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We did an experiment where we put one iron nail in bottled water and another in aquarium salt water made using bottled water. We left each for 2 weeks then filtered each using filter paper and took weights. We expected more rust in the salt water and that the weight would be greater. However the weight was greater in the bottled water than the salt water. Any ideas about why? My guess is that perhaps the nail was treated in some way not to react as much to saltwater.
Answer 1:

Iron in fresh water will actually rust faster than in salt water. After a short time, I would expect the iron in the fresh water has a thin film after initial rust forms and protects it from any further rusting. In the salt water, an electrochemical corrosion reaction is taking place, (the salt water lets electrons flow much easier) and does not create the same type of film as in the fresh water case. This reaction takes longer than in the fresh water. It may take longer than 2 weeks for the salt water to rush more than the fresh water due to the slower reaction.



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