Balloon Expansion – Sick Science!

Witness air growing when it is heated

Is there a way to see air changing? Sure, there are clouds and fog, but those are particles of water. Does anything happen to air when it gets cooled or heated? We've come up with a great way to see how air changes when it gets hotter and colder… and you can do it all with just a few things you probably already have in your house!


  • Glass bottle
  • Balloon
  • Hot plate


  1. Stretch the opening of the balloon around the open end of the bottle.
  2. Set the bottle on the hot plate.
  3. Give the bottle and balloon apparatus a few moments to get warm. This may take a while, but you won't want to take your eyes off of the balloon.
  4. Once the bottle is warm, what happens to the balloon? Wow! It's filling with air. But how?

How Does It Work?

So what made the balloon begin to fill with air? We could be wrong here, but we're pretty sure that there isn't a tiny person in there blowing as hard as he can. Instead, you are seeing the expansion of air. Unlike water, which expands when it is cooled, air expands when it is heated. Since the bottle was full of air when you stretched the balloon over it, it only had one place to go when it began expanding… into the balloon!