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

• A. Propane (C_3H_8)
• B. Hydrogen (H_2)
• C. Carbon monoxide (CO)
• D. Methane (CH_4)

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 that different gases have different LEL values due to their chemical properties, such as bond energies and combustion characteristics. Gases with lower LEL values ignite at lower concentrations in air.
3. Identify the LEL values for each gas: Hydrogen (H_2), Propane (C_3H_8), Carbon monoxide (CO), and Methane (CH_4). These values are typically given as volume percentages in air.
4. Compare the LEL values of the gases. The gas with the smallest LEL value will ignite at the lowest concentration in air.
5. Conclude that Hydrogen (H_2) has the lowest LEL among the given gases, meaning it ignites at the lowest concentration in air compared to Propane, Carbon monoxide, and Methane.