When we heat lead nitrate [Pb(NO₃)₂] in a boiling tube, we observe the emission of brown fumes. Which one of the following is the brown gas?
- (a)NO
- (b)N₂O₂
- (c)NO₃
- (d)NO₂
Correct — D, NO₂. Heating lead(II) nitrate breaks it up into three products at once: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Of these, the lead(II) oxide stays behind in the tube as a yellow residue and the oxygen is colourless, so the only thing that can be giving the brown fumes is the nitrogen dioxide. NO₂ is the standard brown gas of school chemistry, and this reaction is the standard laboratory way of making it. The demonstration is used to teach thermal decomposition — one compound broken by heat into two or more simpler substances — and it is one of the few school reactions where a coloured gas announces the result across the room.
- (a)NO — Nitric oxide is colourless. It does turn brown at the mouth of a tube, but only because it meets the oxygen of the air and becomes NO₂ — the brown is still nitrogen dioxide.
- (b)N₂O₂ — A formula that is not a stable gas of the nitrogen-oxide family. The dimer of nitrogen dioxide is N₂O₄, which is colourless, and it is not what this decomposition releases.
- (c)NO₃ — The nitrate ion is NO₃⁻ and exists in the salt, not as a free gas. A neutral NO₃ molecule is a short-lived radical and never appears as a product in this equation.
Thermal decomposition is a reaction in which heat alone splits a single compound into two or more products. Nitrates of the less reactive metals break down this way, giving the metal oxide, nitrogen dioxide and oxygen. Lead nitrate is the school example because all three products are easy to identify: a coloured solid residue, a brown gas and a gas that relights a glowing splint.
The question is really a colour-recall test with a formula trap. Two of the wrong options are nitrogen oxides that a candidate half-remembers, and the third dresses up the nitrate ion as a gas. The safe route is to write the equation and see what is left: PbO is a solid, O₂ is colourless, and only NO₂ remains to account for the fumes. The one confusion worth clearing is NO. It is genuinely produced in other reactions and it genuinely browns on contact with air — but that brown is NO₂ forming, so naming NO as the brown gas is wrong even in those experiments.
- The equation is 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.
- Nitrogen dioxide is the brown gas; the oxygen released is colourless and the lead(II) oxide residue is a solid.
- Heating lead nitrate is a convenient laboratory means of making nitrogen dioxide.
- Nitric oxide, NO, is colourless and becomes brown only after it reacts with atmospheric oxygen to form NO₂.
- The reaction is a thermal decomposition, not an oxidation — the crackling as the solid breaks up gives it the older name of a decrepitation reaction.
- Naming NO as the brown gas because it browns in air — what browns is the NO₂ it turns into.
- Reading NO₃ as a gas; the nitrate ion belongs to the salt, not to the fumes.
- Calling the reaction an oxidation because oxygen is released; heat alone is doing the work.
As a products-of-heating item, a colour-of-gas item, or as a set of statements about the same demonstration with one false claim slipped in.
CDS_GK_2020_II_Q1152020Lead nitrate on heating gives
- (a) PbO2 and NO2
- (b) PbO and NO2
- (c) PbO and NO
- (d) PbO2 and NO
Answer(b) PbO and NO2
The same demonstration asked for its full product list a session earlier. Getting that one right means holding both halves of the equation — the solid oxide left behind and the brown gas that leaves — while this question asks only for the gas.
Which of the following statements with regard to heating of lead nitrate powder over a flame are correct? 1. Brown fumes of NO are released. 2. Colourless O₂ gas is released. 3. It is an example of oxidation reaction. 4. It is an example of thermal decomposition used for the production of NO₂ gas. Select the correct answer using the code given below.
- (a) 1 and 2
- (b) 2, 3 and 4
- (c) 1, 3 and 4
- (d) 2 and 4 only
Answer(d) 2 and 4 only
The same experiment turned into a statement trap. Its first statement calls the brown fumes NO, which is exactly the mistake option (a) invites here, and that is why the statement has to be rejected.
- practice — not a real PYQ
On heating, ferrous sulphate crystals lose their colour and give off a gas with the smell of burning sulphur. This type of reaction is called
- (a)a combination reaction
- (b)a displacement reaction
- (c)a decomposition reaction
- (d)a neutralisation reaction
Answer(c) a decomposition reaction — heat alone splits one compound into simpler products, here ferric oxide together with sulphur dioxide and sulphur trioxide.
- practice — not a real PYQ
The residue left in the tube after lead nitrate has been heated strongly is
- (a)lead
- (b)lead(II) oxide
- (c)lead nitrite
- (d)lead carbonate
Answer(b) lead(II) oxide — the solid product of the decomposition, left behind after the nitrogen dioxide and oxygen have escaped as gases.