When an iron nail gets rusted, iron oxide is formed

Concept — A chemical change is a rearrangement of atoms, and the law of conservation of mass says that no atom is created or destroyed in it. So whenever a solid combines chemically with matter drawn from its surroundings, the solid product carries the mass of the original solid plus the mass of everything the solid has taken in.

Application — Rusting is exactly such a combination reaction: iron reacts with the oxygen of the air in the presence of moisture.

1. Write the reaction: 4Fe + 3O2 + 2xH2O gives 2Fe2O3·xH2O, hydrated iron(III) oxide, the reddish-brown solid we call rust.

2. Identify where each atom comes from: every iron atom was already in the nail, while the oxygen and the water molecules come from the air and the moisture around it.

3. Track the mass: the nail keeps all of its iron and, in addition, now holds the oxygen and water chemically bonded into the oxide layer.

4. Put a number on it: 56 g of iron, that is one mole of Fe, ends up as half a mole of Fe2O3, which is 80 g — a gain of 24 g, about 43 per cent, even before the water of hydration is counted.

5. Conclude: the nail together with its rust weighs more than the fresh nail did, so the iron oxide is formed with an increase in the weight of the nail.

Cross-check

• Equal weight would require the combined oxygen to contribute nothing to the mass, but oxygen atoms have mass and they were not part of the nail before.

• A lower weight would require iron to leave the nail; rusting instead deposits an extra layer on it. Loose rust flaking off a badly corroded object can leave a lighter remnant, but the oxide that forms is heavier than the iron that produced it.

• Unchanged appearance is ruled out by simple observation: the grey metallic surface turns reddish-brown as the oxide layer forms.