In this activity, students can defy logic by putting flame to a balloon without popping it, thanks to the ability of water to conduct heat.
Water has a high heat capacity. In other words, it takes a lot of heat and energy to change the temperature of water by 1oC.
The high heat capacity of water is due to the fact that it takes a lot of energy to separate water molecules (the physical bonds are very strong). Water has a heat capacity about four times that of air. This means that it takes about four times as much heat to raise the temperature of a balloon full of water than it would a similar sized balloon filled with air.
As the water-filled balloon is put on the flame, the heat of the flame is easily absorbed through the balloon and into the water. The water directly above the hot spot rises, cools, and sinks again, carrying away the heat from the hot spot (this cycle is called a convection current). In other words, the thin rubber surface that is being heated is cooled by the comparatively large volume of water above it. This cooling process continues until either all of the water in the balloon becomes too hot, or until a far more concentrated source of heat, such as a blowtorch, is applied to one small area on the balloon,
When an air-filled balloon is placed in a flame, it bursts. Air is a relatively poor conductor of heat away from the thin layer of rubber. As a result, the rubber overheats and the physical bonds holding the rubber polymers together are broken.