Final answer:
Rust weighs more than the original iron nail because it encompasses the mass of combined oxygen and water along with the iron.
Step-by-step explanation:
When iron corrodes, it combines with oxygen and water from the air to form rust. Rust is a red-brown hydrated metal oxide (Fe₂O₃ · xH₂O), which is heavier than iron alone because it includes the mass of the oxygen and water that have combined with the iron. Therefore, if an iron nail were allowed to rust completely, one should find that the rust weighs more than the nail it came from. This is because the rust formation is a process of corrosion where the initial iron reacts with oxygen and water to form the hydrated iron (III) oxide, thus gaining additional mass from these reactants.