Decomposition of Lead vs Copper Nitrate: Class 10 Science

Quick answer: On strong heating, lead nitrate and copper nitrate both decompose into a metal oxide, brown nitrogen dioxide gas and oxygen: $\mathrm{2Pb(NO_3)_2 \xrightarrow{heat} 2PbO + 4NO_2 + O_2}$ and $\mathrm{2Cu(NO_3)_2 \xrightarrow{heat} 2CuO + 4NO_2 + O_2}$ . Both are thermal decomposition reactions. The residue from lead nitrate is yellow lead oxide (it looks reddish-brown while hot); from copper nitrate, it is black copper oxide.

When some metal nitrates are heated strongly, they break down into simpler substances. A reaction in which a single substance breaks down into two or more simpler substances on heating is called thermal decomposition.

In CBSE Class 10 Science, Chapter 1 (Chemical Reactions and Equations), the heating of lead nitrate (an NCERT activity) is a frequently asked board exam topic, and copper nitrate behaves in a similar way.

What You Will Learn

What thermal decomposition is
The equation and observations when lead nitrate is heated
The equation and observations when copper nitrate is heated
How to identify the two gases formed, nitrogen dioxide and oxygen
A side-by-side comparison of the two reactions

1. Heating Lead Nitrate

Equation:

$\mathrm{2Pb(NO_3)_2(s) \xrightarrow{heat} 2PbO(s) + 4NO_2(g) + O_2(g)}$

Observation	Explanation
Crackling sound	The crystals crack apart on heating (called decrepitation)
Brown fumes	Nitrogen dioxide ( $\mathrm{NO_2}$ ) is given off
Residue: reddish-brown when hot, yellow when cold	The solid left behind is lead(II) oxide, $\mathrm{PbO}$

Key point: The brown fumes are nitrogen dioxide. Oxygen, which is colourless, is also released.

Safety note: Nitrogen dioxide is a toxic gas. This activity should be done only under a teacher's supervision, in a well-ventilated room or fume cupboard.

2. Heating Copper Nitrate

Equation:

$\mathrm{2Cu(NO_3)_2(s) \xrightarrow{heat} 2CuO(s) + 4NO_2(g) + O_2(g)}$

Observation	Explanation
Blue crystals change	The crystals first lose their water of crystallisation, then decompose
Brown fumes	Nitrogen dioxide is given off
Black residue	The solid left behind is copper(II) oxide, $\mathrm{CuO}$

3. Identifying the Gases

Test	Nitrogen dioxide ( $\mathrm{NO_2}$ )	Oxygen ( $\mathrm{O_2}$ )
Colour	Reddish-brown	Colourless
Smell	Sharp, irritating	No smell
Moist blue litmus	Turns red (acidic gas)	No change
Glowing splint	Does not relight it	Relights it

4. Lead Nitrate vs Copper Nitrate

Feature	Lead nitrate	Copper nitrate
Formula	$\mathrm{Pb(NO_3)_2}$	$\mathrm{Cu(NO_3)_2}$
Colour of the salt	White	Blue
Gases formed	$\mathrm{NO_2}$ (brown) and $\mathrm{O_2}$	$\mathrm{NO_2}$ (brown) and $\mathrm{O_2}$
Residue	Lead(II) oxide, $\mathrm{PbO}$	Copper(II) oxide, $\mathrm{CuO}$
Colour of residue	Yellow when cold (reddish-brown when hot)	Black
Type of reaction	Thermal decomposition	Thermal decomposition

5. Summary Table

Reaction	Products
$\mathrm{2Pb(NO_3)_2 \rightarrow}$	$\mathrm{2PbO + 4NO_2 + O_2}$
$\mathrm{2Cu(NO_3)_2 \rightarrow}$	$\mathrm{2CuO + 4NO_2 + O_2}$

6. Exam Tips and Common Mistakes

Exam Tip: If asked to name the brown gas formed when lead nitrate is heated, answer nitrogen dioxide, $\mathrm{NO_2}$ .

Exam Tip: Always balance the equation: $\mathrm{2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2}$ .

Common Mistake: Writing lead nitrate as $\mathrm{PbNO_3}$ . Lead forms $\mathrm{Pb^{2+}}$ and nitrate is $\mathrm{NO_3^-}$ , so the formula is $\mathrm{Pb(NO_3)_2}$ .

Common Mistake: Calling this a combination reaction. One substance breaks down into several, so it is a decomposition reaction.

Frequently Asked Questions

What happens when lead nitrate is heated?

It decomposes into lead oxide, brown nitrogen dioxide gas and oxygen: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.

What is the brown gas formed when lead nitrate is heated?

Nitrogen dioxide, NO₂.

What is the colour of the residue when copper nitrate is heated?

Black, because the residue is copper(II) oxide, CuO.

What type of reaction is the heating of lead nitrate?

It is a thermal decomposition reaction.

How can you test for oxygen?

Oxygen relights a glowing splint.

Heating Lead Nitrate and Copper Nitrate | Class 10 Science