Balloons in Bottles

Do you have a really competitive friend who can’t stand losing? Then here’s a great trick to play on them; a race to blow up balloons in a bottle and you are guaranteed to win!

Australian Curriculum links

  • Science > Chemical Sciences > Year 5 > ACSSU077

You'll need

  • 2 balloons
  • 2 x 1.25 L PET bottles
  • sharp scissors, knife or lit candle
  • adult supervisor

Try this

  1. Get an adult to use the scissors or knife to carefully make a hole in the bottom of one of the PET bottles, or get an adult to light a candle and hold the base of the bottle over the very tip of the flame until a small hole is formed.
  2. Holding the neck of one balloon, push a balloon into the mouth of one of the PET bottles and stretch the neck of the balloon over the mouth of the bottle.
  3. Repeat Step 2 with the other PET bottle.
  4. Find a friend to challenge, give them the bottle without the hole and make sure you keep the bottle with the hole for yourself. Have a race to see who is the fastest at blowing up a balloon inside a bottle. (The person without a hole in their bottle will not be able to blow up their balloon.)

    Hint: If you’re having trouble blowing up the balloon in the bottle with a hole, try blowing up and deflating the balloon a few times before stretching it onto the bottle.

What's happening?

To blow up a balloon normally, air must be forced into the balloon. The same thing needs to happen to the balloon inside a bottle. However, each PET bottle is already filled with air, so to add more air to the bottle the air inside the bottle needs to be compressed (squashed) to take up a smaller space. This puts the air inside the balloon under high pressure. Our lungs aren’t strong enough (they can’t provide enough pressure) to compress the air in the air in the bottle, so it’s impossible to blow up the balloon in the bottle without a hole. The balloon in the bottle with a hole can be blown up because air in the bottle is able to escape through the hole, making space for the balloon to inflate.

Real world links

When you’re flying in an aeroplane at high altitudes, more than 3000 m above sea level, (a.k.a. high up in the sky) the air pressure is very low and this can be dangerous for our health. At high altitudes under low air pressure, humans can suffer from a range of sicknesses such as hypoxia, altitude sickness, decompression sickness and barotrauma. To prevent these sicknesses aeroplanes flying at high altitudes pump air into the cabin to make the air in the cabin more compressed. The air inside the aeroplane is under higher pressure than the air outside the aeroplane. This is called “cabin pressurisation”.