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Question

Which one of the following gas smells like a rotten egg?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
Hydrogen sulphide

Understanding the Smell of Rotten Eggs

Many gases have distinct odors, and some are quite unpleasant. The question asks to identify the specific gas that is commonly known for smelling like rotten eggs. This characteristic smell is a key identifier for certain chemical compounds.

Identifying Hydrogen Sulphide

The gas associated with the smell of rotten eggs is Hydrogen sulphide. The chemical formula for this gas is LaTeX code: \(H_2S\). This compound is naturally produced when organic matter containing sulfur breaks down, especially in environments lacking oxygen (anaerobic conditions). It can be found in sources like natural gas, swamp gas, and volcanic emissions. The smell is noticeable even at very low concentrations.

Comparing Common Gas Odors

It is helpful to compare the odor of Hydrogen sulphide (\(H_2S\)) with the other gases listed:

Gas Chemical Formula Common Odor Description
Ammonia \(NH_3\) Sharp, pungent, irritating (similar to urine)
Hydrogen sulphide \(H_2S\) Rotten eggs
Acetylene \(C_2H_2\) Faint garlic-like odor (often due to impurities)
Sulphur dioxide \(SO_2\) Pungent, suffocating, acrid (like burnt matches)

Conclusion on Gas Identification

Based on established chemical properties and sensory identification, Hydrogen sulphide (\(H_2S\)) is the gas universally recognized for its characteristic rotten egg smell, distinguishing it from Ammonia, Acetylene, and Sulphur dioxide.

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