Stop the Pop

Once you pop, we’ll show you how to stop. A battle arises between an air-filled balloon and a flame: the flame is victorious! The second battle arises between a water-filled balloon and a flame: the balloon is victorious!

Australian Curriculum links

  • Science > Physical Sciences > Year 3 > ACSSU049

You'll need

  • matches or lighter
  • ½ cup water
  • candle with a sturdy base (Blu-Tack on a plate works well)
  • 2 balloons

Try this

Safety precaution: This activity requires adult supervision or assistance to use the lighter or match.

  1. Blow up one balloon with air until it’s fully inflated.
  2. Fill 1/3 of the other balloon with water and then blow up the balloon with air until it’s fully inflated.
  3. Use the lighter or a match to light the candle.
  4. Slowly lower the air-filled balloon onto the candle flame. (The balloon will pop.)
  5. Slowly lower the water- filled balloon onto the candle flame and hold it there. (The balloon will not pop.)

What's happening?

The air inside the air-filled balloon isn’t very good at transferring the heat energy away from the rubber of the balloon. As a result, the heat energy damages the thin layer of rubber of the balloon. This creates a weakness in the balloon. The air inside the balloon is under pressure and is pushing against the balloon in all directions. As the air inside the balloon pushes against the weakened rubber, it makes the weakness in the balloon get larger, until eventually the balloon pops.

Because the balloon is thin, the heat energy from the flame of the candle passes through the water-filled balloon quickly and heats the water. Water has a very high specific heat capacity, meaning it takes a lot of heat energy to raise the temperature of water. This means that water can absorb a lot of heat energy.

While the water is being heated, the small pieces of water, called molecules, are moving, even though you can’t see them moving. The molecules of water at the bottom of the balloon are heated by the flame and become less dense (lighter), so they rise. The cooler molecules of water at the top of the balloon are denser (heavier), so they sink to the bottom of the balloon. Then the cooler molecules absorb the heat from the flame, get less dense and rise again. This type of heat transfer circulation is called convection. Convection means the water-filled balloon doesn’t pop because the heat is constantly being transferred away from the balloon, and the balloon remains undamaged by the flame.

The black mark left on the bottom of the balloon is not because the balloon has been burnt. It is carbon from the candle flame, which you can rub off with your finger or some paper towel.

Real world links

John Bartlett, a fire-fighter from Florida, U.S.A, discovered that a used disposable baby nappy (diaper) didn’t burn in a fire in a rubbish bin. Disposable nappies have special polymers that can soak up and hold a lot of water (or urine in the case of babies). Because a nappy is good at soaking up water, it kept the nappy from burning during the fire. As a result of this discovery, the first fire-blocking gel was created.

People can use a fire-blocking gel coating on their house to protect it during a bushfire. The gel holds a lot of water and the water absorbs the heat energy of the bushfire. The gel can be applied to the outside of a building to stop it being damaged by the heat energy and flying embers of a bushfire.