MadSci Network: Chemistry

Re: Does Iron Rust Faster and More in Salt Water or Fresh Water

Date: Tue Jan 12 14:57:50 2010
Posted By: Todd Whitcombe, Associate Professor, Chemistry
Area of science: Chemistry
ID: 1263146055.Ch

The rusting of iron is an oxidation reaction between molecular oxygen (O2) and iron (Fe) in the presence of water (H2O). All three components are necessary for the reaction:

           Fe   +   O2   +   H2O  ---->   Fe2O3.xH2O

That is, iron exposed to dry air won't rust nor will iron placed in water with no oxygen. Further, the role of salt is to provide an electrolyte which makes it so that the "oxidation and reduction" reactions - the REDOX reactions - involved in this process occur more easily. That is why the literature says that the salt water system should rust more iron. This is all something that I am sure that you already know but it is the basics of what you are doing.

In your case, the fresh water that you got from the refridgerator was, I am assuming, from one of the spiggots in the front. I don't have one of these on my refridgerator but if I recall from the last time I used one, it had an aeration disc in it and the water came out "frothy". That is, it was very oxygen rich water. If that is the case, then having a higher concentration of oxygen would speed up the reaction relative to some water that hadn't been aerated. So, that is one possible answer.

There is also the question of the salt water that you are getting from the fish store. Is it really "salt water" from the ocean or something that they make that resembles salt water from the ocean? If the later, I can see that they might want to use very pure water as the starting point in which case they might use distilled water. And distillation takes a lot of the oxygen out of the water. This would mean that it is oxygen deficient - even relative to just tap water.

It is also possible that if they are getting their salt water from the ocean, that it has been kept for a while and has become oxygen depleted because it has been standing around or because it contains micro- organisms that have used up the oxygen. This wouldn't be a problem for someone that is using the water for salt water fish - because they would have an aeration system in their tank to put the oxygen back into the water so it is something that the store might not even be aware of.

So, one possible answer to your problem may be a lack of oxygen in the salt water or an over-abundance of oxygen in the fresh water. Another possibility is that your results are not really telling you what you think they are. Salt water contains chloride ions which are fairly common and not a big deal. But in the presence of iron, they react with the iron atoms to give a yellow solution. This colouration is probably water you were seeing. The orange that you were observing in the case of the fresh water is iron oxide. Hence, when you say that "the salt water nail is pale yellowish, while the water for the fresh water nail is orange to dark orangish in color", you are not really comparing two of the same thing. It is a bit like comparing two apples, one green and one red, and saying that the red one is more "apple-like" because it has an apple red color. That is not really a fair comparison as the green apple is just as much an apple.

I would suggest that in order to determine just how much rusting has occurred, you will need to compare the rusted nails at the end of your 72 hour test. I would suggest weighing them before and after. And I would suggest trying to scrape them to the same level of cleanliness so that you are really comparing how much of the metal has turned to rust in both cases - not how much rust in on each nail.

Other things that you could try would be to boil both the salt water and the fresh water for a short period of time (but exactly the same length of time) to remove the oxygen in both cases. Then introduce the nail. Since the rate of flow of oxygen from the air back into the solutions will be the same, provided that you use identical containers, you will eliminate the amount of oxygen as one of the possible variables. That is, since the amount of oxygen that enters each solution will be the same for any length of time, then the concentration of oxygen will always be the same for each container and any intrinsic variability between the two types of water will have disappeared.

I hope this helps. Keep up the great work and I hope that you do well on your science project!!

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