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.)
• 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?