Oxidation is a chemical reaction where a substance combines with oxygen or loses electrons. When iron rusts, that's oxidation: the iron reacts with oxygen in the air to form reddish-brown iron oxide. When an apple slice turns brown after being exposed to air, that's oxidation too.

Scientists use oxidation to describe a whole family of chemical reactions, including reactions where substances combine with oxygen and reactions where substances lose electrons. Batteries work through oxidation reactions, and so does the process your body uses to get energy from food.

You can see oxidation everywhere. Silver jewelry tarnishes through oxidation. Copper roofs turn green over many years because of oxidation. Even fire is a rapid oxidation reaction where fuel combines with oxygen so fast that it releases heat and light.

The opposite process is called reduction, where a substance gains electrons. In chemistry class, you might hear about “redox reactions,” which is short for reduction-oxidation. These reactions always happen together, like partners in a dance. When one substance gets oxidized, another gets reduced.

Understanding oxidation helps explain why we store some foods in airtight containers (to slow oxidation), why metals corrode, and how our cells produce energy to keep us alive and active.