions ($NO_3^-$).
Procedure: Add freshly prepared $FeSO_4$ solution to the nitrate solution. Then, carefully add concentrated $H_2SO_4$ along the sides of the test tube.
Observation: A dark brown ring forms at the junction of the two layers.

Complex Formula:
$$ [Fe(H_2O)_5(NO)]^{2+} $$ (Pentaaquanitrosyliron(I) sulfate)
Oxidation state of Fe is +1 here.

5. Passivity of Metals

Metals like Iron (Fe), Aluminum (Al), Chromium (Cr), and Nickel (Ni) do not dissolve in concentrated nitric acid because they form a thin, inert oxide layer on their surface. This phenomenon is called Passivity.

6. Aqua Regia

A mixture of Conc. HCl and Conc. HNO_3 in a 3:1 ratio. It can dissolve noble metals like Gold ($Au$) and Platinum ($Pt$).

$$ Au + 4H^+ + NO_3^- + 4Cl^- \rightarrow AuCl_4^- + NO + 2H_2O $$

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