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This article provides simple suggestions on how to get kids interested in science by using air pressure to crush a soft drink can.
- Rinse soda cans with water and leave a small amount (approximately 2 tbls. ) of water in the bottom of the can.
- Prepare a bowl of ice water.
- Place can up-right on stove and heat until steam is seen escaping from the mouth of can.
- Hold the tongs with your palm facing upward.
- Pick up the hot can, quickly turn over and plunge the top of the can into the bowl of water.
- In a fraction of a second the can will be crushed with a VERY LOUD NOISE! (very impressive)
- DO NOT STOP HERE! This is called an anticipatory activity.
- Next ask "What caused the can to crush?"
- Listen to the children's response. This will help you find out what they already know. Do not reaffirm, or degrade any of the answers. Just acknowledge, accept them, and ask the student to explain their thinking.
- Based on their answers you may need to explain what happened or if they have a good explanation go on to the next step. (See explanations in the "Tips" section)
- Have the children suggest things to change in the activity to test the explanation.
- Now they can really do science. The children should repeat the activity and vary the conditions. They should record their experiments, results and draw conclusions. They should share their findings with one another to see if they agree.
- Holding the tongs palm up makes it much easier to flip the can in to the water.
- Do not flip and let go. Flip the can and put (do not drop) the can in the water.
- Practice before doing this in front of children.
- Possible variables to change:
- Amount of water in can
- Temperature of water in bowl
- Increasing the time between removing the can from the heat and placing in water.
- When the can is heated it becomes filled with water vapor. This vapor occupies approximately 1000 times more space than it did as a liquid. When you turn the can over into the cool water the water vapor turns back to liquid water very quickly creating a partial vacuum in the can.
- Why doesn't the water in the bowl just get pushed up in to the can rather than the can crushing? Water has mass which also means it has inertia. Inertia can be thought of as a resistance to a change in motion. (That is not technically correct but will suffice in this situation) The water must be pulled up and it must go from little or no motion and start moving up. This requires more force than the sides of the can will be able to withstand so the can crushes before the water moves in. Some water will move into the can and will be seen draining out when you lift the can out of the water.
- Lets assume that the can only has a 2/3 vacuum. It could be much lower pressure. Atmospheric pressure is 14 lbs / square inch. A 2/3 vacuum would produce an inside pressure of approximately 5 lbs / square inch. This is a differential of 9 lbs / square inch. That means each square inch of the can feels a force of 11 lbs inward.
- The average coke can has about 49 square inches of surface area. The can would experience a total crushing force of 441 lbs. If a total vacuum is formed the crushing force is about 686 lbs.
- (Numbers are rounded for simplicity, the English system is used to make the forces a little more understandable for the lay person)
- Splash-proof goggles are required for all participants. (Can be purchased at some hardware and home centers. Be careful to make sure they are splash proof.)
- The can and the water inside will be hot. Make sure that as you flip the can into the water no one is standing in a position where the water might fly out toward them.
- Older children (ages 10+) may be able to do the activity themselves with adult supervision but you, as the adult, must watch carefully. You are responsible for safety! Never allow more than one person to do the demonstration at a time!
Things You'll Need
- Empty Aluminum Soda Cans
- Tongs large enough to comfortably handle the hot cans
- Stove or hot plate
- Shallow, clear, bowl of cool water
Sources and Citations
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