Why does rust form on iron?

Answer: Iron reacts with oxygen to form iron oxide

Iron reacts with oxygen to form iron oxide ✓ — Correct! Rust is iron oxide, formed when iron reacts with oxygen in the presence of water. The overall reaction is 4Fe + 3O2 + 6H2O → 4Fe(OH)3, which dehydrates to rust. Both oxygen and water are needed, which is why iron rusts faster in humid air. Painting or oiling the surface blocks oxygen and prevents rust.

Iron absorbs water vapor from the air — Wrong. Iron does not absorb water vapor like a sponge. Rust forms through a chemical reaction between iron, oxygen, and water, not by simple absorption. The water acts as an electrolyte, but it is not absorbed into the metal.

Acid in the air eats away the iron — Wrong. While acids can corrode iron, ordinary rust is not caused by acid in the air. Rust forms through oxidation in the presence of moisture and oxygen, without needing acid. Acid corrosion is a different process that typically produces soluble iron salts, not rust.