Why does this work?
This is all about heat absorption. If you simply heat a balloon filled with air, the heat degrades the rubber to the point at which the air pressure inside ruptures the balloon. However, this experiment has you working with water in the balloon! Water is able to conduct far more easily than air, so in this experiment the heat from the candle quickly passes from the rubber balloon into the water inside... which means the balloon doesn't pop whilst the water is able to absorb the heat!
What happens if you run this over a longer time?
You'll find that you can run this for quite a while longer than the air-filled balloon purely because the water will continue to absorb the heat from the flame. So, how long could you run this? Well, it really depends on how much water you have in the balloon as well as the quality of the rubber balloon itself. Try varying the levels of the water in the balloon and see how long your experiment can last!
BTW; whilst the water balloon survives the flame, the water inside the balloon will start to cycle around and around due to convection. Convection is the movement of either gases or liquids due to uneven heating causing density changes that drive movement. How does this convection work in the balloon then?
- Warmer water rises due to the water expanding and becoming less dense than the surrounding colder water.
- As the warmer water rises, the surrounding colder water moves underneath to replace the warm water.
- Once the warmer water reaches the top of the balloon it is away from the flame... so it contracts as it cools down. This contraction makes this water denser and so this water moves down again.
- In the meantime... the water that had moved near the candle flame also heats up and expands & rises... and the process keeps repeating as a continual circular motion of water within the balloon!