Lead nitrate on heating gives
- (a)PbO2 and NO2
- (b)PbO and NO2
- (c)PbO and NO
- (d)PbO2 and NO
Correct — B, PbO and NO₂. Heating lead(II) nitrate breaks it into three products by the equation 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ — a yellow residue of lead(II) oxide, thick brown fumes of nitrogen dioxide, and colourless oxygen. It is the standard school demonstration of thermal decomposition, a reaction in which a single compound is split by heat into two or more simpler substances, and it is also a convenient laboratory way of making nitrogen dioxide. Both parts of the answer follow from the starting material. The lead in lead nitrate is already in the +2 state, and heat does not oxidise it further, so the oxide formed is PbO and not PbO₂. The brown gas is nitrogen dioxide: nitric oxide, NO, is colourless, and the crackling brown fumes seen in the test tube are the visible signature that identifies the product as NO₂.
- (a)PbO2 and NO2 — Right about the gas, wrong about the residue. Lead dioxide would require the lead to be oxidised from +2 to +4, which heating the nitrate does not do. The solid left in the tube is yellow lead monoxide, litharge.
- (c)PbO and NO — Right about the residue, wrong about the gas. Nitric oxide is colourless and would show nothing; the reaction is recognised in the laboratory precisely by its dense brown fumes, which are nitrogen dioxide.
- (d)PbO2 and NO — Wrong on both counts — it changes the oxidation state of the lead and names a colourless gas for a reaction whose whole visible signature is brown fume.
Thermal decomposition is a reaction in which heat alone splits one compound into two or more products. Nitrates of the less reactive metals — lead, copper, silver — break down to the metal oxide, nitrogen dioxide and oxygen. Nitrates of the most reactive metals, sodium and potassium, decompose only as far as the nitrite and oxygen. The pattern is worth carrying because CDS and NDA both test it through the colour of the residue and the colour of the gas.
The four options are the four combinations of two independent choices — PbO or PbO₂, and NO or NO₂ — so, as with several items in this block, the reliable method is to settle each choice separately. Lead: the nitrate is a lead(II) salt and nothing in the reaction oxidises it, so PbO. Nitrogen: the gas is brown, and of the two oxides offered only NO₂ is brown, so NO₂. A small point of care about NO — it is colourless, but it turns brown the moment it meets air because it is oxidised to NO₂, which is why some accounts of nitrogen chemistry mention brown fumes in reactions that in fact release NO first. In this decomposition the NO₂ is produced directly. Note also that the printed options render the formulae without subscripts, so PbO2 in the paper means PbO₂ and NO2 means NO₂.
- The reaction is 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ on heating.
- The residue is lead(II) oxide, litharge, which is yellow; lead dioxide is not formed.
- Nitrogen dioxide is a brown gas; nitric oxide is colourless and turns brown only on meeting air.
- Heating lead nitrate is a convenient laboratory method of preparing nitrogen dioxide.
- Nitrates of reactive metals such as sodium and potassium decompose only to the nitrite and oxygen, not to the oxide.
Two independent decisions, four options — settle the residue and the gas one at a time and only one option survives.
- Promoting the lead to PbO₂ because the reaction involves oxygen; the lead stays at +2.
- Naming NO for the brown gas — NO is colourless and only browns on contact with air.
- Forgetting that oxygen is also released, which matters when the equation has to be balanced.
As this product-identification item, as an equation-balancing exercise, or through a colour-of-residue and colour-of-gas observation question.
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 reaction, and the same brown-gas trap, tested three years later. Its first statement calls the brown fumes NO and is rejected for exactly the reason option (c) fails here — nitric oxide is colourless, and the brown gas is nitrogen dioxide.
- practice — not a real PYQ
Heating copper carbonate produces
- (a)copper and carbon dioxide
- (b)copper oxide and carbon dioxide
- (c)copper oxide and carbon monoxide
- (d)copper nitrate and oxygen
Answer(b) copper oxide and carbon dioxide — the standard thermal decomposition of a carbonate, leaving a black residue of CuO.
- practice — not a real PYQ
The brown fumes seen when lead nitrate is heated in a test tube are of
- (a)nitric oxide
- (b)nitrous oxide
- (c)nitrogen dioxide
- (d)oxygen
Answer(c) nitrogen dioxide — the only brown gas among the products. Nitric oxide and nitrous oxide are colourless, as is oxygen.