Water molecules are busy dancing around all the time. But the dance really gets interesting when you add heat to them.
The hotter molecules get, the faster they move, turning from water to steam. And when liquid water turns to gas, not only do the molecules move really fast, but they get spaced so much farther apart that they generate pressure by pushing on everything they come into contact with. Take the heat away from that steam, and it becomes liquid water again. This is called condensation.
Ever wonder where those little drops of water that form on the outside of your cold can of soda come from? That’s condensation! The cold aluminum can causes water vapor in the air to cool down and form tiny beads of water on the can’s surface. The molecules in these beads of water are grouped far more closely together than when they were in their gas phase. So, in an enclosed space, what kind of effect do you think condensation would have on pressure?
What happens when we let cooling steam condense in a bottle that has a water-filled balloon placed over its neck?
- 1 3-gallon water bottle with a wide neck (the kind used in office water coolers)
- Empty water balloons (keep more than one handy in case of breakage)
- Oven Mitt
- Pot or tea kettle for boiling water
- Adult Helper
- 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 through gravity alone, but not so large that it would burst were it to get pushed through.
- Once your water reaches a rapid boil, carefully pour the whole kettle into your bottle.
- Place the filled water balloon in the neck of the bottle.
- Stand back and watch what happens. You may need to wait a few minutes.
- Record your results.
- Draw what you think the water molecules looked like at each phase of the experiment.
After a minute or two, the balloon should have been pushed into the bottle.
When the water was heated, the water molecules began to move rapidly, turning into water’s gas phase: steam. When in a gas phase, water molecules are spaced much further apart and take up more space. The pressure inside the bottle and the pressure outside the bottle reach a state of equilibrium, meaning that the pressure is the same. Why? With an open bottle, expanding gas can move from one area into another. There’s nothing to keep the steam inside the bottle from flowing out into the surrounding air.
Here’s when everything changes: when the water stops boiling, and the steam starts cooling down. The water molecules start condensing—that is, they start turning from steam back into 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. But the air outside the bottle didn’t change, and it’s still exerting pressure of its own! Now, we have what is called a pressure gradient instead of a state of equilibrium, because the air can’t flow freely into the bottle with the balloon in its way! So what happens? The air pressure pushes the water balloon through the neck of the bottle.
How can you take this experiment further? Good places to start include changing the size of the balloon or changing the amount of water in the bottle. The great thing about scientific experimentation is that the possibilities are endless.