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?

When iron rusts, it combines chemically with oxygen from the air. The rusted nail’s mass includes both the original iron and the oxygen that became part of the rust, so mass is conserved when all the reactants are counted. The nail alone gains mass because oxygen has been added; it does not gain protons from the air.

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