Which common fuel gas has the widest range where it can become flammable when mixed with air?

Acetylene has the widest range of flammability when mixed with air, making it a notable choice among common fuel gases. The flammability range of a gas indicates the proportions of gas in air that can ignite and sustain combustion.

For acetylene, this range is quite broad, typically between approximately 2.5% and 100% by volume in air. This means a relatively low concentration of acetylene in air is sufficient to support combustion, but it can still be present in much higher concentrations without extinguishing the flame. Such properties make acetylene especially useful in various welding processes where precision and high heat are necessary.

In contrast, other gases such as hydrogen, propane, and methane have narrower ranges of flammability. For example, while hydrogen has a wide range and can ignite easily, its lower explosive limit is higher than that of acetylene. Propane and methane similarly have more limited ranges and specific requirements for ignition and combustion. Thus, acetylene's ability to operate effectively over a wide concentration range in air makes it particularly versatile and valuable in welding applications.