Experiment 13.2: Atmospheric Pressure

What happened in your experiment? When you put the first can in the water upright, nothing exciting happened. The can obviously cooled off, and the water in the bowl got warmer, but there should have been no noticeable change.

However, when you put the second can in the water upside down, the can should have crumpled noticeably. Depending on how much water you put in the can, how long you let it boil, and how cold the water in the bowl was, it might have crumpled only a little, or it might have really been crushed.

What explains the results of the experiment? Well, remember that the air around you is exerting pressure on everything it touches, including the cans. The cans do not crumple from this pressure, however, because there is air inside the cans as well. The air outside each can pushes in on the can, and the air inside the can pushes out on the can. The pressure from the air outside is counteracted by the pressure from the air inside. As a result, the can does not crumple.

In the experiment, we changed this situation a bit. The results, however, depended on how we changed the situation. You see, as the water boiled, the steam rising from the boiling water pushed the gas molecules in the air out of each can. Thus, once a steady stream of steam came billowing out of the cans, each can was mostly full of steam (water vapor) and had only a little air inside. The steam exerted pressure, however, so the cans did not crumple, because the pressure of the steam pushing out on the cans still counteracted the atmospheric pressure pushing in on the cans.

When you placed the first can upright in the water, the steam rapidly condensed and turned back into liquid, but as it did so, air was able to rush into the can through the opening in the top. Thus, steam was replaced by air, which means that the pressure being exerted in the can by the steam was replaced with pressure being exerted by the air that came inside the can. As a result, the pressure pushing out from the inside of the can did not change much. This meant that the air pressure pushing in on the can from the outside was still counteracted by pressure pushing out from the inside of the can, so the can did not crumple.

When you placed the second can upside down into the second bowl, the steam once again rapidly condensed into liquid. Unlike what happened with the first can, however, air could not rush in to replace the steam, because the opening of the can was under water. As a result, there was little air pressure inside the can. This meant that there wasn't much pressure inside counteracting the pressure being exerted on the can by the air outside. Since there was nothing to push against the air pressing in on the can, the pressure exerted by the air outside crushed it.