Why would a balloon expand as it rises underwater?

Answer 1

Archimedes' principle: an object experiences a lifting force (buoyant force) equal to the weight of the fluid being displaced by the object.

Helium (He)and hot air are lighter than the normal air near the ground level. If the balloon is filled with He or heater air, near the ground level, the normal air that the balloon displaced has a heavier weight that the weight of He or hot air inside the balloon. The net force is up, assuming the balloon itself is weightless (the balloon fabric is so thin that the weight is negligible). When the balloon reaches the upper atmosphere where the surrounding air is as light as the hot air, the balloon will stop ascending.

Answer 2

As humans, we are used to living at 1 atmosphere of pressure, so we rarely think about it. We normally don't feel the pressure on us because the human body is primarily made up of liquid, and liquids are basically non compressible. At times, we do notice changes of pressure, in our ears. You may have noticed your ears "popping" when flying, driving in the mountains, or even going up and down in an elevator. This is because our ears have an air space in them, and air, like all other gases, is compressible.

Boyle's Law states a gas will compress proportionately to the amount of pressure exerted on it. For example, if you have a 1 cubic foot balloon and double the pressure on it, it will be compressed to 1/2 cubic foot. Increase the pressure by 4, and the volume will drop to 1/4 the size etc. This theory was discovered by Sir Robert Boyle, a 17th century scientist. The theory known as Boyle's Law and it states: If the temperature remains constant, the volume of a given mass of gas is inversely proportional to the absolute pressure.

The reverse is also true and answers your question more directly. A half filled balloon at 2 atmospheres (14 ft underwater) will double its pressure when it rises to the surface at 1 atmosphere. It would therefore become a full balloon. That is why divers exhale when surfacing on Scuba.