This thread brings to memory an incident in my high school chemistry class. The experiment I was conducting involved dissolving a strip of aluminum in a beaker of nitric acid. I didn't expect a particularly vigorous reaction so I was working on an open lab bench, not under a fume hood.
I put the aluminum into the beaker with no immediate result so I added a couple of more strips. Suddenly, a reaction commenced and clouds of a heavy, reddish brown gas started boiling out of the beaker toward the floor. I later recognized the gas as nitrogen dioxide, but in the instant, we just wanted to clear it from the room so we proceeded to open all the windows on one side of the room and the transom above the door opposite to generate some ventilation. This worked and the visible gas soon dissipated out the windows.
During this time, the two girls in the class had retreated to near the door. Shortly, they said they felt strange and the teacher sent them to see the school nurse. In a few minutes, we heard them in the nurse's room laughing uproariously while they lay on the cots there.
In retrospect, I concluded that nitrous oxide was also being evolved out of the beaker, but being invisible and about the same density as air, had tended to float in the vicinty of the girls and given them a sufficient dose to feel its influence. I attributed the delayed evolution of the gases to the time needed for the nitric acid to eat away the oxide layer on the aluminum and then react with the bare metal.
I'm pretty certain that high school chemistry labs are mostly better supervised today than ours was 60-some years ago.
Big Al
