Simple Science Experiment: Self-inflating Balloon - MetroFamily Magazine
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Simple Science Experiment: Self-inflating Balloon

by Steve Davala

Reading Time: 2 minutes 

Do you know how a hot air balloon works? They put a hot jet of air into that light nylon and it fills up and then off they go. But how does that really work? 

The real answer has to do with density, or a measure of how closely packed a substance is. When comparing two materials and one is less dense, it will rise to the top. Like wood in water. Wood is less dense than water. When air is heated, the small pieces that make it up expand, or grow apart, thus making it be less dense than cooler air. 

The following simple science experiment will demonstrate this concept. 


  • A balloon that has been inflated several times, a small mouthed bottle (I used plastic, but glass will work as well)
  • A container to hold the bottle that can be filled with water (enough to fill water up to the top of the bottle)
  • Warm/hot water 


  • Put a balloon over the lid of a small mouthed bottle (make sure the balloon has been inflated several times)
  • Put the bottle into a container that is deep enough to nearly cover the lid
  • Safety issue: Holding the bottle inside the container, have a parent fill the container with warm/hot water
  • Wait until the balloon partially inflates
  • After you’ve seen it grow, put it somewhere cool to see what happens


The hot water causes the air inside the bottle and balloon to also heat up. The faster moving particles inside the bottle start to move faster and faster and soon they expand to fill the balloon. No, it doesn’t get too full, there isn’t enough heat to get them moving that far apart. 

How can you change things in this experiment? To try and see if something different will affect the outcome? You can try different temperatures of water to see how much the balloons fill or you could try different sized balloons. Different types of containers, sizes, etc. Lots of things to test out for your inquiring mind. Good luck!

Steve Davala is a high school chemistry and physics teacher who likes to write. He’s got two kids of his own and subjects them to these science activities as guinea pigs. 

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