ANS:B - NO₂

Nitrogen dioxide (NO2) is a chemical compound composed of one nitrogen atom (N) bonded to two oxygen atoms (O). It is a reddish-brown gas at room temperature and pressure.

1. Formation: NO2 is primarily formed through the oxidation of nitrogen monoxide (NO) in the presence of oxygen (O2) in the atmosphere. This reaction occurs particularly in combustion processes, such as those in vehicle engines and industrial operations, where high temperatures facilitate the formation of nitrogen oxides.

2𝑁𝑂+𝑂2→2𝑁𝑂22NO+O2​→2NO2​

2. Physical Properties: Nitrogen dioxide has a characteristic reddish-brown color, which is observable in urban smog and industrial pollution. It has a sharp, pungent odor and is highly reactive.
3. Chemical Properties: NO2 is a highly reactive and toxic gas. It can undergo various chemical reactions, including:
   • Dissociation: NO2 can dissociate into nitrogen monoxide (NO) and oxygen gas (O2) under certain conditions.
   • Acid-Base Reactions: NO2 can dissolve in water to form nitric acid (HNO3) and nitrogen monoxide (NO).
   • Reaction with Hydrocarbons: NO2 can react with hydrocarbons in the presence of sunlight to form ozone (O3) and other photochemical smog components.
4. Environmental Impact: Nitrogen dioxide is a major component of air pollution and is harmful to both human health and the environment. Prolonged exposure to NO2 can lead to respiratory issues, aggravate existing lung conditions, and contribute to the formation of smog and acid rain.

In summary, nitrogen dioxide is a reactive gas with significant environmental and health implications. Its presence in the atmosphere is a key indicator of air pollution, and efforts to reduce NO2 emissions are essential for improving air quality and mitigating its adverse effects.