This experiment helps to show kids the power of air pressure by using temperature change to change the density of air. This is an very easy air pressure experiment and only requires a few common supplies to get started.
- 2 clear water bottles with caps
- Cold water
- Hot water
Step 1: Peel the labels off each water bottle so kids can see what is happening inside the bottles during the experiment.
Step 2: Fill one water bottle with cool water.
Step 3: Fill the second water bottle with hot water. Use hot pads, if needed, to avoid burning hands. Hot tap water works fine.
Step 4: Put the lids on the bottles and shake until the temperature of the plastic is consistent with the water inside the bottle.
Step 5: Remove the bottle caps and pour the water down the drain.
Step 6: Quickly screw the bottle caps back on the water bottles.
Step 7: Place both bottles inside of the refrigerator.
Step 8: Wait about 5 minutes and open the refrigerator door and remove the bottles. Look at the shape of the bottles now.
The two water bottles behave completely differently after being placed inside the cold environment of a refrigerator. Nothing happens to the water bottle that was rinsed with cold water before being placed inside the refrigerator. The bottle that was heated by hot water, however, was crushed after being placed inside the cold refrigerator. Why?
The air inside the bottle which was heated with hot water expanded from the higher temperature. The expanded air was then sealed inside the bottle when the cap was tightened on the bottle. As soon as the air inside the hot bottle began to cool, negative air pressure was created as the air inside the bottle began to cool and contract. Placing the bottle inside the refrigerator amplified this result even greater. The air pressure difference between the air outside the bottle and inside the bottle was great enough to pull in the sides of the plastic bottle, crushing the bottle.
You can try to perform this experiment in reverse, by pouring hot water over the bottle to try to get the bottle to expand back to its original shape. This works to some degree, but is difficult because a lot of hot water is needed to heat the air inside the bottle enough. The plastic bottle acts like an insulator, but it will work if enough heat energy is added to bring the temperature of the air back up to the pre-refrigerated level.