Rust forms when iron meets oxygen and water, creating a flaky reddish compound over time.

The longer version

Iron exposed to damp air takes part in a slow chemical reaction. Oxygen from the air and water from the surroundings combine with the metal to form hydrated iron oxide, a reddish brown substance that flakes away. Because the flakes crumble off, fresh metal is continually revealed and the process eats deeper.

Salt speeds the process up because dissolved ions carry charge through the thin film of water on the surface. Dry air slows it almost to a halt, and a coating of paint or oil blocks both the oxygen and the water. Other metals corrode too, but their oxide layers often cling tightly and protect what lies beneath.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.