Key concepts

Ever wonder where those little drops of water on the outside of your cold can of soda pop or bottle of water come from? That’s condensation! Cold surfaces can cause water vapor in the air to cool down, condense and form tiny beads of liquid. The molecules in these miniscule droplets of water are grouped far more closely together than when they were in their gas phase, and exert less pressure—a fact that has some pretty cool physical implications.

Perhaps you have seen the classic science demonstration where a hard-boiled egg is “sucked” into a bottle using a match. The effect is definitely cool, but understanding how it works is tough. Air molecules are spaced differently and exert different levels of pressure depending on how hot or cold they are. This is a fun experiment where the physics are more observable, the effect more dramatic and the pyrotechnics totally unnecessary.

Molecules, which make up everything around us—including air—are in a constant state of motion. The hotter water molecules become, the faster they move, turning from water (their liquid phase) to steam (their gas phase). When liquid water turns to gas, not only do the molecules move much faster, they also are spaced much farther apart. They spread out so much that they generate pressure by pushing on each other and everything else they come into contact with. What happens when we take the heat source away from that steam? The molecules form liquid water again. This is called condensation.

The air in our atmosphere is also a gas that exerts a fairly strong pressure of its own. This experiment will illustrate what can happen when the changing pressure of condensing steam goes up against the pressure of air, which remains relatively constant.

• One large, thick plastic bottle with a wide neck (an empty, 64-ounce fruit juice bottle will work or a three-gallon water-dispenser jug is great). Use caution with thinner plastic containers—hot water can cause them to melt; and avoid glass—boiling water can cause glass to break.
• Small, empty water balloons (Keep more than one handy, in case of breakage.)
• Water
• Stove
• Oven mitt
• Pot or teakettle for boiling water (Use caution and adult help when dealing with hot water.)

• Set a kettle or pot of water to boil on the stove.
• While you’re waiting for your water to boil, fill your balloon full of water using a faucet or a hose. Don’t overinflate the balloon! It should be too large to slip through the neck of the bottle via gravity alone but not so large that it would burst were it to get pushed through.
• Once your water reaches a rapid boil, very carefully pour it into your bottle to about a quarter of the way full.
• Place the filled water balloon in the neck of the bottle.
• Stand back and watch as the balloon gets sucked into the bottle. Knowing what we know now about water and steam pressure, why do you think this happens?
Extra: Try sketching a diagram that includes illustrations of what the air and water molecules look like during each phase of the experiment. Read “Observations and Results” below for some hints.
Extra: Suction is a misleading concept. Condensing steam doesn’t have attractive power of its own, like a magnet does. It doesn’t actually pull or suck the balloon into the bottle. When the steam molecules stop pushing out of the bottle, and stop pushing on the balloon, something else outside the bottle becomes strong enough to push the balloon into the bottle—and it’s not gravity. What might it be?
Extra: What happens if the balloon is too big? Why?

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
Crunch a Can, from
Balloon in a Bottle: An Air Pressure Experiment, from
Balloon Air Pressure Magic, from

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