Subject: What exactly is going on when iron is placed in vinegar?

I am a teacher, who recently did the following lab with my 7th grade students. Steel wool (iron) is placed in a dilute vinegar solution, paper towel dried and then allowed to rust in a test tube in a beaker of water. Over 10 minutes the water level goes up. So far so good, the iron is rusting, using up the oxygen and the water replaces the gas. BUT, when left longer, the water level began to drop and it became clear that a gas was being produced not consumed by the reaction in the tubes. I then set up a gas generator and repeated the procedure, with the same results. Initially there is rapid rusting, and the consumption of a gas (presumable oxygen). After a short while, (presumably once all the oxygen is gone) there is gas production on a reasonable serious scale. My first assumption was that the gas was hydrogen, produced by a single displacement reaction of acetic acid with iron. However, two things suggest that this is not the case. (1) the vinegar is very dilute (2) the gas did not ignite/eplode. Tomorrow I will set it up again, and test for oxygen and/or carbon dioxide, but my chemistry does not extend to understanding what is happening. (The gas has no color or odor.) HELP.

Re: What exactly is going on when iron is placed in vinegar?