What substances form when metal is exposed to oxygen?

• A. Oxide & Aluminum oxide
• B. Hydroxide
• C. Chloride
• D. Sulfide

Answer: (A)

When metal meets oxygen, it undergoes oxidation to form metal oxides. This is the common corrosion product for most metals. The option mentioning oxide—and aluminum oxide as a specific example—captures both the general outcome and a concrete instance: metals typically form oxides like iron oxide or aluminum oxide, depending on the metal. Hydroxides require water to react, chlorides need chlorine, and sulfides require sulfur or a sulfur-containing reactant, so they aren’t the immediate products simply from exposure to oxygen. The oxide layer, especially with aluminum, can even protect the metal from further corrosion. So the substance formed when metal is exposed to oxygen is oxide (with aluminum oxide as a specific example).