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 acid 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?

Nitrogen dioxide is brown colour

The correct answer is option 4. Nitrogen dioxide is brown colour.

Nitrogen dioxide (\(NO_2\)) is a gas that is known for its characteristic brown color. The color arises due to the absorption of light in the visible region of the spectrum. Here's a brief explanation of why nitrogen dioxide appears brown:

1. Dimerization:  In the atmosphere, nitrogen dioxide exists as a mixture of monomers (\(NO_2\)) and dimers (\(N_2O_4\)). The equilibrium between these two forms is influenced by factors like temperature and pressure.

2. Absorption of Light: Nitrogen dioxide absorbs light in the blue and violet regions of the electromagnetic spectrum. This absorption leads to a complementary color being observed, which is in the orange to brown range. The absorption spectrum of nitrogen dioxide includes wavelengths in the blue and violet regions, resulting in the perception of a brown color.

3. Complex Chemistry: The brown color is associated with the complex chemistry of nitrogen dioxide and its dimer (\(N_2O_4\)). The presence of unpaired electrons and the formation of charge-transfer complexes contribute to the absorption of specific wavelengths of light.

4. Environmental Significance: Nitrogen dioxide is a component of air pollution, often emitted from combustion processes such as vehicle engines and industrial activities. The brown color of nitrogen dioxide is one visible indicator of air pollution in urban areas.

In summary, the brown color of nitrogen dioxide is a consequence of its molecular structure and the absorption of specific wavelengths of light. This color is often noticeable in polluted air or environments with elevated nitrogen dioxide concentrations.