Meteorologist Sherri Pugh demonstrates the "can crushing experiment" at Silk Hope Elementary School in October 2006.
Air has weight and is always pushing down on us. This experiment demonstrates strength of air pressure in the earth's atmosphere. The cans resting on the hot plate contain water. As the cans heat up, the water is brought to a boil and begins to evaporate. When water evaporates, it absorbs heat and changes from a less excited liquid state to a more excited gas state. These excited molecules are "bumping" into each other more, resulting in higher pressure inside the can.
While heating the can will result in higher pressure on the inside, a switch to cold will have the opposite effect. When the hot can full of gaseous water vapor is put into ice cold temperatures, the gaseous water vapor condenses into liquid water. The excited gas becomes less excited liquid, dropping the pressure inside the can. As the atmosphere is pushing down on us at all times, the low pressure inside the can makes it easy for the atmosphere outside of the can to to crush it.
The strong temperature gradient is very important in this experiment. If the can is not heated enough or the water is not cold enough, the can will only crush slightly, or not crush at all. This large temperature gradient and fast temperature change is not typical in our atmosphere.
Consider what would happen if the can went from cold to hot in an opposite experiment. The pressure inside the can would change from low to high, forcing the air inside the can to push outwards. In theory, this could cause the can to explode. However, it's important to note that soda cans are made to withstand high pressure.
A video demonstration of this experiment can be found below, as well as a list of materials and procedures. Please note that an adult should be present at all times during experimentation.
Aluminum cans (diet sodas cans do not work as well; Mountain Dew and Dr Pepper cans work better)
Hot plate or other heat source that can boil water
Container for water (big enough for top of can to be immersed)
Tongs or heat resistant gloves
Add cold water to the container. Do not completely fill the container; leave room to add ice and not spill.
Start the hot plate early to get best results; turn to High.
Add a small amount of water to the cans. The amount of water is not largely important, but there should be enough so that it does not boil away before you begin. I estimate 1-2 centimeters is sufficient.
Arrange the container and hot plate for best flipping (container on left side if right handed and if facing the students with the experiment in between).
Space container away from the hot plate to keep bucket cool.
Check that the cans are releasing steam and that the water inside is bubbling.
Add enough ice to the container to cover water's surface.
Move the container closer to the hot plate to quicken flip speed and reduce spill.
Grasp can with tongs or heat resistant glove.
QUICKLY move can over to bucket, flip it over, and immerse the top of the can in the ice cold water. Wait for the can to crush (this may take a few seconds).
--> This final step should be completed in one fluid motion. Speed is important to keep the temperature change more immediate. However, if the can is hot enough, you shouldn't have to race to flip it over.
Cloud in a Bottle Experiment
There are only three ingredients required to make a cloud: water, condensation particles, and a drop in temperature to the saturation point. As a part of the water cycle, the water is evaporated from the earth's surface in preparation for making a cloud.
Condensation particles are required for the water vapor to having something to condensate onto. Examples of such particles are dust, clay, soot, sea salt, pollen, and aerosols. For this experiment, we will use smoke as our condensation particles.
The final ingredient is a drop in temperature. Warm air can hold more water vapor than cold air, so dropping the temperature causes water to condense. In this experiment, we will use changes in pressure to change the temperature. Warmer temperatures yield higher pressure, for the water molecules become excited as result of the heat. Therefore a drop in pressure would result in a drop in temperature.
Water vapor condenses on condensation particles when the temperature cools to a saturation point. When the clouds we see in the sky are formed, an updraft (air moving upward) pushes water vapor upward. The pressure decreases as the air rises higher, resulting in a temperature drop. When the saturation point (or dew point) is met, the water vapor condenses on condensation particles in the atmosphere and forms clouds.
Fog, a surface level cloud, is seen in cold temperatures and often over ponds. The warm pond contrasts to the cold air and provides a source of moisture.
The temperature and pressure relationship can be described with a simple classroom analogy. If there were twice as many students in the room, everyone would bump into each other and generate a lot of heat. This would be an example of a high pressure situation. But, as it is with a regular class size, the students have "elbow room" and are in a more low pressure situation, thus generating less heat.
A list of materials and procedures for the "cloud in a bottle" experiment can be found below.
2-liter bottle with cap
Optional: a black background (poster board, notebook, etc)
Add water to the bottom of the bottle. An inch or two should be enough; and more water will not necessarily yield different or better results.
Strike a match and let most of the stick burn.
Blow out the match and drop into the bottle, closing the cap quickly.
Squeeze the bottle as tightly as possible. This is the increase in pressure.
Release the bottle to drop the pressure and the temperature.
Repeat steps 4 and 5. Squeezing will make the cloud disappear while releasing displays the cloud. Releasing in front of a black background makes the cloud more visible.
Open the bottle and squeeze it in short bursts to allow the cloud to escape out of the top of the bottle. Often this is easily visible.