Hot air balloon
How to blow up a balloon with warm air
Everyone knows that hot air rises, right? It feels like a fact that I have always known and always assumed to be true. When I think about it, I have seen examples of hot air rising such as steam rising from a cup of tea or the steam that collects on the ceiling of the bathroom when I forget to turn the exhaust fan on – but what kind of experiment would you run that can show kids really clearly that hot air goes up when it’s heated?
Well, I’ve got a super easy, simple and safe experiment that you can set up and do in under five minutes to show kids really clearly that air rises when it gets warm!
1 round balloon
1.25L soft drink bottle or larger
2 large bowls or dishes that you can submerge at least the bottom ¼ of the bottle in
Tray of ice-cubes and cold tap water
- Fit the balloon over the opening of the bottle.
- Fill one bowl with the boiling water but leave space at the top so that it does not overflow when the bottle is submerged.
- Fill the other bowl with the ice cubes and then fill the bowl with water leaving room again to submerge the bottle.
- Place the bottle into the bowl with the hot water and wait for the balloon to stop expanding.
- Move the bottle to the cold water and watch the balloon reduce in size.
The science behind this experiment is pretty straight forward. When air is heated it rises. When the bottle is placed in hot water, the air inside heats up and rises, moving into the balloon and inflating it.
When the bottle is moved to the cold water, the air cools and shift down out of the balloon.