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Topic Thermal decomposition
Asked 4 times across 2 UPSC exams over 3 years (2020–2023), peaking in 2020.
Key insights
- Most tested in CDS — 3 of 4 PYQs
- Most asked in 2020
- Recurs across 3 years (2020–2023)
How it travelled across exams
First appearance in each exam — the same idea, reframed.
CDS 2020 → NDA 2020
How the concept evolved
Rising — more than half its appearances (4 of 4) are since 2016.
Sample questions
You've seen the pattern — here are a few of the actual PYQs.
CDS2023
Which of the following statements with regard to heating of lead nitrate powder over a flame are correct?
- Brown fumes of NO are released
- Colourless O2 gas is released
- It is an example of oxidation reaction
- 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 answer: (D)
Science & Technology › Physics & Chemistry
CDS2021
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 answer: (D)
Science & Technology › Physics & Chemistry
CDS2020
Lead nitrate on heating gives
- A PbO₂ and NO₂
- B PbO and NO₂ ✓
- C PbO and NO
- D PbO₂ and NO
✓ Correct answer: (B)
Science & Technology › Physics & Chemistry
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