SSC CGL 2024 Tier-I · 25 September 2024, Shift 2 · General Awareness · Question 11 of 99

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 10 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

When you take Lead nitrate powder in a boiling tube and heat it, you will observe the emission of brown fumes that are of _____________________.

(a)Nitrogen dioxide
(b)Nitric oxide
(c)Nitrous oxide
(d)Dinitrogen trioxide

Answer

Why

Correct — A. Heating lead nitrate breaks it apart, and the brown gas driven off is nitrogen dioxide.

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

NO₂ is reddish-brown, and it is the only coloured gas in that equation: the oxygen is colourless and the lead(II) oxide stays behind as a yellow solid in the tube.

Colour is the whole identification here, so the brown fumes are the nitrogen dioxide of option (a).

Why the others are wrong

(b)
Nitric oxide — Nitric oxide, NO, is colourless. It does turn brown on contact with air, but only by oxidising into NO₂ — so the brown you see is still the dioxide.
(c)
Nitrous oxide — Nitrous oxide, N₂O, is the colourless gas known as laughing gas, and it is not among the products of this decomposition.
(d)
Dinitrogen trioxide — Dinitrogen trioxide, N₂O₃, is deep blue as a liquid and comes apart readily into NO and NO₂. It is not the brown fume seen on heating lead nitrate.

Concept

This is a thermal decomposition — one reactant broken into more than one product by heat — and it is a standard laboratory demonstration.

Lead nitrate gives three products: lead(II) oxide as a yellow residue, nitrogen dioxide as brown fumes, and oxygen.

The chapter's other set pieces are worth holding beside it. Calcium carbonate decomposes to quicklime and carbon dioxide. Ferrous sulphate crystals lose water, turn white, and on further heating leave a brown residue of ferric oxide with sulphur dioxide and sulphur trioxide.

This experiment is worth revising by its colours — the brown gas, the yellow residue, the white anhydrous salt.

Get the colours and the equation will be there when you need it.

Key facts

Heating lead nitrate gives 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.
Nitrogen dioxide is a reddish-brown gas, while nitric oxide and nitrous oxide are both colourless.
The residue left in the boiling tube, lead(II) oxide, is a yellow solid.
Thermal decomposition breaks one reactant into more than one product using heat.

Study next

Thermal, electrolytic and photolytic decomposition with one example of eachColour changes used to identify gases and residues in school experimentsWhich decomposition reactions are also redox reactions

Common traps

Naming nitric oxide because it too turns brown, when it does so only after oxidising in air.
Answering with the residue instead of the gas: the yellow solid left behind is lead(II) oxide.

Decomposition reactions are asked either as an observation from the experiment, as here, or as a classification of the reaction itself.

Which decomposition reaction is NOT a redox reaction is asked on 9 Sep 2024, 09:00 at General Awareness Q.11, from the same chapter.

Related PYQs

No directly related past PYQ was found.