Which statement about rust formation is true?

Which statement about rust formation is true?

• A. Rust forms only in dry air.
• B. Rust forms when iron reacts with oxygen in absence of water.
• C. Rust is a general term for any metal oxide.
• D. Rust is iron oxide formed when iron reacts with water and oxygen.

Answer: (D)

Rust forms when iron corrodes in the presence of both oxygen and water. Water acts as an electrolyte, letting ions move and speeding the redox reaction where iron is oxidized and oxygen is reduced. Oxygen by itself in dry air doesn’t produce the characteristic rust quickly, because there isn’t the watery medium to facilitate the ion flow. With water and oxygen, the iron ends up as hydrated iron oxide, the substance we call rust. So the statement that rust is iron oxide formed when iron reacts with water and oxygen captures both the need for oxygen and water and the specific product produced.

Rust forms when iron corrodes in the presence of both oxygen and water. Water acts as an electrolyte, letting ions move and speeding the redox reaction where iron is oxidized and oxygen is reduced. Oxygen by itself in dry air doesn’t produce the characteristic rust quickly, because there isn’t the watery medium to facilitate the ion flow. With water and oxygen, the iron ends up as hydrated iron oxide, the substance we call rust. So the statement that rust is iron oxide formed when iron reacts with water and oxygen captures both the need for oxygen and water and the specific product produced.