Which substance forms instantly when aluminum is exposed to air?

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

Answer: (B)

When aluminum meets air, oxygen immediately reacts with the surface to form a very thin layer of aluminum oxide (Al2O3). This oxide film forms essentially instantly and sticks tightly to the metal, creating a passivating, protective coating that slows further oxidation. That’s why aluminum is so corrosion-resistant in air.

The other options don’t form just from exposure to air: aluminum carbide would require carbon at high temperatures or specific carburizing conditions; aluminum hydroxide forms in water or moisture; aluminum sulfate forms in the presence of sulfuric acid or sulfate ions.