Balloon Expansion

How Does It Work

This activity shows that air really is matter (has weight; takes up space). What made the balloon get bigger? It’s simply the expansion of air trapped in the bottle and the balloon. Air expands when it is heated; it spreads out and either fills a larger space or raises the pressure in a closed space. Since the bottle and the balloon were full of air molecules when you stretched the balloon over the opening and sealed them inside, they began to expand upward into the balloon as the bottle was heated. The air molecules moved faster and faster as the temperature rose and they could spread out and fill a larger space as a result. The expanding balloon showed that this was happening. When the heat was removed, the air molecules contracted (got closer together), took up less space, and the balloon collapsed. However, add a little heat again and it starts all over.

Heat energy can be moved from place to place by conduction, radiation, and/or convection. The bottle on the hot plate gets hot because glass conducts (carries or moves) heat from one place to another. If you support the bottle over the hot plate (so the two don’t touch), then the bottle gets hot due to heat radiating from the hot plate (like sunshine). Convection occurs when a fluid (like air) is heated and rises in its center. It cools as it rises and then falls back down the outside of the rising, hotter fluid. At the bottom, it’s heated again and rises in the center in a steady up and down “convective” flow. It’s likely that convection is occurring inside the sealed bottle as heated air rises into the balloon, cools, and then falls back into the bottle to be heated again.

Take It Further

You’re working with a limited amount of air inside the sealed bottle. What happens if you add a tablespoon of water before putting on the balloon? What have you added to the system? What might be the result of adding the water?

What does immersing the warm bottle in room temperature water cause the balloon to collapse even faster? What might be happening if the balloon gets pulled into the bottle a little bit? What does that tell you about differences in air pressure inside and outside of the sealed bottle?

Safety Information

• Using a hot plate will require adult help. Make sure the heat setting you use is in the low to medium range. The hot plate will be very hot even though you don’t see any change in the heat coil. Be careful!
• Here’s a reminder that the bottle will be hot, too! You won’t see a change in it, either, but don’t touch it! Use the hot pad or the tongs to handle the bottle after heating.
• Wear safety glasses! Be aware that breaking the bottle on the hot plate is possible. Always start with a cold hot plate and a cold bottle. Let them heat up together. A cold bottle placed on a hot coil will result in shattering glass.
• Using a microwave or hot water to heat the bottle are not a good alternatives for heating the bottle.