Q1: The gas evolved in sewers is

ANS:D - All of these.

1. Hydrogen sulfide (H2S):
   • Hydrogen sulfide is a colorless, toxic gas with a characteristic foul odor of rotten eggs.
   • It is produced during the anaerobic decomposition of organic matter by sulfate-reducing bacteria.
   • In sewers, organic materials such as human waste, food scraps, and other debris undergo decomposition, leading to the release of hydrogen sulfide gas.
   • Hydrogen sulfide is highly toxic and can pose serious health risks to humans, including respiratory irritation, headaches, nausea, and even death at high concentrations.
2. Methane (CH4):
   • Methane is a colorless, odorless gas that is flammable and lighter than air.
   • It is produced by the anaerobic decomposition of organic matter, similar to hydrogen sulfide.
   • In sewers, methane is formed as a byproduct of microbial activity during the breakdown of organic materials.
   • While methane itself is not typically as toxic as hydrogen sulfide, it is highly flammable and can pose explosion risks in confined spaces if allowed to accumulate in high concentrations.
3. Carbon dioxide (CO2):
   • Carbon dioxide is a colorless, odorless gas that is naturally present in the atmosphere and is produced by various natural processes, including respiration and combustion.
   • In sewers, carbon dioxide can be generated through microbial respiration during the decomposition of organic matter.
   • While carbon dioxide is not typically considered toxic in small concentrations, it can displace oxygen in confined spaces, leading to asphyxiation if present in high enough concentrations.