Observations and Results
When the water was heated, its molecules began to move rapidly, turning some into its gas phase: steam. When in a gas phase, water molecules are spaced much farther apart and take up more space. The pressures inside and outside the bottle reach a state of equilibrium, meaning that they are the same. Why? With the neck of the bottle unobstructed, the expanding steam can move from inside the bottle out into the surrounding air.
Here’s when everything changes: When the steam in the bottle starts cooling down and we place the balloon in the bottle’s neck. Without heat, the water molecules inside the bottle start condensing—that is, they start turning from steam back into liquid water. When matter turns from its gas phase back into its liquid phase, the molecules take up much less space and exert far less pressure. In fact, the condensing steam creates a partial vacuum—a region of much lower pressure than that of the surrounding atmosphere—inside the bottle. Remember, unlike the condensing steam the air outside the bottle doesn’t change, and still exerts a pressure of its own. We call the resulting difference between these two areas a pressure gradient. The pressures aren’t able to equalize easily because the balloon blocks the gases from flowing from one area into another. So what happens? The gas on the outside (air) pushes harder than gas on the inside (the condensing steam), so the balloon gets pushed—and pulled—into the bottle.
Another way to describe what happened is to use the word “suction,” because the water balloon was sucked through the neck and into the bottle. But suction can be a misleading concept! What we’re really talking about when we talk about “suction” is a liquid or gas force that pushes on something in the absence of an equal force pushing back. You can crunch an empty water bottle simply by sucking the air out of it. The outside air pressure is what causes the bottle to collapse, because you’ve removed the air inside that was pushing back!
More to explore
Condensation Balloon Trick , from ScienceFix.com
Crunch a Can, from Education.com
Balloon in a Bottle: An Air Pressure Experiment, from Education.com
Balloon Air Pressure Magic, from Education.com
This activity brought to you in partnership with Education.com