Which of the following gases will ignite at the lowest concentration in air?

• A. Methane (CH_4)
• B. Hydrogen (H_2)
• C. Ammonia (NH_3)
• D. Carbon monoxide (CO)

Answer: (B)

Verified step by step guidance

1. Understand that the question is asking about the minimum concentration of a gas in air at which it can ignite, known as the Lower Explosive Limit (LEL) or Lower Flammability Limit (LFL). This is the lowest concentration of the gas in air that can propagate a flame when an ignition source is present.
2. Recall or look up the typical LEL values for each gas: Methane (CH_4), Hydrogen (H_2), Ammonia (NH_3), and Carbon monoxide (CO). These values are usually expressed as a percentage by volume in air.
3. Compare the LEL values for each gas. The gas with the smallest LEL value ignites at the lowest concentration because it requires less gas mixed with air to become flammable.
4. Note that Hydrogen (H_2) has a very low LEL, typically around 4% by volume in air, which is lower than Methane (~5%), Ammonia (~15%), and Carbon monoxide (~12.5%).
5. Conclude that Hydrogen ignites at the lowest concentration in air among the given gases because it has the lowest Lower Explosive Limit.