Hot air expands, cold air contracts



Difficulty: ★★☆☆☆

Air is all around us, but it is hard to see it, right? With the aid of balloon and color powder, this science activity visualizes how hot air expands when heated and how cold air contracts when cooled!


Kit contains:

  • Dropper
  • Bottle with cap
  • Plastic tubes x 2
  • Balloons x 2
  • Cups x 2
  • Colored powder
  • Rubber band
  • Small amount of cold water with ice cube and hot water (Please prepare)



(Order this project / Subscribe to Activity Box to get this step-by-step guide! Free sample guide available HERE)


What's happening?

Why does the balloon expand & contract?

Air is made up of molecules (分子) that are in motion constantly. As hot water heats up the air in the bottle, the molecules in the bottle move faster and spread out more, resulting in an increased pressure in the bottle, which causes the balloon to inflate– a phenomenon called ‘thermal expansion’.

The opposite happens when air cools. As cold water cools down the air, the speed of molecules slows down and takes up less space, the air shrinks and deflates the balloon – this is ‘thermal contraction.


Why does the level of colored fluid change?

The fluid shares similar reasoning with the balloon experiment. As hot water rises the temperature of the colored fluid, the molecules of fluid inside the bottle expands, and some is pushed up from the bottle into the narrow tube, just like a thermometer.


Level up the game!

Ask your kids to list some real life application of heat gain/loss (e.g. lid of a bottle, space between railroad tracks)


Acknowledgements & disclaimer: These guides are developed by Moinàrchy. All activities are intended for use with children under adult supervision with due diligence.


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