Question

Nitrogen fixation is the conversion of

• A. Nitrate to nitrogen gas
• B. Nitrogen gas to ammonia
• C. Ammonia to nitrite
• D. Nitrite to nitrate

Hint:

Remember: \[ \boxed{N_2 \rightarrow NH_3} \] Nitrogen fixation converts atmospheric nitrogen into ammonia, making nitrogen available for living organisms.

Answer 1

The Correct Option is B

Concept: Nitrogen fixation is an important step in the nitrogen cycle. Atmospheric nitrogen (\(N_2\)) is highly stable due to the presence of a triple bond between the two nitrogen atoms. Most organisms cannot directly utilize atmospheric nitrogen. Therefore, it must first be converted into a usable form such as ammonia (\(NH_3\)).

Step 1: Understand atmospheric nitrogen.
About \(78\%\) of Earth's atmosphere consists of nitrogen gas (\(N_2\)). \[ \boxed{N \equiv N} \] The triple bond makes nitrogen chemically inert and unavailable to most living organisms.

Step 2: Understand nitrogen fixation.
Nitrogen-fixing bacteria such as Rhizobium, Azotobacter, and cyanobacteria convert atmospheric nitrogen into ammonia. \[ N_2 + 8H^+ + 8e^- \longrightarrow 2NH_3 + H_2 \] This process makes nitrogen available for plants and subsequently for animals.

Step 3: Eliminate incorrect options.

• Nitrate to nitrogen gas is denitrification.
• Ammonia to nitrite is nitrification.
• Nitrite to nitrate is also a step of nitrification.
• Nitrogen gas to ammonia is nitrogen fixation. Final Answer: \[ \boxed{\text{Nitrogen fixation = Conversion of } N_2 \text{ into } NH_3} \] Hence, \[ \boxed{Option (B) is correct \]