Measuring oxygen content of air-failed

[lfo12363]

Hello. for my 7th grade science fair I performed the steel wool in the inverted test tube experiment. I used fine stool wool from hardware store, six test tubes (3 with steel wool, 3 without), instead of individual beakers I inverted all six (attached to peg board), over a tub of water....after six days....nothing. What could be wrong? Halfway through the experiment, I did take one test tube and briefly fill it with water and reinverted...thinking maybe the wetting steel wool would accelerate oxygen use...and if you look real hard, the water may have risen a tiny amount in this tube? I don't know...help

[carolinethorn]

Don't be disheartened, getting experiments to work is a difficult thing, even ones that have been tested by others before. Lets think about what is supposed to be happeneing in the experiment to see if we can find where the problem might be.
In the experiment the steel wool is supposed to rust or "oxidise", this is a chemical reaction that will use the oxygen from the air and combine it with the iron in the steel wool to form rust or iron oxide. When the oxygen is used the space that it took up in the air in the tube is replaced by water.
So, first of all, did the steel wool rust? If it hasn't rusted yet then the oxygen hasn't been used in the reaction so you would not see the water in the tube move.
Another possible problem is that instead of water replacing the oxygen some air managed to sneak in. Do you think this was possible?
Thirdly, you mentioned that you have all of the tubes in one large tub of water. This makes it harder to see a change in the levels - if you take a few spoonfuls of water out of a small beaker you notice it much more than if you took the same amount from a large tub wouldn't you? Also it makes all of the tubes connected by the water so they are not independent systmes. If you have each tube in a separate beaker then perhaps one may get some air leak in but the others could be airtight, increasing your chances of getting the experiment to work as planned.

POst back with some of your additional observations and lets see if we can figure out together where it is going wrong.

Best of luck,
Caroline

[lfo12363]

Caroline,
Thanks for the reply. You're right, with the exception of a small amount of orange rust on the bottom of the steel wool in the tube that we got wet....I see no rust otherwise. We are going to redo the experiment. I will use the beakers this time...what should I do about the steel wool? Should I try a different brand or something else?

[carolinethorn]

Thats good that you have figured out the problem. If you have the time you could leave your apparatus longer to allow the rust more time to develop. Or you could think about ways to accelerate the rusting process. There are a number of ways that you could encourage your steel wool to rust - one of them you already obsevred - that if you got it wet first that seemed to make it rust faster. Try doing a google search for accelerating rust and see what comes up. Post back with your ideas.

-Caroline

[Louise]

Thats good that you have figured out the problem. If you have the time you could leave your apparatus longer to allow the rust more time to develop. Or you could think about ways to accelerate the rusting process. There are a number of ways that you could encourage your steel wool to rust - one of them you already obsevred - that if you got it wet first that seemed to make it rust faster. Try doing a google search for accelerating rust and see what comes up. Post back with your ideas.

-Caroline

Also try to get the finest (thinnest) steel wool possible. This will rust faster. Also, some steel wool has coatings to prevent rust, so make sure that isn't the problem!

Good luck.
Louise

[MelissaB]

I agree, it could be a coating that's your problem...I suggest rubbing the steel wool with sandpaper or something similar first, and, like Louise said, trying to use the finest wool possible. The more surface area you expose, the faster the rusting process.