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
Correct — D, 2 and 4 only. Heating lead nitrate breaks it apart into three products, and the equation is 2Pb(NO3)2 giving 2PbO plus 4NO2 plus O2. Statement 2 is therefore right: oxygen is among the products and it is a colourless gas. Statement 4 is right too — this is a thermal decomposition, one compound pulled apart by heat into simpler ones, and it is a standard laboratory way of obtaining nitrogen dioxide. Statement 1 gets the colour right and the formula wrong: the brown fumes are nitrogen dioxide, NO2, whereas nitric oxide, NO, is colourless. Statement 3 misnames the reaction type; nothing here is being oxidised by the addition of oxygen, the compound is simply coming apart under heat.
- (a)1 and 2 — Accepts the wrong formula for the brown gas. Nitric oxide is colourless when it forms; it only turns brown after meeting air, and what it turns into is the nitrogen dioxide named in statement 4.
- (b)2, 3 and 4 — Right on 2 and 4 but adds statement 3. Calling this an oxidation reaction misreads it — a single compound splitting under heat into an oxide and two gases is a decomposition, and the label the paper wants is the one in statement 4.
- (c)1, 3 and 4 — Carries both errors at once, the wrong gas formula and the wrong reaction type, while dropping the oxygen that is genuinely released.
Thermal decomposition is a reaction in which a single compound is broken down by heat into two or more simpler substances. Lead nitrate is the classic school demonstration because everything about it is visible: a white powder in a boiling tube turns yellow as lead oxide forms, thick brown fumes of nitrogen dioxide come off, and oxygen goes off invisibly with them. The full equation is 2Pb(NO3)2 giving 2PbO plus 4NO2 plus O2.
Two of the four statements are true and two are near-misses, and both near-misses are the kind that survive careless reading. NO and NO2 differ by a single subscript but not by a single property — nitric oxide is a colourless gas, nitrogen dioxide is reddish-brown and choking, and it is the second that fills the tube. The other slip is a category error. Adding oxygen to a substance is oxidation; taking a compound apart with heat is decomposition, and the lead in this reaction does not change its oxidation state at all. Note that the same nitrogen dioxide is the reason heating lead nitrate is done in a fume cupboard: NO2 is toxic and is one of the oxides of nitrogen tracked in air-quality monitoring.
- Heating lead nitrate gives 2Pb(NO3)2 to 2PbO + 4NO2 + O2 — lead oxide, nitrogen dioxide and oxygen.
- Nitrogen dioxide is a reddish-brown gas; nitric oxide is colourless and turns brown only on meeting air.
- Heating lead nitrate is a convenient laboratory route to nitrogen dioxide.
- A single compound broken apart by heat is a thermal decomposition, not an oxidation.
- Heating limestone to give calcium oxide and carbon dioxide is the same reaction type on an industrial scale.
- Reading NO and NO2 as interchangeable; only one of them is brown.
- Labelling any reaction involving oxygen an oxidation, when here oxygen is a product of a decomposition.
- Missing that the residue in the tube changes colour too, from white nitrate to yellow oxide.
As this statements item, as a straight what-are-the-products item, or as an identify-the-brown-gas item.
CDS_GK_2021_I_Q972021When 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
Answer(d) NO₂
The identical observation asked directly two years earlier, with NO offered as the first temptation and rejected. Anyone who had met that item would strike out statement 1 here on sight.
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 decomposition asked for its products. Getting PbO and NO2 out of it fixes both the residue and the brown gas, which between them settle three of the four statements in this item.
- practice — not a real PYQ
The residue left in the boiling tube after lead nitrate has been strongly heated is
- (a)lead
- (b)lead oxide
- (c)lead nitrite
- (d)lead carbonate
Answer(b) lead oxide — the nitrate decomposes to yellow PbO, releasing nitrogen dioxide and oxygen.
- practice — not a real PYQ
Which one of the following changes is a thermal decomposition reaction?
- (a)Iron reacting with copper sulphate solution
- (b)Calcium carbonate heated to give calcium oxide and carbon dioxide
- (c)Sodium hydroxide reacting with hydrochloric acid
- (d)Magnesium burning in air
Answer(b) Calcium carbonate heated to give calcium oxide and carbon dioxide — a single compound is split by heat into simpler substances.