Rust results from iron’s reaction to oxygen. An iron nail gains mass when it rusts. How does this reaction support the law of conservation of mass? The mass of the rusted nail equals the mass of iron and the oxygen from the air it reacted with to form the rust. The mass of the rusted nail increases because iron attracts more protons from the air. The increased mass of the rusted nail is an exception to the law of conservation of mass since the rusted nail’s mass increases. The increased mass of the rusted nail results from the rearrangement of protons and neutrons within oxygen, according to the law of conservation of mass.