Pupil with plasma ball

Kit List

  • 2 litre empty clear plastic drinks bottle
  • Balloon
  • Drill / sharp knife / scissors


Safety Notice

Ask an adult to help you with this experiment

What to do

  1. Find all of the items listed above in the equipment list.
  2. Using the drill/knife/scissors, make a small hole in the bottle, around 2 cm from the base. This can be quite tricky, as the bottle will be smooth and slippery plastic, so ask an adult to help rather than risk a cut.
  3. Stretch the balloon over the mouth of the bottle, as pictured.
  4. Try to blow the balloon up inside the bottle, and observe what happens.
  5. Next, cover the hole in the bottle with your finger, and then try to blow up the balloon inside the bottle. Observe what happens.

What's happening

Air is a collection of different gases, and gases take up space in the same way as liquids and solids, it’s just usually a little bit harder to see. The first time you tried to blow up the balloon, it would have worked as normal, and the balloon would have got bigger. The air you blew into the balloon forced the air already inside the bottle out of the hole at the bottom. The next time however, when you tried to blow the air into the balloon, there was nowhere for the air in the bottle to flow, so you couldn’t force any air into the balloon. This is because your finger trapped the air inside the bottle from flowing out, which meant there was no room for the air you were trying to force into the balloon.

You can extend this activity by investigating what happens when you blow the balloon up inside the bottle, and then hold your finger over the hole. The air will stay in the balloon, and not rush out of the top of the bottle. Why is this?

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