Why does the mass of steel wool increase when it burns in air?

• A. The iron oxide formed has more mass than the iron alone.
• B. The mass of oxygen leaves the system.
• C. No reaction occurs and mass is unchanged.
• D. The mass of the iron decreases.

Answer: (A)

When steel wool burns, iron reacts with oxygen from the air to form iron oxide. The oxygen that is part of the air becomes bound to the iron, so the new solid includes both the original iron and the added oxygen. That extra oxygen increases the mass, which is why the material weighs more after burning. The other ideas don’t fit because oxygen isn’t leaving the system, a reaction does occur, and the iron doesn’t simply lose mass—it binds with oxygen to form a heavier oxide.