Why does a balloon pop when it stepped on?

Answer 1

It depends upon the strength of the rubber making up the balloon and how much air is already in it, which will increase pressure in certain regions of balloon when stepped on.

The assumption is the one is stepping on the balloon with a wide shoe bottom, rather than a stiletto heel.

Sometimes, you can step on a balloon, and the part under the foot gets flattened as the air gets displaced into the stretching parts of the balloon coming out from under the sides of the foot. If the air content is low enough to not overcome the tensile strength of the rubber, then the balloon will not pop.

However, if there is enough air in the balloon to overcome the tensile strength of the rubber as the air pressure increases when you step on the balloon, then the balloon will pop.

What happens is the part of the balloon under the foot remains still, and the air is forced away from that region, placing more air pressure on the rest of the balloon. This stretches the balloon so that it comes out the sides of the foot. In these regions, the air pressure is much greater. Now, due to the special intermolecular bonds in rubber, the molecules can remain bound but still stretch and bend, allowing for rubber to stretch. As air pressure builds, it produces a force against the inside of the rubber, causing the rubber to stretch. If the rubber stretches past the point where the molecules separate enough to form a small tear (air hole) in the rubber, then the overall forces placed on the rubber cause a VERY rapid failure in the adjacent regions of the rubber to maintain integrity. They rip, and the rubber next to it rips, and so forth in what is called cascade failure. In all, a large rip forms along the rubber as the air escapes its container (the balloon).