help in analyzing science fair project results??? pllzz?

Question

ok i did my SFP on bio rusting i took 4 jars with distilled water and put two rust nails in each. all the nails are sanded for no protection from bacteria and i put antibacterial soap in four of those jars. I took another 4 jars and put a large amount of dirt into them with also two nails in each. I also had another jar that was the control. only filled with distilled water and just two nails (all the nails are sanded*) the results were the control nails had rusted the most. the ones in the dirt less and the antibacterial ones like more than the bacterial ones .. these results confuse me do you think there is some type of error in this procedure or should i have another trial and test with something other than dirt, and something other than antibacterial soap..?

Answer 1

Bacterial action wouldn't necessarily cause the nails to rust. That's a product of oxidation. The jar with the dirt probably rusted less because the microbes in the soil (at least those that could survive in the water) used some of the oxygen that was available, so less rust was formed. Decomposing leaves or any organic material uses oxygen too.

The soap itself (not any antibiotic in it) may have kept as much oxygen from being absorbed into the water from the air above it, so I would expect this one to be intermediate.

There are bacteria that will cause iron to rust (see this: http://en.wikipedia.org/wiki/Iron_bacteria ) but that doesn't necessarily mean they'll be present in all soils. And if the water you used was straight from the tap (containing chlorine) the chlorine may have killed the bacteria. If you repeat this experiment, you might want to let the water stand for a few days, or use the stuff they put in fish tanks to take the chlorine out.

Answer 2

if i understand the question correctly
perhaps the nails in the dirt were Biodegradable due to the high oxygen and Co2 content in the dirt. mabey sand would do better. And perhaps the antibacterial soap would be an agent like detol or something, that would be a catalyst to rust the nail because it could contain, (although the percentage would be low) some corrosive material?
Mabey you should try Sand instead of Dirt, and Mineral water instead of anti-bacterial water

Answer 3

D' and B' had some interesting responses.

Adding my additional comments:
What kind of "dirt" did you use?
High organic or inorganic content?
pH: acidic or alkaline?

pH, second to oxygen,
is the factor that contributes most to "rustablity".
If anything you exposed the nails to has a low pH (acidic),
rusting will be accelerated.
Purhaps your soil more alkaline (and had less oxygen as mentioned by D' ) than the water?