What substance forms instantly when aluminum is exposed to air?

• A. Aluminum hydroxide
• B. Aluminum sulfate
• C. Aluminum carbide
• D. Aluminum oxide

Answer: (D)

Aluminum readily reacts with oxygen in the air, so a very thin, adherent layer of aluminum oxide forms on its surface immediately. This oxide layer, Al2O3, develops at once because oxygen from the air is always present and aluminum has a strong affinity for it. The resulting film is protective (passivating), slowing further oxidation and giving aluminum its notable corrosion resistance in ordinary conditions. Other substances—like hydroxide, sulfate, or carbide—would require water, acids, carbon sources, or high temperatures to form, not just exposure to air. Therefore, aluminum oxide is the substance that forms instantly.