Fire safety — Which one of the following substances is effectively incombustible under ordinary conditions and is used as a non-flammable solvent?

Introduction:
Understanding flammability is vital for safe storage and handling of chemicals. Some compounds are highly flammable, while others are used precisely because they resist combustion. This question asks you to identify a substance that is essentially incombustible under normal conditions.

Given Data / Assumptions:

• Ambient pressure and temperature.
• Ignition sources are typical (sparks, flames).
• No exotic oxidizers are present.

Concept / Approach:
Hydrogen, methane, and acetylene are classic combustible gases; they burn readily in air across defined flammability limits. Sulfur is combustible as a solid and burns with a blue flame, forming sulfur dioxide. Carbon tetrachloride (CCl₄), however, is a halogenated solvent with negligible flammability—it was historically used in some fire extinguishers (now obsolete due to toxicity and environmental concerns). The heavy halogenation suppresses combustion by both chemical and physical mechanisms.

Step-by-Step Solution:
Screen options for known fuels: H₂, CH₄, C₂H₂ are combustible.Note sulfur’s known flammability in air.Recognize CCl₄ as a non-flammable, incombustible solvent under ordinary conditions; select it.

Verification / Alternative check:
Safety data sheets list CCl₄ as non-flammable with no flash point, while the others have defined flash/flammability characteristics.

Why Other Options Are Wrong:

• H₂, CH₄, C₂H₂: fuel gases with broad explosive limits (especially H₂, C₂H₂).
• S: ignites and burns to SO₂; used in flame tests historically.

Common Pitfalls:
Assuming all hydrocarbons/elemental nonmetals behave similarly; overlooking halogenated solvent behavior and toxicity concerns (never use CCl₄ as a modern extinguisher).

Final Answer:
Carbon tetrachloride (CCl₄)