Answer the question on the basis of the passage given below:
Molecular nitrogen is very little reactive chemically. During lightning discharge, atmospheric nitrogen and oxygen combine to form nitric oxide which also combines with oxygen to form nitrogen dioxide. Both \(NO\) and \(NO_2\) constitute smog which is very poisonous. It can cause a burning sensation in the eyes and adversely affect the respiratory system.
What is the colour of nitrogen dioxide?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Nitrogen dioxide is brown colour
Nitrogen dioxide is an odd-electron molecule (17 valence electrons), and the unpaired electron makes it absorb visible light: it is a brown gas, acidic in nature. It is prepared by heating lead nitrate:
\( \ce{2Pb(NO3)2} \xrightarrow{673\,\mathrm{K}} \ce{4NO2 + 2PbO + O2} \)
On cooling, two \( \ce{NO2} \) molecules pair up their odd electrons and form colourless N2O4:
\( \ce{2NO2 <=> N2O4} \)
The colours of the oxides of nitrogen are: \( \ce{N2O} \) and NO colourless gases, \( \ce{N2O3} \) a blue solid, \( \ce{NO2} \) a brown gas, \( \ce{N2O4} \) colourless, \( \ce{N2O5} \) a colourless solid.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Oxides of Nitrogen – …
