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

• A. Nitrogen (N_2)
• B. Carbon dioxide (CO_2)
• C. Oxygen (O_2)
• D. Hydrogen (H_2)

Answer: (D)

Verified step by step guidance

1. Understand that the question is about the minimum concentration of a gas in air required for it to ignite, which relates to the concept of the Lower Explosive Limit (LEL) or Lower Flammability Limit (LFL). This is the lowest concentration of a gas or vapor in air capable of producing a flash of fire in presence of an ignition source.
2. Recall that gases like Nitrogen (N_2), Carbon dioxide (CO_2), and Oxygen (O_2) are not flammable. Nitrogen and carbon dioxide are inert or non-flammable gases, and oxygen supports combustion but does not ignite itself.
3. Recognize that Hydrogen (H_2) is a highly flammable gas with a very low LEL, meaning it can ignite at very low concentrations in air compared to other gases.
4. Compare the flammability properties of the gases: since N_2 and CO_2 are non-flammable and O_2 is an oxidizer, Hydrogen (H_2) will ignite at the lowest concentration in air among the options given.
5. Conclude that the key to answering this question is understanding the flammability and explosive limits of gases, and knowing that Hydrogen has a much lower ignition concentration than the others listed.