|MadSci Network: Chemistry|
Interesting question. When iron or any other metal oxidizes, oxygen from the air combines with the iron to make iron oxide or rust. So the first part of the answer is that the weight of the bar increases, due to the increased weight of oxygen which has combined with the iron. The second part of the answer has to do with the nature of the oxide that is formed. Iron oxide is non-adherent; either the volume of the oxide is greater or less than the volume of iron it replaced. So over time the rust will fall off of the bar. So if you look at something which has rusted for a long time (like my 75 Chevy pickup truck), the ground surrounding the iron part is coated with a layer of rust which has blown or washed off the iron part. So, over time, the weight of the iron bar will tend to decrease, due to factors related more to erosion than to chemistry. Some metals form adherent oxides, such as aluminum and chromium. If you were able to expose a perfectly clean surface of either to the air, it would initially gain weight (although the increase would be very small since we are only talking about a couple molecules thick oxide coating) and then the weight would remain essentially constant. So, my 75 Chevy pickup ought to gradually get better gas milage as I leave pieces of fender, truck bed, and floorboard on the sides of the road in the form of iron powder.
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