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Rust, dirt, water

I'm helping my daughter with a science project for school. She is in 4th grade and we did a rust experiment. We took two iron nails & put one in clean water & the other in dirt & water. The one in the dirt & water hasn't rusted at all. The one in the clean water is really rusted. My daughter has looked all over the internet & we can't seem to find an answer about why this happens. Can you help?

Thanks,

Julia Chapman
- Las Vegas, NV

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I am a little surprised by this result, but "dirt" is not a very exacting description of the chemistry. There are many possibilities:
1. There could be oil and grease in the dirt that float to the top of the dirty water, and then get absorbed on the surface of the nail and protect it.
2. There could be minerals in the water which, by the common ion effect, are discouraging the iron from going into solution.
3. There could be a difference between the two nails.
4. Probably most likely is that there is alkalinity (lime) in the dirt that is raising the pH of the water and limiting the acidity of the water.

I think I'm going to try this one myself since the result surprised me.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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What did you find out Ted? This kind of sparked my curiosity too.

Marc Green
anodizer - Boise, Idaho
 

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I learned that my pyrex dessert bowls were still out in the garage engaged in the previous experiment of cutting a penny and seeing if the zinc would sacrificially dissolve. I don't have plain iron nails but I'm trying galvanized nails and we'll see what happens. I suspect that 'dirt' is such an uncontrolled variable that I won't learn much--but I will report back in a week or so.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

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As they say, an experiment is worth a thousand guesses.

In my limited experiment I have obtained the same result as Ms. Chapman and her daughter so far. I put 2 galvanized nails each in two pyrex desert bowls, one with tap water in it, the other with tap water and a small scoop of garden dirt. One of the nails in the plain water has begun rusting on the chiseled point. The galvanizing also looks rougher and more corroded in the plain water.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey