Explosion limits of gases in air: Which gas among the following has the widest flammability (explosion) range in air—approximately 2 to 81 percent by volume—making it particularly hazardous over a broad concentration span?

Introduction / Context:
Explosion (flammability) limits define the range of fuel concentrations in air that will support flame propagation. A wide range implies greater hazard because more mixtures encountered in leaks or vents can ignite.

Given Data / Assumptions:

• Air at ambient pressure/temperature.
• Typical textbook limit values are being referenced.
• The approximate 2–81% figure in the prompt is a stylized teaching value.

Concept / Approach:
Hydrogen is notorious for very wide limits, commonly cited around 4–75% by volume in air (values vary modestly with conditions). Many question banks round these to “about 2–81%” for emphasis. Acetylene also has very broad limits but is often reported as 2.5–100% with oxygen and narrower in air; the prompt’s stated band aligns most closely with hydrogen in air as classically taught.

Step-by-Step Solution:
Compare each gas’s lower explosive limit (LEL) and upper explosive limit (UEL).Hydrogen’s in-air range is among the widest in standard tables.Therefore, select “Hydrogen”.

Verification / Alternative check:
Process safety references consistently flag hydrogen for its large flammability envelope and low ignition energy, requiring rigorous leak prevention and ventilation.

Why Other Options Are Wrong:
Acetylene: extremely hazardous, but the exact “2–81% in air” phrasing aligns with hydrogen in many syllabi.Carbon monoxide: narrower flammability range.Ammonia: limited flammability in air.Methane: typical 5–15% in air—much narrower.

Common Pitfalls:
Mixing “in oxygen” limits with “in air.” Some fuels show huge ranges in oxygen but much narrower in air.

Final Answer:
Hydrogen