Which gas has a specific gravity of 0.555 and requires 12.5% oxygen to ignite?

The specific gravity of a gas measures its density compared to air, with values less than 1 indicating that the gas is lighter than air. Methane has a specific gravity of approximately 0.555, which confirms that it is indeed lighter than air and will rise in the atmosphere.

Additionally, the ignition requirement of 12.5% oxygen means that the gas can ignite and sustain combustion with a relatively low concentration of oxygen compared to air, which contains about 21% oxygen. Methane is known to ignite at this oxygen level, making it a common fuel in various applications.

In contrast, while hydrogen is also lighter than air and has a lower ignition requirement, its specific gravity is closer to 0.069. Propane has a higher specific gravity at about 1.5, meaning it is heavier than air and would have a different ignition profile. Butane similarly has a specific gravity around 2.0. Thus, both propane and butane do not match the specific gravity or ignition requirements for the question asked.

Therefore, the attributes of methane regarding its specific gravity and ignition requirements align perfectly with the details provided in the question.