Rust will form on an iron nail when which of the following has occurred?

• A. Iron has been oxidized in dry air without any moisture.
• B. Iron has been oxidized in the presence of oxygen and water.
• C. Iron has been reduced by reacting with carbon dioxide.
• D. Iron has been dissolved in hydrochloric acid.

Answer: (B)

Verified step by step guidance

1. Understand that rust formation is a chemical process involving iron reacting with its environment.
2. Recall that rust is primarily iron oxide, which forms when iron undergoes oxidation.
3. Recognize that for rust to form, iron must be oxidized in the presence of both oxygen and water (moisture).
4. Note that dry air alone (oxygen without moisture) does not cause rust because water is necessary to facilitate the electrochemical reactions.
5. Conclude that the correct condition for rust formation is when iron has been oxidized in the presence of oxygen and water.