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

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

Answer: (C)

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 that each gas has a characteristic LEL value, usually expressed as a percentage by volume in air. The lower the LEL, the easier it is for the gas to ignite at low concentrations.
3. Look up or recall the typical LEL values for the gases in question: Hydrogen (H_2), Ammonia (NH_3), Methane (CH_4), and Carbon monoxide (CO). These values are experimentally determined and can be found in standard chemical safety references.
4. Compare the LEL values of the gases. The gas with the smallest LEL value ignites at the lowest concentration in air, meaning it is the most easily ignitable among the options.
5. Conclude that Hydrogen (H_2) has the lowest LEL among the given gases, which explains why it ignites at the lowest concentration in air compared to Ammonia, Methane, and Carbon monoxide.