Properties of Matter

Unbalanced balloons

Enrichment Activity for 5-11 11-14

What you need

  • Two similar balloons
  • Two sports drinks bottles
  • Tape
  • Something to cut the ends off the bottles


  1. Cut the lock and screw ends off both bottles, keep the ends unlocked and stretch a balloon over each one
  2. Blow up one balloon a little and lock the cap. Blow up the other a lot more and lock
  3. Tape the caps together and unlock – air flows from the small balloon to the larger one and inflates it even more!

Results and Explanation

The small balloon’s surface is very curved, so most of the tension in the rubber squeezes down on the air, increasing its pressure. The big balloon has a flatter surface, so the tension in the rubber mostly pulls on other rubber.

IOP DOMAINS Physics CPD programme

Electricity CPD videos

Our new set of videos gives teachers and coaches of physics a preview of the training we offer ahead of this term's live support sessions.

Find out more