Exposure of chlorine in air in the presence of sunlight releases a gas which is poisonous. Identify the gas.

Phosgene

The correct answer is option 3. Phosgene.

When chlorine gas (\(\text{Cl}_2\)) is exposed to air in the presence of sunlight, a series of chemical reactions can occur, especially if there are traces of organic compounds or carbon monoxide (\(\text{CO}\)) present in the environment. One of the most significant and dangerous products of these reactions is phosgene (\(\text{COCl}_2\)).

Formation of Phosgene (\(\text{COCl}_2\))

Reaction Overview:

Phosgene is produced through the reaction between chlorine gas (\(\text{Cl}_2\)) and carbon monoxide (\(\text{CO}\)) in the presence of sunlight. The reaction is as follows:

\(\text{Cl}_2 + \text{CO} \overset{\text{sunlight}}{\longrightarrow} \text{COCl}_2\)

Sunlight acts as a catalyst by providing the energy needed to initiate the reaction.

Source of Carbon Monoxide:

Carbon monoxide (\(\text{CO}\)) can be present in the air due to incomplete combustion of carbon-containing fuels. It is a common pollutant in urban environments, especially near vehicle exhausts or industrial processes.

Properties of Phosgene:

Phosgene (\(\text{COCl}_2\)) is a highly toxic and colorless gas. It has a smell that is often described as similar to musty hay or freshly cut grass, which can be deceiving because it might not immediately be recognized as harmful. Phosgene is dangerous because it can cause severe damage to the lungs if inhaled, leading to pulmonary edema, which can be fatal.

Historical Context:

Phosgene was infamously used as a chemical warfare agent during World War I due to its deadly effects when inhaled. Even in small quantities, it can cause severe respiratory damage.

Why the Other Options are Incorrect:

Phosphene (Phosphine, \( \text{PH}_3 \)): Phosphine is a toxic gas, but it is unrelated to chlorine exposure in air. It is typically formed in different chemical processes, not by the reaction of chlorine in sunlight.

Chloric Acid (\( \text{HClO}_3 \)): Chloric acid is an aqueous solution and not a gas. It is formed by the reaction of chlorine with water, but it is not produced by the exposure of chlorine gas to air in the presence of sunlight.

Chlorous Acid (\( \text{HClO}_2 \)): Similar to chloric acid, chlorous acid is also an aqueous solution and not a gas. It is another oxyacid of chlorine but is not involved in the formation of phosgene.